# Musings about complex numbers

1. Aug 27, 2013

### meBigGuy

I have a view of complex numbers and the way they are taught. I think the whole concept of i as the sqrt(-1) is a terrible place to start. And calling it "imaginary" is worse yet. They should be called blue numbers, or vertical numbers, or something. They are anything but imaginary. It is about simple 2 dimensional numbering systems. The sqrt(-1) is actually an advanced topic, not a starting point. Let me explain.

I view complex numbers as the general case and real numbers as a special (but simple) case.

Things in nature generally have a magnitude and a direction, like impedance and phase, or momentum and direction. If you draw a magnitude and phase "arrow" on a two dimensional graph, you can expresses it as a vertical and horiziontal component. That's the "complex" plane. What is so complex about that. It's pretty simple. Why not call them simple numbers? And what is imaginary about the vertical component? In some representations the horizontal component is just as "imaginary", or hard to visualize.

If you don't case about the phase or directionality you can just deal with 1 quantity, typically a real number. In reality there may be underlying 2-dimensional components but you can treat the whole as a real number.

The whole idea of 2 dimensional numbers can be extended to 3 and more dimensions. what do you call the 3rd dimension? The really imaginary number?

I'm not saying I have a cirriculum to teach complex math in Jr High, but sqrt(-1) is imaginary is not the place to start.

2. Aug 27, 2013

### Mentallic

When I was first exposed to complex numbers, I - as well as my peers - had a good grasp of polynomials and their roots. The biggest amazement for me was when I learnt about the fundamental theorem of algebra and seeing that the field of complex numbers are algebraically closed was very enlightening.

While seeing rotations with complex arithmetic was also pretty amazing in itself, I can't deny that since I had been working with polynomials for many years prior to taking that class, finding out that every n degree polynomial has n roots with this new set of numbers was just that much more impressive.

Now, since it makes no sense to relate vectors to the roots of polynomials, and since most students would be exposed to polynomials a lot more than vectors at that stage, $i=\sqrt{-1}$ which is clearly not found on the real (not imaginary) number line would suffice.

Challenge a student to find where the graph $y=x^2+1$ cuts the x-axis, and once they say that it never cuts the x-axis, try to convince them that it does in fact intersect, and that you aren't imagining it

3. Aug 27, 2013

### pwsnafu

Bad naming is nothing new in mathematics. See "irrational numbers".

All numbers are imaginary, i.e. a figment of our imagination. It's the name "reals" that's the problem, because it makes everything else look like not-real, so to speak.

No, its a 2 dimensional algebraically closed field. That's why it's important and not just a 2D vector space.

I don't know about what education system you came from, but I learned 2d vectors before complex numbers. Multiplication (and hence polynomials) was the key separating point.

General case and special case of what? Where do the quaternions fit in this? What about the p-adics?

Phase is a great example for complex numbers, but momentum is a vector quantity.

Complex does not mean "difficult to understand" rather "more than one part". Compare "complex carbohydrate".

Does not exist due to Huritz's theorem. 3d vectors exist, but again, no multiplication (technically invertible multiplication).

4. Aug 27, 2013

### D H

Staff Emeritus
The square root of -1 is *exactly* the place to start. It is fundamental.

As far as the name "imaginary numbers" is concerned, look no further than the negative numbers, zero, and the irrationals. You have no problem with -1, 0, or √2. That these are "numbers" just makes sense. That has not always been the case. The qualifier "negative" has a rather derogatory meaning. Negative number quite literally means a quantity that is not a number. Zero is perhaps just a name to you, but it has the same root as cipher. Zero was a very puzzling concept not all that long ago. Finally, the term irrational has an even more derogatory meaning: "numbers that don't make a bit of sense".

Imaginary is yet another of those apparently derogatory labels that no longer have a negative meaning. Just get over it.

5. Aug 27, 2013

### SteamKing

Staff Emeritus
I look at irrational numbers as those which cannot be expressed as the ratio of two integers, or 'non-rational' numbers, if you will.

6. Aug 27, 2013

### Staff: Mentor

Vulgar fractions are universally offensive.

7. Aug 27, 2013

### Tobias Funke

I don't necessarily agree with your entire post, but you're right on the money here; i^2=-1 is a better place to start. When students are introduced to the sqrt function, it's stressed that we take the positive root to get a single-valued function. So naturally, since both i and -i squared result in -1, and i=sqrt(-1), then i is positive and -i is negative. At least, that's what any reasonable person would think, but not only is it not true, it doesn't even make sense. I don't know why anyone would prefer i=sqrt(-1) over i^2=-1 (and I don't think the difference is as minor as it may at first seem). Maybe someone else has some thoughts?

I also think the correct history of complex numbers should be taught, or at least briefly mentioned. Mathematicians didn't just decide one day that they really wanted solutions to x^2+1=0, and therefore invented them. The real history is more interesting (and it's, you know, real, ie not a lie). I suppose one could argue that the books and teachers just say "well, suppose we want to solve this", or "we can actually define new numbers to solve more equations", and don't necessarily claim that that's how it happened historically, but they should not assume the students will grasp this (and why would they?).

Lastly, I wish the geometric view of complex number arithmetic was given more exposure early on. If they can't prove it, fine, tell them they will in trig, but it's helpful to know, it appeals to "geometric learners", and it can even give them an early view of linear transformations.

I'm sure that many high school books and teachers actually do these things, but from what I've seen it's not very common. Also, sorry if I hijacked your thread with my own musings (rants?) about complex numbers.

8. Aug 27, 2013

### HallsofIvy

Staff Emeritus
It's the distinction between "rational" and "irrational" mathematicians that we need to make!

9. Aug 27, 2013

### meBigGuy

pwsnafu beat me up pretty bad (obviously I'm not a mathematition nor a physicist). But the overall point remains that i is simply part of the "No, its a 2 dimensional algebraically closed field." As I remember, the two dimensional aspect is not addressed initially (in high school) and that is the basic concept, not the significance of i^2 = -1 and its usefullness in solving x^2+1 = 0. Once you learn the 2 dimensional stuff, i^2 = -1 can be derived. Instead you first learn to blindly manipulate this imaginary "i" thing to find roots to equations you really don't care about.

Everybody had good stuff to say and I appreciate all the corrections.

10. Aug 27, 2013

### Curious3141

Not as much as improper ones.

11. Aug 27, 2013

### Jorriss

If your point is that one should start teaching complex numbers from a geometric point of view, that's not a bad idea.

12. Aug 27, 2013

### HallsofIvy

Staff Emeritus
This is simply not correct. "i^2= -1" does NOT follow from two-dimensionality. There exist other two-dimensional models in which that is not true.

13. Aug 29, 2013

### LCKurtz

14. Aug 29, 2013

### Mandelbroth

Nah. The term "imaginary" gives us an excuse to make fun of electrical engineers. :tongue:

Actually, I'm pretty sure $i^2=-1$ is the starting point.

cf. Definition 1.

15. Aug 29, 2013

### cosmik debris

16. Aug 29, 2013

### meBigGuy

Yet another way to say it.
"This article gives a pedagogical approach to introducing complex numbers to students who haven't seen them before. Arithmetic on the number line is generalized in a natural way to arithmetic on the plane. The terms "imaginary" and "complex" are not used during the development, and the "number" whose square is -1 arises naturally. At the end of the development, the standard complex number notation is given. "

I don't think so. Not any more than 1^2 = 1 is the starting point for real numbers. Can one derive all of complex analysis from just the statement $i^2=-1$?

I talked to a colleague today who learned complex numbers by the geometric approach, in High School. So I guess it depends on the teacher.

17. Aug 30, 2013

### pwsnafu

Just to elaborate on this there are precisely three 2-dimensional algebra systems: the complex numbers (i2=-1), the split-complex numbers (j2=1) and dual numbers2=0).
Proof is on Wikipedia.

Depends. If you already have the real numbers and its properties, then the complex numbers are constructed either as in the article that was linked, or (better) as polynomials with real coefficients. This allows one to rigorously define i, and then you discard the notation you had for a+bi with i2=-1.

And yes that's enough to derive all complex analysis, provided you have real analysis already defined.

Definitely. A friend of mine was doing high school tutoring and he said the girls he taught understood algebraic concepts better, while boys understood geometric concepts better. So both should be taught.

18. Aug 30, 2013

### deiki

Don't go with i = √(-1). It gives you awful stuff like the following :

i = √(-1) = √(-1/1) = √(1/-1) = √(1)/√(-1) = 1/i ... ouch.

i i = -1 works better as a starting point.

Baby Rudin starts this way : Let a, b, c, d be real numbers. By the definition, x = (a,b) and y = (c,d) are complex numbers if x+y = (a+c,b+d) and x y = (a c-b d,a d+b c). If e is a real number, e = (e,0). At last you define i = (0,1).

19. Aug 30, 2013

### Mandelbroth

Well, if you give me some other basic-ish concepts (limits, etc.) and, for the sake of making things easy for me, the generalized Stokes' Theorem, I suppose I could. :tongue:

The point is you'd need to recognize $i^2=-1$ in order to construct the complex plane, unless you just assumed that $i$ is "orthogonal" to the real numbers. In the latter case, you could just use vectors. You don't really gain anything special about complex numbers from a geometric point of view unless you can use it in tandem with $i^2=-1$. For example, vectors in $\mathbb{R}^2$ can all be represented by $\begin{bmatrix}x \\ y\end{bmatrix}=\sqrt{x^2+y^2}\begin{bmatrix}\cos\theta \\ \sin\theta\end{bmatrix}$, but complex numbers can be represented by $x+iy=\sqrt{x^2+y^2}e^{i\theta}$ because $i^2=-1$.

20. Aug 30, 2013

### meBigGuy

I guess I'm saying i is orthogonal to the complex plane, and i^2 = -1 develops naturally from that. It feel better than "let's hypothesize something you can square to get -1" and from that you develop the complex plane.