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'''Number theory''' is a branch of [[pure mathematics]] devoted primarily to the study of the [[integers]]. Any attempt to conduct such a study naturally leads to an examination of the properties of [[prime numbers]] (the building blocks of integers) as well
as the properties of objects made out of integers (such as [[rational number]]s) or defined as generalisations of the integers (such as, for example, [[algebraic integers]]).


'''Number theory''' is a branch of mathematics devoted primarily to the study of the integers. Any attempt to conduct such a study naturally leads to an examination of the properties of that which integers are made of (namely, prime numbers) as well
Integers can be considered either in themselves or as solutions to equations  
as the properties of objects made out of integers (such as rational numbers) or defined as generalisations of the integers (such as, for example, algebraic integers).
([[diophantine geometry]]). Questions in number theory are often best understood through
 
the study of [[Complex analysis|analytical]] objects (e.g., the [[Riemann zeta function]]) that encode properties of the integers, primes or other number-theoretic objects in some fashion ([[analytic number theory]]). One may also study real numbers in relation to rational numbers, e.g., as approximated by the latter ([[diophantine approximation]]).
Integers can be considered either as such or as solutions to equations  
(''diophantine geometry''). Some of the main questions are those of distribution: questions, say,
on patterns or their absence (in the primes or other sequences) or, more generally,
questions on size, number and growth. Such matters are often best understood through
the study of analytical objects (e.g., the [[Riemann zeta function]]) that encode
them in some fashion (''analytical number theory''). One may also study real numbers
in relation to rational numbers, e.g., as approximated by the latter (''diophantine approximation'').


The older term for number theory is ''arithmetic''; it was superseded by "number theory"
The older term for number theory is ''arithmetic''; it was superseded by "number theory"
in the nineteenth century, though the adjective ''arithmetical'' is still fully current.  
in the nineteenth century, though the adjective ''arithmetical'' is still fully current.  
By 1921,
By 1921,
[[Sir Thomas Heath|T. Heath]] had to explain: "By arithmetic Plato meant, not arithmetic
[[T. L. Heath]] had to explain: "By arithmetic Plato meant, not arithmetic
in our sense, but the science which considers numbers in themselves, in other words,
in our sense, but the science which considers numbers in themselves, in other words,
what we mean by the Theory of Numbers."<ref name=HeathAr>Sir Thomas Heath, A History of Greek Mathematics, vol. 1, Dover,
what we mean by the Theory of Numbers."<ref name=HeathAr>Sir Thomas Heath, A History of Greek Mathematics, vol. 1, Dover,
Line 22: Line 18:
viz., as an older synonym for ''number theory''. (The use of the term ''arithmetic''  
viz., as an older synonym for ''number theory''. (The use of the term ''arithmetic''  
for ''number theory'' has regained
for ''number theory'' has regained
some ground since Heath's time, arguably in part due to French influence.)
some ground since Heath's time, arguably in part due to French influence.<ref>Take, e.g.,
[[Jean-Pierre Serre|Serre]]'s ''A Course in Arithmetic'' (1970; translated into
English in 1973). In 1952, [[Harold Davenport|Davenport]] still had to specify that he
meant ''The Higher Arithmetic''. [[G. H. Hardy|Hardy]] and Wright wrote in the introduction to ''An Introduction to the Theory of Numbers'' (1938): "We proposed at one time to change [the title] to ''An introduction to arithmetic'', a more novel and in some ways a more appropriate title; but it was pointed out that this might lead to misunderstandings about the content of the book."</ref> In particular, ''arithmetical'' is preferred as an adjective to ''number-theoretic''. Moreover, "the arithmetic of" is used, whereas
"the number theory of" is not; thus, for example, the ''[[arithmetic of elliptic curves]]''.)


==Origins==
== History ==


===The dawn of arithmetic===
{{main|History of number theory}}


The first historical
===The beginnings===
find of an arithmetical nature is a fragment of a table: the broken clay tablet
[[Plimpton 322]] (Larsa, Mesopotamia, ca. 1800 BCE) contains a list of "[[Pythagorean triples]]", i.e., integers
<math>\scriptstyle (a,b,c)</math> such that <math>\scriptstyle a^2+b^2=c^2</math>.
The triples are too many and too large to have been obtained by brute force.


[[Image:800px-Plimpton 322.jpg|right|thumb|300px|{{#ifexist:Template:800px-Plimpton 322.jpg/credit|{{800px-Plimpton 322.jpg/credit}}<br/>|}}The Plimpton 322 tablet.]]  
While there are elements of what in retrospect can be seen as number theory
in [[Babylonian mathematics|Babylonian]] and ancient Chinese mathematics
(see [[Plimpton 322]] and the [[Chinese Remainder Theorem]], respectively), the history of number theory truly starts with the Greek and Indian traditions.


The table's outlay suggests it was constructed by means of what amounts, in modern language, to the identity
The [[irrational number|irrationality]] of <math>\scriptstyle \sqrt{2}</math> is credited to
the early [[Pythagoreans]].<ref>Plato, Theaetetus, p. 147 B, cited in: Kurt von Fritz, "The discovery of incommensurability by Hippasus of Metapontum", p. 212, in: J. Christianidis (ed.), Classics in the History of Greek Mathematics, Kluwer, 2004. [[Plato]] reports on further work by Theodorus on
irrationality.</ref> [[Euclid]] gave an algorithm for computing the
greatest common divisor of two numbers ([[Euclid's Elements]], Prop. VII.2) and a proof
that there are infinitely many primes (Elements, Prop. IX.20).
Much later in the Hellenistic period, [[Diophantus]] studied rational solutions to equations
and systems of equations.


<center><math>\left(\frac{1}{2} \left(x - \frac{1}{x}\right)\right)^2 + 1 =
Results in number theory within [[Indian mathematics]] date from the period that would correspond to the medieval era in Europe. [[Aryabhata]] gave an algorithm for solving<ref name="Aryabhata">Āryabhaṭa,
\left(\frac{1}{2} \left(x + \frac{1}{x}\right)\right)^2;</math></center>
''Āryabhatīya'', Chapter 2, verses 32-33, cited in: K. Plofker, ''Mathematics in India'', Princeton University Press, 2008,
pp. 134-140.
See also W. E. Clark, ''The Āryabhaṭīya of Āryabhaṭa: An ancient Indian work on Mathematics and Astronomy'', University of Chicago Press, 1930, pp. 42-50. A slightly more explicit description of the ''kuṭṭaka'' was later given in [[Brahmagupta]],
''Brāhmasphuṭasiddhānta'', XVIII, 3-5 (in Colebrooke, ''Algebra, with Arithmetic and Mensuration, from the Sanscrit of Brahmegupta and Bháscara'', London, 1817, p. 325, cited in: Clark, op. cit., p. 42).</ref>
pairs of [[congruences]]
<math>\scriptstyle n\equiv a_1 \text{ mod } m_1</math>,
<math>\scriptstyle n\equiv a_2 \text{ mod } m_2</math>,
apparently with astronomical applications in mind.<ref name="Plofker">K. Plofker, ''Mathematics in India'', Princeton University Press, 2008, p. 119.</ref>
[[Brahmagupta]] started the systematic study of indefinite quadratic equations, including
what would later be misnamed [[Pell's equation]]. A general procedure (the [[chakravala method|chakravala]], or "cyclic method") for solving Pell's equation was
finally found by Jayadeva (cited in the eleventh century; his work is otherwise lost); the earliest surviving exposition appears in [[Bhāskara II]]'s
Bīja-gaṇita (twelfth century).<ref name="PlofBha">Plofker, op. cit., p. 194</ref>
Unfortunately, these achievements were largely unknown in the West until the late eighteenth century.<ref name="Ploper">Plofker, op. cit., p. 283</ref>


this identity is implicit in routine Old Babylonian exercises<ref name=Robson>Eleanor Robson, "Neither Sherlock Holmes nor Babylon: a reassessment of Plimpton 322", ''Historia Math.'' '''28''' (3), p. 184.</ref>.
Much Greek mathematics and some Indian mathematics was available to Arabic scholars from the early ninth century onwards. Part of the treatise ''al-Fakhri'' (by [[al-Karaji|al-Karajī]], 953 - ca. 1029) builds on Diophantus's work to some extent.
If some other method was used, the triples were first
constructed and then reordered by <math>c/a</math>, presumably for actual use as a "table",
i.e., with a view to applications.


We do not know what these applications may have been, or whether there could have been any; [[Babylonian astronomy]], for example, truly flowered only later. It has been suggested instead that the table was a source of numerical examples for school problems. Alternatively, the table could have served to demonstrate a method for solving a problem
=== Modern number theory ===
of interest to one's students or fellow scribes.


While Babylonian number theory - or what survives of Babylonian mathematics that can be called thus - consists of this single, striking fragment,  
Modern number theory begins with [[Pierre de Fermat]], inspired in part by his study of Diophantus. Continuous activity on the subject started almost a century later with [[Euler]].<ref>A. Weil, 'Number theory: an approach through history - from Hammurapi to Legendre'', Birkhäuser, 1984, pp. 1-2.</ref> [[Lagrange]] provided proofs of some of Fermat's and Euler's key statements. He and [[Legendre]] also set the basis
Babylonian algebra (in the
of the study of quadratic forms; Legendre was the first to state the law of [[quadratic reciprocity]]. In [[Disquisitiones Arithmeticae]], Gauss gave the first valid proof of this law, developed the theory of quadratic forms further, and started the modern study of [[cyclotomy]].
secondary-school sense of "algebra") was exceptionally well developed. [[Iamblichus]] states that [[Pythagoras]] learned mathematics from the Babylonians, and
there is no very strong reason to believe otherwise. (Much earlier sources attest to
the travels and studies of [[Thales]] and [[Pythagoras]] in [[Egypt]].)


Pythagoras was a mystic who gave great importance to the odd and the even. Euclid IX 21--34
Starting early in the nineteenth century, the following developments gradually took place:
is very probably Pythagorean; it is very simple material
* The rise to self-consciousness of number theory (or ''higher arithmetic'') as a field of study.<ref>See the discussion in section 5 of C. Goldstein and N. Schappacher, "A book in search of a discipline (1801-1860)', in C. Goldstein, N. Schappacher and J. Schwermer (eds.), "The shaping of arithmetic after C. F. Gauss's Disquisitiones Arithmeticae", Springer, 2007. Early signs of self-consciousness are present already in letters by Fermat: thus his remarks on what number theory is, and how "Diophantus's work [...] does not really belong to [it]" (quoted in A. Weil, op. cit., p. 25).</ref>
("odd times even is odd", "if an odd number measures [= divides] an even number, then it also measures [= divides] half of it"), but it is all that is needed to prove that <math>\scriptstyle \sqrt{2}</math>
* The development of much of modern mathematics necessary for basic modern number theory: complex analysis, group theory, Galois theory -- accompanied by greater rigor in analysis and abstraction in algebra.
is irrational. The discovery that <math>\scriptstyle \sqrt{2}</math> is irrational is credited
* The rough subdivision of number theory into its modern subfields - in particular, [[analytic number theory|analytic]] and [[algebraic number theory]].
to the early Pythagoreans (pre-[[Theodorus of Cyrene|Theodorus]]). By revealing (in modern
terms) that numbers could be irrational, this discovery seems to have
provoked the first foundational crisis in mathematical history; its proof or its divulgation
are sometimes credited to [[Hippasus of Metapontum|Hippasus]], who was expelled or split from
the Pythagorean sect. It is only here that we can start to speak of a clear, conscious division between
''numbers'' (integers and the rationals - the subjects of arithmetic) and ''lengths'' ([[real numbers]], whether rational or not).


The Pythagorean tradition spoke also of so-called [[polygonal number|polygonal numbers]] and [[perfect number|perfect numbers]]. The study of the latter made some look at the divisors of integers. While square numbers, cubic numbers, etc., are seen now as more natural than triangular numbers, square numbers, pentagonal numbers, etc., the study of the sums
Algebraic number theory may be said to start with the study of reciprocity and cyclotomy, but truly came into its own with the development of abstract algebra and early ideal theory and valuation theory; see below. An obvious conventional starting point for analytic number theory would be [[Riemann]]'s memoir on the [[Riemann zeta function]] (1859); there is also
of triangular and pentagonal numbers would prove very fruitful in the early modern period (17th to early 19th century).
[[Dirichlet's theorem on arithmetic progressions]], which preceded it in the study of the
[[Riemann zeta function|zeta function]] and even [[L-functions]] (for <math>\scriptstyle Re(s)>1</math>), or
[[Jacobi]]'s work on the four square theorem, which connected arithmetical questions with [[elliptic functions]]. The first use of analytical arguments in number theory goes further back,
to [[Euler]].<ref>H. Iwaniec and E. Kowalski, Analytic number theory, AMS Colloquium Pub., Vol. 53, 2004, p. 1.</ref>


We know of no clearly arithmetical material in [[Egyptian mathematics|ancient Egyptian]] or [[Vedic mathematics]], though there is some algebra in both. Imperial China has left us a single result in arithmetic, namely, the
The history of each subfield is sketched in its own section below. Many of the most interesting questions in each area remain open and are being actively worked on.
basic statement known as the [[Chinese remainder theorem]] to all students of number theory.
The result appears as an exercise in [[Sun Zi]]'s [[Suan Ching]] (also known Sun Tzu's Mathematical Classic; 3rd, 4th or 5th century CE). A sketch of a method of solution is given: Sun Zi finds a number given its residues
much as a modern would, though we do not know whether he had a good way of taking an important intermediate step - namely, finding the inverse of a number modulo another. (The earliest full solution known to this last problem is [[Āryabhata]]'s; see below.)


There is also some numerical mysticism in Chinese mathematics, but, unlike that of the Pythagoreans, it seems to have
== Approaches and subfields ==
led nowhere. Like the Pythagoreans' perfect numbers, [[magic squares]] have passed from superstition into recreation.


=== Plato and Euclid ===
===Introductory texts and elementary tools===


Aside from a few fragments, the mathematics of Classical Greece is known to us either through the reports of contemporary non-mathematicians or through mathematical works from the early Hellenistic period. In the case of number theory, this
Two of the most popular introductions to the subject are:
means, large and by, ''Plato'' and ''Euclid'', respectively.
* [[G_H_Hardy|G. H. Hardy]] and E. M. Wright, ''An introduction to the theory of numbers'', 6th ed., rev. by D. R. Heath-Brown and J. H. Silverman, Oxford University Press, Oxford, 2008 (first published in 1938).
* [[Ivan_Matveyevich_Vinogradov|I. M. Vinogradov]], ''Elements of Number Theory'', Mineola, NY: Dover Publications, 2003, reprint of the 1954 edition.


[[Plato]] had a keen interest in mathematics, and distinguished clearly between arithmetic and calculation. (By ''arithmetic'' he meant, in part, theorising on number, rather than what ''arithmetic'' or ''number theory'' have come to mean.) It is through one of Plato's dialogues -- namely,
Hardy and Wright's book is a comprehensive classic, though its clarity sometimes suffers due to the authors' insistence on elementary methods.<ref name="MR0568909">T. M. Apostol,
''Theaetetus'' -- that we know that [[Theodorus of Cyrene|Theodorus]] had proven that <math>\scriptstyle \sqrt{3}, \sqrt{5}, \dots, \sqrt{17}</math> are irrational. [[Theaetetus of Athens|Theaetetus]] was, like Plato, a disciple of Theodorus's; he worked on distinguishing different kind of inconmensurables, and was thus arguably a pioneer in the study of [[number systems]]. (Book X  of [[Euclid's Elements|Euclid]] is described by [[Pappus of Alexandria|Pappus]] as being largely based on Theaetetus's work.)
Review of ''An introduction to the theory of numbers'', Mathematical Reviews, MR0568909.</ref>
Vinogradov's main attraction consists in its set of problems, which quickly lead to Vinogradov's own research interests; the text itself is very basic and close to minimal.


... [[Euclid]] devoted part of his Elements to prime numbers and divisibility, topics that belong unambiguously to number theory and are basic thereto;
The term ''[[elementary proof|elementary]]'' generally denotes a method that does not use [[complex analysis]]. For example, the [[prime number theorem]] was first proven in 1896, but an elementary proof was found only in 1949. The term is somewhat ambiguous: for example, proofs based on [[Tauberian theorem|Tauberian theorems]] are often seen as quite enlightening but not elementary, in spite of using Fourier analysis, rather than complex analysis as such. Here as elsewhere, an ''elementary'' proof may be longer and more difficult for most readers than a non-elementary one.
in particular, he gave the first known proof of the [[infinitude of primes]] ...


=== Diophantus ===
Number theory has the reputation of being a field many of whose results can be stated to the layperson. At the same time, the proofs of these results are not particularly accessible, in part because the range of tools they use is, if anything, unusually broad within mathematics.


Very little is known about [[Diophantus of Alexandria]]; he probably lived in the third century CE, that is, about five hundred years after Euclid. Diophantus's [[Arithmetica]] - of which only six out of thirteen chapters survive - is a collection of worked-out problems where the task is invariably to find rational solutions to a system of polynomial equations, usually of the form <math>\scriptstyle f(x,y)=z^2</math> or <math>\scriptstyle f(x,y,z)=w^2</math>. Thus, nowadays, we speak of ''Diophantine equations'' when we speak of polynomial equations to which rational or integer solutions must be found.  
Popular choices for a second textbook include [[Borevich]] and [[Igor_Shafarevich|Shafarevich]]'s ''Number theory'' and [[Jean-Pierre_Serre|Serre]]'s ''Cours d'arithmetique''. Textbooks for later stages in one's study tend to branch into analytic and algebraic number theory, among other subfields.


One may say that Diophantus was studying rational points -- i.e., points whose coordinates are rational --
===Main fields===
on [[curve|curves]] and [[variety|varieties]]; however, unlike the Greeks of the Classical period, who did what we would now call basic algebra in geometrical terms, Diophantus did what we would now call basic algebraic geometry in purely algebraic terms. In modern terms, what Diophantus does is to find rational parametrisations of many varieties, in other words, he shows how to obtain all rational numbers satisfying a system of equations by giving a procedure that can be made into an algebraic expression
====Analytic number theory====
(say, <math>\scriptstyle x=f(r,s)</math>, <math>\scriptstyle y=g(r,s)</math>, <math>\scriptstyle z=h(r,s)</math>,
where <math>\scriptstyle f</math>, <math>\scriptstyle g</math> and <math>\scriptstyle h</math> are polynomials
or quotients of polynomials).


When Diophantus does not succeed in giving a rational parametrisation, it is usually because - as one can nowadays show - no rational parametrisation is possible for the variety <math>\scriptstyle V</math> in question. When he is in such a situation, Diophantus contents himself with a procedure for finding some rational points on <math>\scriptstyle V</math>. This generally means that he gives a rational parametrisation for a subvariety of <math>\scriptstyle V</math>. However, he also studies the equations of some non-rational curves, for which no rational parametrisation is possible, and for which there are no rational subvarieties of dimension <math>\scriptstyle 0</math>. He manages to find some rational points on these curves -- [[elliptic curves]], as it happens, in what seems to be their first known occurence -- by means of what amounts (in geometrical terms) to a tangent construction. (He also resorts to what could be called a special case of a secant construction.)
{{main|Analytic number theory}}


While Diophantus is concerned largely with rational solutions, he assumes some results on integer numbers; in particular, he seems to assume that every integer is the sum of four squares, though he never states as much explicitly.
''Analytic number theory'' is generally held to denote the study of problems in number theory by analytic means, i.e., by the tools of
[[calculus]]. Some would emphasize the use of [[complex analysis]]: the study of the [[Riemann zeta function]] and other [[L-functions]] can be seen as the epitome of analytic number theory. At the same time, the subfield is often held to cover studies of elementary problems by elementary means, e.g., the study of the divisors of a number without the use of analysis, or the application of [[sieve methods]]. A problem in number theory can be said to be ''analytic'' simply if it involves statements on quantity or distribution, or if the ordering of the objects studied (e.g., the [[prime numbers|primes]]) is crucial. Several different senses of the word ''analytic'' are thus conflated in the designation ''analytic number theory'' as it is commonly used.


=== The Indian school: Āryabhaṭa, Brahmagupta, Bhāskara ===
The following are examples of problems in analytic number theory: the [[prime number theorem]], the [[Goldbach conjecture]] (or the [[twin prime conjecture]], or the [[Hardy-Littlewood conjectures]]), the [[Waring problem]] and the [[Riemann Hypothesis]]. Some of the most important tools of analytic number theory are the [[circle method]], [[sieve methods]] and [[L-functions]] (or, rather, the study of their properties).


... [[Brahmagupta]] (628 CE) started the systematic study of indefinite quadratic equations -- in particular, the misnamed
One may ask analytic questions about [[algebraic numbers]], and use analytic means to answer such questions; it is thus that algebraic and analytic number theory intersect. For example, one may define [[prime ideals]] (generalisations of [[prime number|prime numbers]] living in the field of algebraic numbers) and ask how many prime ideals there are up to a certain size. This question can be answered by means of an examination of [[Dedekind zeta function]]s, which are generalisations of the [[Riemann zeta function]], an all-important analytic object that controls the distribution of prime numbers.
[[Pell's equation|Pell equation]], in which [[Archimedes]] may have first been interested. Later Sanskrit authors would
follow, using Brahmagupta's technical terminology. A general method (the [[cakravāla]]) for solving Pell's equation was
finally found by [[Jayadeva]] (cited in the eleventh century; his work is otherwise lost) and [[Bhāskara II|Bhāskara]] (twelfth century).


=== Arithmetic in the Islamic golden age ===
====Algebraic number theory====


In the early ninth century, the caliph Al-Ma'mun ordered translations of many Greek mathematical works and at least one Sanskrit work (generally presumed to be Brahmagupta's), thus giving rise to the rich tradition of [[Islamic mathematics]].
{{main|Algebraic number theory}}
Diophantus's main work, the ''Arithmetica'', was translated into Arabic in the 10th century; [[al-Karajī]] would build on it
within a generation. Al-Karajī's contemporary
[[Ibn al-Haytham]] knew<ref name=Rashed>Roshdi Rashed, Ibn al-Haytham el le théorème de Wilson, Arch. Hist. Exact Sci. 22 (1980), no. 4, pp. 305-321.</ref> what would later be called [[Wilson's theorem]], which, arguably, was thus the first clearly non-trivial result on [[congruences]] to prime moduli ever known.


Other than a treatise on squares in arithmetic progression by
''Algebraic number theory'' studies algebraic properties and algebraic objects of interest in number theory. (Thus, analytic and algebraic number theory can and do overlap:
[[Fibonacci]] - who lived and studied in north Africa and Constantinople during his formative
the former is defined by its methods, the latter by its objects of study.)
years, ca. 1175-1200 - no number theory to speak of was done in western Europe while it went through the Middle Ages.
A key topic is that of the [[algebraic number|algebraic numbers]], which are generalisations of the rational numbers. Briefly, an ''algebraic number'' is any complex number that is a solution to some polynomial equation <math>\scriptstyle f(x)=0</math> with rational coefficients;
Matters started to change in Europe in the late [[Rennaissance]], thanks to a renewed study of the works of Greek antiquity.  
for example, every solution <math>x</math> of <math>\scriptstyle x^5 + (11/2) x^3 - 7 x^2 + 9 = 0
The key catalyst was the textual emendation and translation into Latin of Diophantus's ''Arithmetica''.
</math> (say) is an algebraic number. Fields of algebraic numbers are also called ''[[algebraic number field]]s''.


===Early modern number theory===
It could be argued that the simplest kind of number fields (viz., quadratic fields) were already studied by [[Gauss]], as the discussion of quadratic forms in ''Disquisitiones arithmeticae'' can be restated in terms of [[Ideal (mathematics)|ideals]] and
[[Norm (mathematics)|norms]] in quadratic fields. (A ''quadratic field'' consists of all
numbers of the form <math>\scriptstyle a + b \sqrt{d}</math>, where
<math>a</math> and <math>b</math> are rational numbers and <math>d</math>
is a fixed rational number whose square root is not rational.)
For that matter, the 11th-century [[chakravala method]] amounts - in modern terms - to an algorithm for finding the units of a real quadratic number field. However, neither [[Bhāskara II|Bhāskara]] nor Gauss knew of number fields as such.


==Subfields==
The grounds of the subject as we know it were set in the late nineteenth century, when ''ideal numbers'', the ''theory of ideals'' and ''valuation theory'' were developed; these are three complementary ways of dealing with the lack of unique factorisation in algebraic number fields. (For example, in the field generated by the rationals
and <math>\scriptstyle \sqrt{-5}</math>, the number <math>6</math> can be factorised both as <math>\scriptstyle 6 = 2 \cdot 3</math> and
<math>\scriptstyle 6 = (1 + \sqrt{-5}) ( 1 - \sqrt{-5})</math>; all of <math>2</math>, <math>3</math>, <math>\scriptstyle 1 + \sqrt{-5}</math> and
<math>\scriptstyle 1 - \sqrt{-5}</math>
are irreducible, and thus, in a naïve sense, analogous to primes among the integers.)
The initial impetus for the development of ideal numbers (by [[Ernst Kummer|Kummer]]) seems to have come from the study
of higher reciprocity laws,<ref name="Edwards">H. M. Edwards, Fermat's Last Theorem: a genetic introduction to algebraic number theory, Springer Verlag, 1977, p. 79.</ref> i.e., generalisations of [[quadratic reciprocity]].


===Analytic number theory===
Number fields are often studied as extensions of smaller number fields: a field ''L'' is said to be an ''extension'' of a field ''K'' if ''L'' contains ''K''.
(For example, the complex numbers ''C'' are an extension of the reals ''R'',
and the reals ''R'' are an extension of the rationals ''Q''.)
Classifying the possible extensions of a given number field is a difficult and partially open problem. Abelian extensions -- that is, extensions ''L'' of ''K'' such that the [[Galois group]]<ref>The Galois group
of an extension ''K/L'' consists of the operations ([[isomorphisms]]) that send elements
of L to other elements of L while leaving all elements of K fixed.
Thus, for instance, ''Gal(C/R)'' consists of two elements: the identity element
(taking every element ''x+iy'' of ''C'' to itself) and complex conjugation
(the map taking each element ''x+iy'' to ''x-iy'').
The Galois group of an extension tells us many of its crucial properties. The study of Galois groups started with [[Evariste Galois]]; in modern language, the main outcome of his work is that an equation ''f(x)=0'' can be solved by radicals
(that is, ''x'' can be expressed in terms of the four basic operations together
with square roots, cubic roots, etc.) if and only if the extension of the rationals by the roots of the equation ''f(x)=0'' has a Galois group that is  [[solvable]]
in the sense of group theory. ("Solvable", in the sense of group theory, is
a simple property that can be checked easily for finite groups.)</ref> ''Gal(L/K)'' of ''L'' over ''K'' is an [[abelian group]] -- are relatively well understood.
Their classification was the object of the programme of [[class field theory]], which was initiated in the late 19th century (partly by [[Kronecker]] and [[Eisenstein]]) and carried out largely in 1900--1950.


''Analytic number theory'' is generally held to denote the study of problems in number theory by analytic means, i.e., by the tools of
The [[Langlands program]], one of the main current large-scale research plans in mathematics, is sometimes described as an attempt to generalise class field theory to non-abelian extensions of number fields.
[[calculus]]. Some would emphasize the use of complex analysis: the study of the [[Riemann zeta function]] and other L-functions can be seen as the epitome of analytic number theory. At the same time, the subfield is often held to cover studies of elementary problems by elementary means, e.g., the study of the divisors of a number without the use of analysis, or the application of [[sieve methods]]. A problem in number theory can be said to be ''analytic'' simply if it involves statements on quantity or distribution, or if the ordering of the objects studied (e.g., the primes) is crucial. Several different senses of the word ''analytic'' are thus conflated in the designation ''analytic number theory'' as it is commonly used.
 
====Diophantine geometry====
 
{{main|Diophantine geometry}}
{{main|Glossary of arithmetic and Diophantine geometry}}


The following are examples of problems in analytic number theory: the [[prime number theorem]], the [[Goldbach conjecture]] (or the [[twin prime conjecture]], or the [[Hardy-Littlewood conjectures]]), the [[Waring problem]] and the [[Riemann Hypothesis]]. Some of the most important tools of analytic number theory are [[the circle method]], [[sieve methods]] and [[L-functions]] (or, rather, the study of their properties).
The central problem of ''Diophantine geometry'' is to determine when a [[Diophantine equation]] has solutions, and if it does, how many. The approach taken is to think of the solutions of an equation as a geometric object.


One may ask analytic questions about algebraic numbers, and use analytic means to answer such questions; it is thus that algebraic and analytic number theory intersect. For example, one may define [[prime ideals]] (generalisations of [[prime number|prime numbers]] living in the field of algebraic numbers) and ask how many prime ideals there are up to a certain size. This question can be answered by means of an examination of [[Dedekind zeta function]]s, which are generalisations of the [[Riemann zeta function]], an all-important analytic object that controls the distribution of prime numbers.
For example, an equation in two variables defines a curve in the plane. More generally, an equation, or system of equations, in two or more variables defines a [[algebraic curve|curve]], a [[algebraic surface|surface]] or some other such object in ''n''-dimensional space. In Diophantine geometry, one asks whether there are any ''[[rational points]]'' (points all of whose coordinates are rationals) or
''integral points'' (points all of whose coordinates are integers) on the curve or surface. If there are any such points, the next step is to ask how many there are and how they are distributed. A basic question in this direction is: are there finitely
or infinitely many rational points on a given curve (or surface)? What about integer points?


===Algebraic number theory===
An example here may be helpful. Consider the equation <math>x^2+y^2 = 1</math>;
we would like to study its rational solutions, i.e., its solutions
<math>(x,y)</math> such that
''x'' and ''y'' are both rational. This is the same as asking for all integer solutions
to <math>a^2 + b^2 = c^2</math>; any solution to the latter equation gives
us a solution <math>x = a/c</math>, <math>y = b/c</math> to the former. It is also the
same as asking for all points with rational coordinates on the curve
described by <math>x^2 + y^2 = 1</math>. (This curve happens to be a circle of radius 1 around the origin.)


''Algebraic number theory'' studies fields of [[algebraic number|algebraic numbers]], which are generalisations of the rational numbers. (Briefly, an ''algebraic number'' is any complex number that is a solution to some polynomial equation with rational coefficients.) Fields of algebraic numbers are also called ''number fields''.
The rephrasing of questions on equations in terms of points on curves turns out to be felicitous. The finiteness or not of the number of rational or integer points on an algebraic curve - that is, rational or integer solutions to an equation <math>f(x,y)=0</math>, where <math>f</math> is a polynomial in two variables - turns out to depend crucially on the ''genus'' of the curve. The ''genus'' can be defined as follows:<ref>It may be useful to look at an example here. Say we want to study the curve <math>y^2 = x^3 + 7</math>. We allow ''x'' and ''y'' to be complex numbers: <math>(a + b i)^2 = (c + d i)^3 + 7</math>. This is, in effect, a set of two equations on four variables, since both the real
and the imaginary part on each side must match. As a result, we get a surface (two-dimensional) in four dimensional space. After we choose a convenient hyperplane on which to project the surface (meaning that, say, we choose to ignore the coordinate ''a''), we can
plot the resulting projection, which is a surface in ordinary three-dimensional space. It
then becomes clear that the result is a [[torus]], i.e., the surface of a doughnut (somewhat
stretched). A doughnut has one hole; hence the genus is 1.</ref> allow the variables in <math>f(x,y)=0</math> to be complex numbers; then <math>f(x,y)=0</math> defines a 2-dimensional surface in (projective) 4-dimensional space (since two complex variables can be decomposed into four real variables, i.e., four dimensions). Count
the number of (doughnut) holes in the surface; call this number the ''genus'' of <math>f(x,y)=0</math>. Other geometrical notions turn out to be just as crucial.


It could be argued that the simplest kind of number fields (viz., those of degree two over the rationals) were already studied by [[Gauss]], as the discussion of quadratic forms in ''Disquisitiones arithmeticae'' can be restated in terms of ideals and norms in quadratic fields. For that matter, the 11th-century [[cakravāla]] method amounts - in modern terms - to an algorithm for finding the units of a real quadratic number field. However, neither [[Bhāskara II|Bhāskara]] nor Gauss knew of number fields as such.
There is also the closely linked area of [[diophantine approximations]]: given a number <math>x</math>, how well can it be approximated by rationals? (We are looking for approximations that are good relative to the amount of space that it takes to write the rational: call <math>a/q</math> (with <math>gcd(a,q)=1</math>) a good approximation to <math>x</math> if <math>\scriptstyle |x-a/q|<\frac{1}{q^c}</math>, where <math>c</math> is large.) This question is of special interest if <math>x</math> is an algebraic number. If <math>x</math> cannot be well approximated, then some equations do not have integer or rational solutions. Moreover, several concepts (especially that of [[height]]) turn out to be crucial both in diophantine geometry and in the study of diophantine approximations.


The grounds of the subject as we know it were set in the late nineteenth century, when ''ideal numbers'', the ''theory of ideals'' and ''valuation theory'' were developed; these are three complementary ways of dealing with the lack of unique factorisation in algebraic number fields. (For example, in the field generated by the rationals
Diophantine geometry should not be confused with the [[geometry of numbers]], which is a collection of graphical methods for answering certain questions in algebraic number theory. ''Arithmetic geometry'', on the other hand, is a contemporary term
and <math>\scriptstyle \sqrt{-5}</math>, the number <math>6</math> can be factorised both as <math>\scriptstyle 6 = 2 \cdot 3</math> and
for much the same domain as that covered by the term ''diophantine geometry''. The term ''arithmetic geometry'' is arguably used
<math>\scriptstyle 6 = (1 + \sqrt{-5}) ( 1 - \sqrt{-5})</math>; all of <math>2</math>, <math>3</math>, <math>\scriptstyle 1 + \sqrt{-5}</math> and
most often when one wishes to emphasise the connections to modern algebraic geometry (as in, for instance, [[Faltings' theorem]]) rather than to techniques in diophantine approximations.
<math>\scriptstyle 1 - \sqrt{-5}</math>
are irreducible, and thus, in a naïve sense, analogous to primes among the integers.)
A failure of awareness of this lack had led to an early erroneous "proof" of [[Fermat's Last Theorem]] by G. Lamé; the realisation that this proof was erroneous made others study the consequences of this lack, and ways in which it could be alleviated.


Number fields are often studied as extensions of smaller number fields: a number field ''L'' is said to be an ''extension'' of a number field ''K'' if ''L'' contains ''K''. Classifying the possible extensions of a given number field is a difficult and partially open problem. Abelian extensions -- that is, extensions ''L'' of ''K'' such that the [[Galois group]] ''Gal(L/K)'' of ''L'' over ''K'' is an abelian group -- are relatively well understood. Their classification was the object of the programme of [[class field theory]], which was initiated in the late 19th century (partly by [[Kronecker]] and [[Eisenstein]]) and carried out largely in 1900--1950.
===Recent approaches and subfields===


The [[Langlands program]] is sometimes described as an attempt to generalise class field theory to non-abelian extensions of number fields.
The areas below date as such from no earlier than the mid-twentieth century, even if they are based
on older material. For example, as is explained below, the matter of algorithms in number theory is very old, in some sense older than the concept of proof; at the same time, the modern study of computational complexity dates only from the 1930s and 1940s.


===Diophantine geometry===
====Probabilistic number theory ====


Consider an equation or system of equations. Does it have rational or integer solutions, and if so, how many? This is the central question of ''Diophantine geometry''.
{{main|Probabilistic number theory}}


We may think of this question in the following graphic way. An equation in two variables defines a curve in the plane; more generally, an equation, or system of equations, in two or more variables defines a curve, a surface or some other such object in ''n''-dimensional space. We are asking whether there are any ''rational points'' (points all of whose coordinates are rationals) or
Take a number at random between one and a million. How likely is it to be prime? This is just another way of asking how many primes there are between one and a million. Very well; ask further: how many prime divisors will it have, on average? How many divisors will it have altogether, and with what likelihood? What is the probability that it have many more or many fewer divisors or prime divisors than the average?
''integer points'' (points all of whose coordinates are integers) on the curve or surface. If there are any such points on the curve or surface, we may ask how many there are and how they are distributed. Most importantly: are there finitely
or infinitely many rational points on a given curve (or surface)? What about integer points?


The rephrasing of questions on equations in terms of points on curves turns out to be felicitous. The finiteness or not of the number of rational or integer points on an algebraic curve - that is, rational or integer solutions to an equation <math>f(x,y)=0</math>, where <math>f</math> is a polynomial in two variables - turns out to depend crucially on the ''genus'' of the curve. The ''genus'' can be defined as follows: allow the variables in <math>f(x,y)=0</math> to be complex numbers; then <math>f(x,y)=0</math> defines a 2-dimensional surface in 4-dimensional surface; count the number of (doughnut) holes in the surface; call this number the ''genus'' of <math>f(x,y)=0</math>. Other geometrical notions turn out to be just as crucial.
Much of probabilistic number theory can be seen as an important special case of the study of variables that are almost, but not quite, mutually [[statistical independence|independent]]. For example, the event that a random integer between one and a million be divisible by two and the event that it be divisible by three are almost independent, but not quite.


There is also the closely linked area of ''diophantine approximations'': given a number <math>x</math>, how well can it be approximated by rationals? (We are looking for approximations that are good relative to the amount of space that it takes to write the rational: call <math>\scriptstyle a/q</math> (with <math>\scriptstyle gcd(a,q)=1</math>) a good approximation to <math>x</math> if <math>\scriptstyle |x-a/q|<\frac{1}{q^c}</math>, where <math>c</math> is large.) This question is of special interest if <math>x</math> is an algebraic number. If <math>x</math> cannot be well approximated, then some equations do not have integer or rational solutions. Moreover, several concepts (especially that of [[height]]) turn out to be crucial both in diophantine geometry and in the study of diophantine approximations.
It is sometimes said that probabilistic combinatorics uses the fact that whatever happens with probability greater than <math>0</math> must happen sometimes; one may say with equal justice that many applications of probabilistic number theory hinge on the fact that whatever is unusual must be rare. If certain algebraic objects (say, rational or integer solutions to certain equations) can be shown to be in the tail of certain sensibly defined distributions, it follows that there must be few of them; this is a very concrete non-probabilistic statement following from a probabilistic one.


Diophantine geometry should not be confused with the ''geometry of numbers'', which is a collection of graphical methods for answering certain questions in algebraic number theory.
====Arithmetic combinatorics====


===Arithmetic combinatorics===
{{main|Arithmetic combinatorics}}


Let <math>A</math> be a set of integers. Consider the set <math>A+A</math> consisting of all sums of two elements of <math>A</math>. Is <math>A+A</math> much larger than A? Barely larger? If <math>A + A</math> is barely larger than <math>A</math>, must <math>A</math> have plenty of arithmetic structure - e.g., does it look like an arithmetic progression?  
Let <math>A</math> be a set of integers. Consider the set <math>A+A</math> consisting of all sums of two elements of <math>A</math>. Is <math>A+A</math> much larger than A? Barely larger? If <math>A + A</math> is barely larger than <math>A</math>, must <math>A</math> have plenty of arithmetic structure - e.g., does it look like an arithmetic progression?  


If we begin from a fairly "thick" infinite set <math>A</math> (say, the primes), does it contain many elements in arithmetic progression: <math>a</math>,   
If we begin from a fairly "thick" infinite set <math>A</math>, does it contain many elements in arithmetic progression: <math>a</math>,   
<math>a+b</math>, <math> a+2 b</math>, <math>a+3 b</math>,  ... , <math>a+10b</math>, say? Should it be possible to write large integers as sums of elements of <math>A</math>?
<math>a+b</math>, <math> a+2 b</math>, <math>a+3 b</math>,  ... , <math>a+10b</math>, say? Should it be possible to write large integers as sums of elements of <math>A</math>?


Line 177: Line 211:
compared.
compared.


===Probabilistic number theory ===
====Computations in number theory====


Take a number at random between one and a million. How likely is it to be prime? This is just another way of asking how many primes there are between one and a million. Very well; ask further: how many prime divisors will it have, on average? How many divisors will it have altogether, and with what likelihood? What is the probability that it have many more or many fewer divisors or prime divisors than the average?
{{main|Computational number theory}}


Much of probabilistic number theory can be seen as an important special case of the study of variables that are almost, but not quite, mutually [[statistical independence|independent]]. For example, the event that a random integer between one and a million be divisible by two and the event that it be divisible by three are almost independent, but not quite.
While the word ''algorithm'' goes back only to certain readers of [[al-Khwārizmī]], careful descriptions of methods of solution are older than proofs: such methods - that is, algorithms - are as old as any recognisable mathematics - ancient Egyptian, Babylonian, Vedic, Chinese - whereas proofs appeared only with the Greeks of the classical period.


It is sometimes said that probabilistic combinatorics uses the fact that whatever happens with probability greater than <math>0</math> must happen sometimes; one may say with equal justice that many applications of probabilistic number theory hinge on the fact that whatever is unusual must be rare. If certain algebraic objects (say, rational or integer solutions to certain equations) can be shown to be in the tail of certain sensibly defined distributions, it follows that there must be few of them; this is a very concrete non-probabilistic statement following from a probabilistic one.
There are two main questions: "can we compute this?" and "can we compute it rapidly?". Anybody can test whether a number is prime or, if it is not, split it into prime factors; doing so rapidly is another matter. We now know fast algorithms for [[primality test|testing primality]], but, in spite of much work, no truly fast algorithm for factoring.
 
===Computations in number theory===
 
While the word ''algorithm'' goes back only to certain readers of [[Al-Kwarismi]], careful descriptions of methods of solution are older than proofs: such methods - that is, algorithms - are as old as any recognisable mathematics - ancient Egyptian, Babylonian, Vedic, Chinese - whereas proofs appeared only with the Greeks of the classical period.
 
There are two main questions: "can we compute this?" and "can we compute it rapidly?". Anybody can test whether a number is prime or, if it is not, split it into prime factors; doing so rapidly is another matter. We now know fast algorithms for testing primality, but, in spite of much work, no truly fast algorithm for factoring.


The difficulty of a computation can be useful: modern protocols
The difficulty of a computation can be useful: modern protocols
for [[cryptography|encrypting messages]] depend on functions that are known to all, but whose inverses (a) are known only to a chosen few, and (b) would take one too long a time to figure
for [[cryptography|encrypting messages]] (e.g., [[RSA]])
depend on functions that are known to all, but whose inverses (a) are known only to a chosen few, and (b) would take one too long a time to figure
out on one's own. For example, these functions can be such that their inverses can be computed only if certain large integers are factorised. While many difficult computational problems outside number theory are known, most working encryption protocols nowadays are based on the difficulty of a few number-theoretical problems.
out on one's own. For example, these functions can be such that their inverses can be computed only if certain large integers are factorised. While many difficult computational problems outside number theory are known, most working encryption protocols nowadays are based on the difficulty of a few number-theoretical problems.
   
   
Line 198: Line 227:
was proven that there is [[Hilbert's 10th problem|no algorithm]] for solving any and all Diophantine equations. There are thus some problems in number theory that will never be solved. We even know the shape of some of them, viz., Diophantine equations in nine variables; we simply do not know, and cannot know, which coefficients give us equations for which  
was proven that there is [[Hilbert's 10th problem|no algorithm]] for solving any and all Diophantine equations. There are thus some problems in number theory that will never be solved. We even know the shape of some of them, viz., Diophantine equations in nine variables; we simply do not know, and cannot know, which coefficients give us equations for which  
the following two statements are both true: there are no solutions, and we shall never know that there are no solutions.
the following two statements are both true: there are no solutions, and we shall never know that there are no solutions.
== Problems solved and unsolved ==
=== The beginnings ===
What are the integers ''x'', ''y'', ''z'' such that <math>\scriptstyle x^2 + y^2 = z^2</math>?
<small> A scribe from [[Larsa]] (1800 BCE) almost certainly had a full solution. The late source [[Proclus]] credits [[Pythagoras]] with the partial solution <math>\scriptstyle (x,(x^2-1)/2,(x^2+1)/2)
</math>, where <math>x</math> ranges on the odd integers. He also credits [[Plato]] with a closely related rule. A general solution makes its first fully explicit appearance in the work of [[Diophantus]].</small>
Are there incommensurable line segments? (In our language: are there [[irrational numbers]]?)
<small> Yes (early [[Pythagoreans]], before [[Plato]]'s day). The question belongs to the history
of [[number system|number systems]] at least as much as it belongs here. The proof in [[Euclid]]'s [[Euclid's Elements|Elements]] is purely arithmetical; nothing besides the "theory of the odd and the even" (likely early [[Pythagorean]]) is needed.</small>
Are there infinitely many [[prime number|prime numbers]]?
<small> Yes ([[Euclid]]). </small>
Given two integers, find the largest integer that divides them both.
<small>[[Euclid's algorithm]] does the job. It also provides the basis for the standard method for finding integer solutions to linear equations in two variables. Such equations, however, were not addressed by Euclid; the first algorithm found for solving them was [[Āryabhaṭa]]'s [[kuṭṭaka]] (see below).
=== Diophantus ===
=== India ===
=== Fermat ===
=== Questions directing current research ===


== References ==
== References ==
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<references/>
<references/>


== Bibliography ==
{{Citizendium}}
 
B. Datta and A. N. Singh, History of Hindu mathematics: a source book, Parts I and II,
Asia publishing house, Bombay, 1962.
 
T. Heath, A history of Greek mathematics, Vol. I: From Thales to Euclid, Dover, New York, 1981.
 
T. Heath, A history of Greek mathematics, Vol. II: From Aristarchus to Diophantus, Dover, New York, 1981.


Y. Mikami, The development of mathematics in China and Japan, Chelsea, New York, 1974.
==External links==
{{Portal|Number theory}}
* [http://www.numbertheory.org Number Theory Web]
* [http://www.math.niu.edu/~rusin/known-math/index/11-XX.html The Mathematical Atlas - 11: Number theory]


E. Robson, "Neither Sherlock Holmes nor Babylon: a reassessment of Plimpton 322",
{{Mathematics-footer}}
''Historia Math.'' '''28''' (3), pp. 167-206, 2001.
{{Number theory-footer}}


B. L. van der Waerden, Science awakening, Oxford University Press, New York, 1961.
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A. Weil, Number theory.
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An approach through history. From Hammurapi to Legendre. Birkhäuser, Boston, MA, 1984.

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Number theory is a branch of pure mathematics devoted primarily to the study of the integers. Any attempt to conduct such a study naturally leads to an examination of the properties of prime numbers (the building blocks of integers) as well as the properties of objects made out of integers (such as rational numbers) or defined as generalisations of the integers (such as, for example, algebraic integers).

Integers can be considered either in themselves or as solutions to equations (diophantine geometry). Questions in number theory are often best understood through the study of analytical objects (e.g., the Riemann zeta function) that encode properties of the integers, primes or other number-theoretic objects in some fashion (analytic number theory). One may also study real numbers in relation to rational numbers, e.g., as approximated by the latter (diophantine approximation).

The older term for number theory is arithmetic; it was superseded by "number theory" in the nineteenth century, though the adjective arithmetical is still fully current. By 1921, T. L. Heath had to explain: "By arithmetic Plato meant, not arithmetic in our sense, but the science which considers numbers in themselves, in other words, what we mean by the Theory of Numbers."[1] The general public now uses arithmetic to mean elementary calculations, whereas mathematicians use arithmetic as this article shall, viz., as an older synonym for number theory. (The use of the term arithmetic for number theory has regained some ground since Heath's time, arguably in part due to French influence.[2] In particular, arithmetical is preferred as an adjective to number-theoretic. Moreover, "the arithmetic of" is used, whereas "the number theory of" is not; thus, for example, the arithmetic of elliptic curves.)

History

For more information, see: History of number theory.


The beginnings

While there are elements of what in retrospect can be seen as number theory in Babylonian and ancient Chinese mathematics (see Plimpton 322 and the Chinese Remainder Theorem, respectively), the history of number theory truly starts with the Greek and Indian traditions.

The irrationality of is credited to the early Pythagoreans.[3] Euclid gave an algorithm for computing the greatest common divisor of two numbers (Euclid's Elements, Prop. VII.2) and a proof that there are infinitely many primes (Elements, Prop. IX.20). Much later in the Hellenistic period, Diophantus studied rational solutions to equations and systems of equations.

Results in number theory within Indian mathematics date from the period that would correspond to the medieval era in Europe. Aryabhata gave an algorithm for solving[4] pairs of congruences , , apparently with astronomical applications in mind.[5] Brahmagupta started the systematic study of indefinite quadratic equations, including what would later be misnamed Pell's equation. A general procedure (the chakravala, or "cyclic method") for solving Pell's equation was finally found by Jayadeva (cited in the eleventh century; his work is otherwise lost); the earliest surviving exposition appears in Bhāskara II's Bīja-gaṇita (twelfth century).[6] Unfortunately, these achievements were largely unknown in the West until the late eighteenth century.[7]

Much Greek mathematics and some Indian mathematics was available to Arabic scholars from the early ninth century onwards. Part of the treatise al-Fakhri (by al-Karajī, 953 - ca. 1029) builds on Diophantus's work to some extent.

Modern number theory

Modern number theory begins with Pierre de Fermat, inspired in part by his study of Diophantus. Continuous activity on the subject started almost a century later with Euler.[8] Lagrange provided proofs of some of Fermat's and Euler's key statements. He and Legendre also set the basis of the study of quadratic forms; Legendre was the first to state the law of quadratic reciprocity. In Disquisitiones Arithmeticae, Gauss gave the first valid proof of this law, developed the theory of quadratic forms further, and started the modern study of cyclotomy.

Starting early in the nineteenth century, the following developments gradually took place:

  • The rise to self-consciousness of number theory (or higher arithmetic) as a field of study.[9]
  • The development of much of modern mathematics necessary for basic modern number theory: complex analysis, group theory, Galois theory -- accompanied by greater rigor in analysis and abstraction in algebra.
  • The rough subdivision of number theory into its modern subfields - in particular, analytic and algebraic number theory.

Algebraic number theory may be said to start with the study of reciprocity and cyclotomy, but truly came into its own with the development of abstract algebra and early ideal theory and valuation theory; see below. An obvious conventional starting point for analytic number theory would be Riemann's memoir on the Riemann zeta function (1859); there is also Dirichlet's theorem on arithmetic progressions, which preceded it in the study of the zeta function and even L-functions (for ), or Jacobi's work on the four square theorem, which connected arithmetical questions with elliptic functions. The first use of analytical arguments in number theory goes further back, to Euler.[10]

The history of each subfield is sketched in its own section below. Many of the most interesting questions in each area remain open and are being actively worked on.

Approaches and subfields

Introductory texts and elementary tools

Two of the most popular introductions to the subject are:

  • G. H. Hardy and E. M. Wright, An introduction to the theory of numbers, 6th ed., rev. by D. R. Heath-Brown and J. H. Silverman, Oxford University Press, Oxford, 2008 (first published in 1938).
  • I. M. Vinogradov, Elements of Number Theory, Mineola, NY: Dover Publications, 2003, reprint of the 1954 edition.

Hardy and Wright's book is a comprehensive classic, though its clarity sometimes suffers due to the authors' insistence on elementary methods.[11] Vinogradov's main attraction consists in its set of problems, which quickly lead to Vinogradov's own research interests; the text itself is very basic and close to minimal.

The term elementary generally denotes a method that does not use complex analysis. For example, the prime number theorem was first proven in 1896, but an elementary proof was found only in 1949. The term is somewhat ambiguous: for example, proofs based on Tauberian theorems are often seen as quite enlightening but not elementary, in spite of using Fourier analysis, rather than complex analysis as such. Here as elsewhere, an elementary proof may be longer and more difficult for most readers than a non-elementary one.

Number theory has the reputation of being a field many of whose results can be stated to the layperson. At the same time, the proofs of these results are not particularly accessible, in part because the range of tools they use is, if anything, unusually broad within mathematics.

Popular choices for a second textbook include Borevich and Shafarevich's Number theory and Serre's Cours d'arithmetique. Textbooks for later stages in one's study tend to branch into analytic and algebraic number theory, among other subfields.

Main fields

Analytic number theory

For more information, see: Analytic number theory.


Analytic number theory is generally held to denote the study of problems in number theory by analytic means, i.e., by the tools of calculus. Some would emphasize the use of complex analysis: the study of the Riemann zeta function and other L-functions can be seen as the epitome of analytic number theory. At the same time, the subfield is often held to cover studies of elementary problems by elementary means, e.g., the study of the divisors of a number without the use of analysis, or the application of sieve methods. A problem in number theory can be said to be analytic simply if it involves statements on quantity or distribution, or if the ordering of the objects studied (e.g., the primes) is crucial. Several different senses of the word analytic are thus conflated in the designation analytic number theory as it is commonly used.

The following are examples of problems in analytic number theory: the prime number theorem, the Goldbach conjecture (or the twin prime conjecture, or the Hardy-Littlewood conjectures), the Waring problem and the Riemann Hypothesis. Some of the most important tools of analytic number theory are the circle method, sieve methods and L-functions (or, rather, the study of their properties).

One may ask analytic questions about algebraic numbers, and use analytic means to answer such questions; it is thus that algebraic and analytic number theory intersect. For example, one may define prime ideals (generalisations of prime numbers living in the field of algebraic numbers) and ask how many prime ideals there are up to a certain size. This question can be answered by means of an examination of Dedekind zeta functions, which are generalisations of the Riemann zeta function, an all-important analytic object that controls the distribution of prime numbers.

Algebraic number theory

For more information, see: Algebraic number theory.


Algebraic number theory studies algebraic properties and algebraic objects of interest in number theory. (Thus, analytic and algebraic number theory can and do overlap: the former is defined by its methods, the latter by its objects of study.) A key topic is that of the algebraic numbers, which are generalisations of the rational numbers. Briefly, an algebraic number is any complex number that is a solution to some polynomial equation with rational coefficients; for example, every solution of (say) is an algebraic number. Fields of algebraic numbers are also called algebraic number fields.

It could be argued that the simplest kind of number fields (viz., quadratic fields) were already studied by Gauss, as the discussion of quadratic forms in Disquisitiones arithmeticae can be restated in terms of ideals and norms in quadratic fields. (A quadratic field consists of all numbers of the form , where and are rational numbers and is a fixed rational number whose square root is not rational.) For that matter, the 11th-century chakravala method amounts - in modern terms - to an algorithm for finding the units of a real quadratic number field. However, neither Bhāskara nor Gauss knew of number fields as such.

The grounds of the subject as we know it were set in the late nineteenth century, when ideal numbers, the theory of ideals and valuation theory were developed; these are three complementary ways of dealing with the lack of unique factorisation in algebraic number fields. (For example, in the field generated by the rationals and , the number can be factorised both as and ; all of , , and are irreducible, and thus, in a naïve sense, analogous to primes among the integers.) The initial impetus for the development of ideal numbers (by Kummer) seems to have come from the study of higher reciprocity laws,[12] i.e., generalisations of quadratic reciprocity.

Number fields are often studied as extensions of smaller number fields: a field L is said to be an extension of a field K if L contains K. (For example, the complex numbers C are an extension of the reals R, and the reals R are an extension of the rationals Q.) Classifying the possible extensions of a given number field is a difficult and partially open problem. Abelian extensions -- that is, extensions L of K such that the Galois group[13] Gal(L/K) of L over K is an abelian group -- are relatively well understood. Their classification was the object of the programme of class field theory, which was initiated in the late 19th century (partly by Kronecker and Eisenstein) and carried out largely in 1900--1950.

The Langlands program, one of the main current large-scale research plans in mathematics, is sometimes described as an attempt to generalise class field theory to non-abelian extensions of number fields.

Diophantine geometry

For more information, see: Diophantine geometry.
For more information, see: Glossary of arithmetic and Diophantine geometry.


The central problem of Diophantine geometry is to determine when a Diophantine equation has solutions, and if it does, how many. The approach taken is to think of the solutions of an equation as a geometric object.

For example, an equation in two variables defines a curve in the plane. More generally, an equation, or system of equations, in two or more variables defines a curve, a surface or some other such object in n-dimensional space. In Diophantine geometry, one asks whether there are any rational points (points all of whose coordinates are rationals) or integral points (points all of whose coordinates are integers) on the curve or surface. If there are any such points, the next step is to ask how many there are and how they are distributed. A basic question in this direction is: are there finitely or infinitely many rational points on a given curve (or surface)? What about integer points?

An example here may be helpful. Consider the equation ; we would like to study its rational solutions, i.e., its solutions such that x and y are both rational. This is the same as asking for all integer solutions to ; any solution to the latter equation gives us a solution , to the former. It is also the same as asking for all points with rational coordinates on the curve described by . (This curve happens to be a circle of radius 1 around the origin.)

The rephrasing of questions on equations in terms of points on curves turns out to be felicitous. The finiteness or not of the number of rational or integer points on an algebraic curve - that is, rational or integer solutions to an equation , where is a polynomial in two variables - turns out to depend crucially on the genus of the curve. The genus can be defined as follows:[14] allow the variables in to be complex numbers; then defines a 2-dimensional surface in (projective) 4-dimensional space (since two complex variables can be decomposed into four real variables, i.e., four dimensions). Count the number of (doughnut) holes in the surface; call this number the genus of . Other geometrical notions turn out to be just as crucial.

There is also the closely linked area of diophantine approximations: given a number , how well can it be approximated by rationals? (We are looking for approximations that are good relative to the amount of space that it takes to write the rational: call (with ) a good approximation to if , where is large.) This question is of special interest if is an algebraic number. If cannot be well approximated, then some equations do not have integer or rational solutions. Moreover, several concepts (especially that of height) turn out to be crucial both in diophantine geometry and in the study of diophantine approximations.

Diophantine geometry should not be confused with the geometry of numbers, which is a collection of graphical methods for answering certain questions in algebraic number theory. Arithmetic geometry, on the other hand, is a contemporary term for much the same domain as that covered by the term diophantine geometry. The term arithmetic geometry is arguably used most often when one wishes to emphasise the connections to modern algebraic geometry (as in, for instance, Faltings' theorem) rather than to techniques in diophantine approximations.

Recent approaches and subfields

The areas below date as such from no earlier than the mid-twentieth century, even if they are based on older material. For example, as is explained below, the matter of algorithms in number theory is very old, in some sense older than the concept of proof; at the same time, the modern study of computational complexity dates only from the 1930s and 1940s.

Probabilistic number theory

For more information, see: Probabilistic number theory.


Take a number at random between one and a million. How likely is it to be prime? This is just another way of asking how many primes there are between one and a million. Very well; ask further: how many prime divisors will it have, on average? How many divisors will it have altogether, and with what likelihood? What is the probability that it have many more or many fewer divisors or prime divisors than the average?

Much of probabilistic number theory can be seen as an important special case of the study of variables that are almost, but not quite, mutually independent. For example, the event that a random integer between one and a million be divisible by two and the event that it be divisible by three are almost independent, but not quite.

It is sometimes said that probabilistic combinatorics uses the fact that whatever happens with probability greater than must happen sometimes; one may say with equal justice that many applications of probabilistic number theory hinge on the fact that whatever is unusual must be rare. If certain algebraic objects (say, rational or integer solutions to certain equations) can be shown to be in the tail of certain sensibly defined distributions, it follows that there must be few of them; this is a very concrete non-probabilistic statement following from a probabilistic one.

Arithmetic combinatorics

For more information, see: Arithmetic combinatorics.


Let be a set of integers. Consider the set consisting of all sums of two elements of . Is much larger than A? Barely larger? If is barely larger than , must have plenty of arithmetic structure - e.g., does it look like an arithmetic progression?

If we begin from a fairly "thick" infinite set , does it contain many elements in arithmetic progression: , , , , ... , , say? Should it be possible to write large integers as sums of elements of ?

These questions are characteristic of arithmetic combinatorics. This is a presently coalescing field; it subsumes additive number theory (which concerns itself with certain very specific sets of arithmetic significance, such as the primes or the squares) and, arguably, some of the geometry of numbers, together with some rapidly developing new material. Its focus on issues of growth and distribution make the strengthening of links with ergodic theory likely. The term additive combinatorics is also used; however, the sets being studied need not be sets of integers, but rather subsets of non-commutative groups, for which the multiplication symbol, not the addition symbol, is traditionally used; they can also be subsets of rings, in which case the growth of and · may be compared.

Computations in number theory

For more information, see: Computational number theory.


While the word algorithm goes back only to certain readers of al-Khwārizmī, careful descriptions of methods of solution are older than proofs: such methods - that is, algorithms - are as old as any recognisable mathematics - ancient Egyptian, Babylonian, Vedic, Chinese - whereas proofs appeared only with the Greeks of the classical period.

There are two main questions: "can we compute this?" and "can we compute it rapidly?". Anybody can test whether a number is prime or, if it is not, split it into prime factors; doing so rapidly is another matter. We now know fast algorithms for testing primality, but, in spite of much work, no truly fast algorithm for factoring.

The difficulty of a computation can be useful: modern protocols for encrypting messages (e.g., RSA) depend on functions that are known to all, but whose inverses (a) are known only to a chosen few, and (b) would take one too long a time to figure out on one's own. For example, these functions can be such that their inverses can be computed only if certain large integers are factorised. While many difficult computational problems outside number theory are known, most working encryption protocols nowadays are based on the difficulty of a few number-theoretical problems.

On a different note - some things may not be computable at all; in fact, this can be proven. For instance, Turing showed in 1936 that there is no algorithm for deciding in finite time whether a given algorithm ends in finite time. In 1970, it was proven that there is no algorithm for solving any and all Diophantine equations. There are thus some problems in number theory that will never be solved. We even know the shape of some of them, viz., Diophantine equations in nine variables; we simply do not know, and cannot know, which coefficients give us equations for which the following two statements are both true: there are no solutions, and we shall never know that there are no solutions.

References

  1. Sir Thomas Heath, A History of Greek Mathematics, vol. 1, Dover, 1981, p. 13.
  2. Take, e.g., Serre's A Course in Arithmetic (1970; translated into English in 1973). In 1952, Davenport still had to specify that he meant The Higher Arithmetic. Hardy and Wright wrote in the introduction to An Introduction to the Theory of Numbers (1938): "We proposed at one time to change [the title] to An introduction to arithmetic, a more novel and in some ways a more appropriate title; but it was pointed out that this might lead to misunderstandings about the content of the book."
  3. Plato, Theaetetus, p. 147 B, cited in: Kurt von Fritz, "The discovery of incommensurability by Hippasus of Metapontum", p. 212, in: J. Christianidis (ed.), Classics in the History of Greek Mathematics, Kluwer, 2004. Plato reports on further work by Theodorus on irrationality.
  4. Āryabhaṭa, Āryabhatīya, Chapter 2, verses 32-33, cited in: K. Plofker, Mathematics in India, Princeton University Press, 2008, pp. 134-140. See also W. E. Clark, The Āryabhaṭīya of Āryabhaṭa: An ancient Indian work on Mathematics and Astronomy, University of Chicago Press, 1930, pp. 42-50. A slightly more explicit description of the kuṭṭaka was later given in Brahmagupta, Brāhmasphuṭasiddhānta, XVIII, 3-5 (in Colebrooke, Algebra, with Arithmetic and Mensuration, from the Sanscrit of Brahmegupta and Bháscara, London, 1817, p. 325, cited in: Clark, op. cit., p. 42).
  5. K. Plofker, Mathematics in India, Princeton University Press, 2008, p. 119.
  6. Plofker, op. cit., p. 194
  7. Plofker, op. cit., p. 283
  8. A. Weil, 'Number theory: an approach through history - from Hammurapi to Legendre, Birkhäuser, 1984, pp. 1-2.
  9. See the discussion in section 5 of C. Goldstein and N. Schappacher, "A book in search of a discipline (1801-1860)', in C. Goldstein, N. Schappacher and J. Schwermer (eds.), "The shaping of arithmetic after C. F. Gauss's Disquisitiones Arithmeticae", Springer, 2007. Early signs of self-consciousness are present already in letters by Fermat: thus his remarks on what number theory is, and how "Diophantus's work [...] does not really belong to [it]" (quoted in A. Weil, op. cit., p. 25).
  10. H. Iwaniec and E. Kowalski, Analytic number theory, AMS Colloquium Pub., Vol. 53, 2004, p. 1.
  11. T. M. Apostol, Review of An introduction to the theory of numbers, Mathematical Reviews, MR0568909.
  12. H. M. Edwards, Fermat's Last Theorem: a genetic introduction to algebraic number theory, Springer Verlag, 1977, p. 79.
  13. The Galois group of an extension K/L consists of the operations (isomorphisms) that send elements of L to other elements of L while leaving all elements of K fixed. Thus, for instance, Gal(C/R) consists of two elements: the identity element (taking every element x+iy of C to itself) and complex conjugation (the map taking each element x+iy to x-iy). The Galois group of an extension tells us many of its crucial properties. The study of Galois groups started with Evariste Galois; in modern language, the main outcome of his work is that an equation f(x)=0 can be solved by radicals (that is, x can be expressed in terms of the four basic operations together with square roots, cubic roots, etc.) if and only if the extension of the rationals by the roots of the equation f(x)=0 has a Galois group that is solvable in the sense of group theory. ("Solvable", in the sense of group theory, is a simple property that can be checked easily for finite groups.)
  14. It may be useful to look at an example here. Say we want to study the curve . We allow x and y to be complex numbers: . This is, in effect, a set of two equations on four variables, since both the real and the imaginary part on each side must match. As a result, we get a surface (two-dimensional) in four dimensional space. After we choose a convenient hyperplane on which to project the surface (meaning that, say, we choose to ignore the coordinate a), we can plot the resulting projection, which is a surface in ordinary three-dimensional space. It then becomes clear that the result is a torus, i.e., the surface of a doughnut (somewhat stretched). A doughnut has one hole; hence the genus is 1.

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