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### Why do we Bother to Find Roots of Unity?

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Date: 7/23/96 at 21:25:29
From: Alan Yu
Subject: Why do we Bother to Find Roots of Unity?

Dr. Math,

As a math teacher, sometimes I think from the students' point of
view.  Next academic year I am going to touch on complex numbers. One
of the topics is about roots of unity. The technicality of solving
those problems may be difficult.

But the first question in my mind is, why do we bother to find the
roots of unity?  What is the point in looking at the cube root, the
fourth root, fifth root, etc. of the unity, just a simple number?
Any significance there?

I just can't figure out even after searching some libraries. I look

Best wishes,
Alan
```

```
Date: 7/24/96 at 5:33:54
From: Doctor Pete
Subject: Re: Why do we Bother to Find Roots of Unity?

I'm not quite sure what you mean by "finding" - do you refer to actual
computation, such as

z[3] = -1/2 + i*Sqrt[3]/2
= Cos[2*Pi/3] + i*Sin[2*Pi/3] ,

or are you more interested in representation and manipulation?

Either way, there are very good reasons to "find" roots of unity.  For
example,

"What regular n-gons are constructible by the classical Greek
methods; i.e., with straightedge and compass?"

This question has an intriguing answer:  When n is precisely the 2^k
times a product of distinct Fermat primes, for nonnegative integers k
(recall a Fermat prime is a prime of the form 2^(2^m) + 1). The
surprising result is that regular 3-, 5-, 17-, 257-, and 65537-gons
can be constructed using only straightedge and compass. This, in turn,
is related to the fact that the tools of straightedge and compass
correspond to the algebraic operations of addition, multiplication,
the extraction of square roots, and no others.  Therefore, the value
of cos(2*Pi/17) is expressible as a series of the above operations on
a finite number of integers. So the computation of the 17th root of
unity is extremely important in devising an actual method of
construction, for example. Conversely, if an n(th) root of unity is
expressible using only these operations, a regular polygon of n sides
is constructible (so from the value I gave for z[3], the equilateral
triangle is constructible). The math behind all of this is intimately
related to Galois theory; see a book on abstract algebra for more.

There are a number of other interesting features of the n(th) roots of
unity. For example, the function x^n - 1 has as roots the n(th) roots
of unity. A common method for numerical approximation of these roots
is Newton's method, generalized to the complex plane. But since there
are multiple roots for n > 1, the process will converge on different
roots depending on the initial guess. People familiar with Newton's
method might be led to believe that the closer the initial guess is to
a particular root, the more likely its iteration will converge to that
root. But the story isn't as simple as that - in the complex plane,
what do the boundaries look like?  That is, if we plot the set of all
points in the complex plane that, when used as an initial guess,
converge to one particular root, what will the boundary of that set
look like?  Indeed, the boundary is not smooth, but fractal.

These are just a few things that are related to n(th) roots of unity;
perhaps other Math Doctors can add more connections.

-Doctor Pete,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```

```
Date: 7/24/96 at 13:50:41
From: Doctor Robert
Subject: Re: Why do we Bother to Find Roots of Unity?

The reason that we look at the roots of unity is that mathematicians
desire to know the solution sets to equations.  If we look at the
equation   x^6 = 1 , mathematicians know that there are 6 roots to
this equation.  (Only two of them are real roots).  It's nice to know
the other roots.

-Doctor Robert,  The Math Forum
Check out our web site!  http://mathforum.org/dr.math/
```
Associated Topics:
College Imaginary/Complex Numbers

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