View source for Roots of unity

The '''Roots of unity''' are a topic closely related to [[trigonometry]]. Roots of unity come up when we examine the [[complex number|complex]] [[root]]s of the [[polynomial]] <math> x^n=1 </math>.

== Solving the Equation ==

First, we note that since we have an '''n'''th degree polynomial, there will be '''n''' complex roots.

Now, we can convert everything to [[polar form]] by letting <math>x = re^{i\theta} </math>, and noting that <math>1 = e^{2\pi ik}</math> for <math> k\in \mathbb{Z}</math>, to get <math>r^ne^{ni\theta} = e^{2\pi ik}</math>. The magnitude of the RHS is 1, making <math>r^n=1\Rightarrow r=1</math> (magnitude is always expressed as a positive real number).  This leaves us with <math>e^{ni\theta} = e^{2\pi ik}</math>.

Taking the [[natural logarithm]] of both sides gives us <math> ni\theta = 2\pi ik</math>.  Solving this gives <math> \theta=\frac{2\pi k}n </math>.  Additionally, we note that for each of <math> k=0,1,2,\ldots,n-1 </math> we get a distinct value for  <math> \theta </math>, but once we get to <math> k > n-1 </math>, we start getting [[coterminal]] angles.

Thus, the solutions to <math>x^n=1 </math> are given by <math>x = e^{2\pi k i/n} </math> for <math> k=0,1,2,\ldots,n-1</math>.  We could also express this in trigonometric form as <math>x=\cos\left(\frac{2\pi k}n\right) + i\sin\left(\frac{2\pi k}n\right) = \mathrm{cis }\left(\frac{2\pi k}n\right). </math>

== Geometry ==

All of the roots of unity lie on the [[unit circle]] in the complex plane.  This can be seen by considering the magnitudes of both sides of the equation <math> x^n = 1</math>.  If we let <math> x = re^{i\theta} </math>, we see that <math> r^n = 1</math>, since the magnitude of the RHS of <math> x^n=1 </math> is 1, and for two complex numbers to be equal, both their magnitudes and arguments must be equivalent.

Additionally, we can see that when the '''n'''th roots of unity are connected in order (more technically, we would call this their [[convex hull]]), they form a regular '''n'''-sided polygon.  This becomes even more evident when we look at the arguments of the roots of unity.

== Properties ==
Listed below is a quick summary of important properties of roots of unity.

* They occupy the vertices of a regular ''n''-gon in the [[complex plane]].
* For <math> n>1 </math>, the sum of the ''n''th roots of unity is 0. More generally, if <math>\zeta</math> is a primitive ''n''th root of unity (i.e. <math>\zeta^m\neq 1</math> for <math>1\le m\le n-1</math>), then <math>\sum_{k=0}^{n-1} \zeta^{km}=\begin{cases} n & {n\mid m}, \\ 0 & \mathrm{otherwise.}\end{cases} </math>
** This is an immediate result of [[Vieta's formulas]] on the polynomial <math> x^n-1 = 0 </math> and [[Newton sums]].
* If <math>\zeta</math> is a primitive ''n''th root of unity, then the roots of unity can be expressed as <math> 1, \zeta, \zeta^2,\ldots,\zeta^{n-1}</math>.
* Also, don't overlook the most obvious property of all!  For each <math>n</math>th root of unity, <math>\zeta</math>, we have that <math> \zeta^n=1 </math>

== Uses ==
Roots of unity show up in many surprising places.  Here, we list a few:

* [[Geometry]]
* [[Factoring]]
* [[Number theory]]

== See Also ==
* [[Complex number]]s

[[Category:Definition]]
[[Category:Complex numbers]]