Talk:Complex number/Draft

Article Checklist for "Complex number/Draft"

Workgroup category or categories	Mathematics Workgroup [Categories OK]
Article status	Developing article: beyond a stub, but incomplete
Underlinked article?	No
Basic cleanup done?	Yes
Checklist last edited by	- Versuri 07:55, 2 April 2007 (CDT)

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Definition

I reworked the text a bit. So this is why.

• I think ${\sqrt {-1}}$ is an objectionable notation...
• The definition hardly matches my understanding... The imaginary unit can be really understood only within the field of complex numbers (defined independently). Otherwise, what is "i"? A square root of (-1)? Then which one? (there are usually two square roots; BTW, have you ever seen an independent definition of a square root of a negative number?). So let's define it by "i^2=1". Then, does it exist? Does it deserve to be called a number? (operations are possible?) The same question arise if we define "i" as a solution of "x^2+1=0". In practice we can use any of these well known properties, but how can we understand it as a definition?

At best, we can say "i" is "just a formal symbol" with no meaning. We define some operations on formal sums "a+bi". Basically, that's OK. The point is that it explains nothing and it can be done in a more elegant way, where we really define all is needed in terms of elementary well-known objects:

Complex numbers are just ordered pairs of reals -as simple as this - with appropriate addition and multiplication. BTW, these operations are enlisted in the article with the "formal" use of "i". Then i=(0,1). And for computational convenience we discover that i^2=-1, and use it.

I think your revision is a good one. I had considered using the term "formal expression" for $a+bi$, but decided not to. But, in truth, I didn't spend a great deal of time on this. It just seemed an obvious omission, giving that there was already an article on real numbers! A possible revision/addition I had considered was adding a section on how the definition can be formalized by saying $\mathbb {C}$ is the splitting field of $x^{2}+1$ over $\mathbb {R}$. Without context, though, that seems like a bit of overkill. Of course, it's formally the same as the definition of algebraic number fields such as $\mathbb {Q} [{\sqrt {-1}}]$ or $\mathbb {Q} [{\sqrt {2}}]$. But I suppose that's a topic for another article. Greg Woodhouse 06:21, 2 April 2007 (CDT)

The bottom line is that I do not object use of "i" in the informal intro, just to give an outline of the idea, there must be, however, a definition that really explains where it logically comes from. --AlekStos 03:01, 2 April 2007 (CDT)

sketch of a plan

The status of the ${\sqrt {-1}}$ notation seems to vary according to different cultures. In French high schools ands colleges, it tends to be a taboo, because of the objections pointed out by Alek Stos here. I have heard its usage is far more common in English speaking countries. The problem is that there is a canonical way to choose which square root of a positive real number $x$ we call ${\sqrt {x}}$ (the positive one), but there is not such a canonical way to choose amongst the two square roots of -1. Once $\mathbb {C}$ is defined, one can choose some convention, but still a determination of the square root over the complex plane cannot be continuous everywhere. On the other hand, using ${\sqrt {-1}}$ in an informal way just because it is easy to understand what is meant by it can be defended, as soon as one is warned of not considering it as anything else but a mere notation. As Greg Woodhouse recalls to us, this notation is quite common for algebraic number theory specialists, to denote some quadratic fields. I still think it is a bit dangerous to use it without comment for beginner readers.

Now I come to a (somewhat vague) suggestion of structure for the article. I like to introduce complex numbers to my students with the example of the resolution of the cubic equation $x^{3}=15x+4$ with the so called Gerolamo Cardano's method (in fact it is due to Scipione del Ferro and Niccolò Tartaglia). Computations are quite easy, and the striking fact is that during them, one has to use some imaginary number which square would be -1, but once the computations are finished, one gets the three real solutions of the equation! At this stage, one can denote the mysterious number by ${\sqrt {-1}}$, as we make anyway only purely formal calculations without giving any legitimate sense to them. They just suggest there might be something which square is -1.

Next we need a model to legitimate this mysterious number, and then, Alek Stos's suggestion is best : considering that $\mathbb {C}$ is $\mathbb {R} ^{2}$ with appropriate addition and multiplication laws is the more elementary way to construct complex numbers. Here we can introduce the "i" notation. Moreover, this allows to have a geometric representation of those counterintuitive numbers, with the complex plane. It is still possible to link this with history : the geometrical viewpoint is due to Robert Argand, and the complete construction was achieved by the great Carl Friedrich Gauss. This section may not only show how complex numbers can be illustrated by geometry, but show too how, reversely, plane geometrical problems can be solved with the power of calculation with complex numbers.

Then, another section may deal with a more abstract point of view, that is $\mathbb {C} =\mathbb {R} [X]/\left(X^{2}+1\right)$, and more generally, introduce the notions of splitting fields, algebraic closure and so on: thats seems to be Greg Woodhouse's idea. Only an introduction, but it has a legitimate place in our article I think.

Finally, some applications of complex numbers must be cited : a few words about complex analysis and holomorphic functions, etc. Separate articles are needed for the details of course. It also may be emphasized in the applications part than those seemingly purely abstract numbers are very useful in physics.

What I like in this sketch of plan for this article is that it begins with a simple, intuitive but not properly formalized idea to end with more precise and more subtle aspects of the theory. Also, I think it is important in this article to stress the historical evolution of the ontological view of complex numbers (how they were little by little accepted from mere calculation artifices to true numbers). Please let me know your opinion. If you think it is a good idea, I can write the cubic equation part quite soon. But if you have better ideas, please share them!

--Sébastien Moulin (talk me) 11:21, 2 April 2007 (CDT)