# Prove that the $n$th roots of unity form a group under multiplication

Let $n$ be a positive integer and let $z$ be the complex number $\cos\left(\frac{2\pi}{n}\right)+i\sin\left(\frac{2\pi}{n}\right)$. Prove that $G=\{1,z,z^2,...,z^{n-1}\}$ is a group under multiplication in $\mathbb{C}$.

This is a reference to Moivre's formula, because $z^n = 1$. But I do not know how to apply this result so that it complies with the group properties. I have thought about doing induction, but it becomes more extensive.

According to the definition of Group. A group is a nonempty set $G$ equipped with a binary operation $*$ that satisfies the following axioms:

1. Clasure: if $a\in{G}$ and $b\in{g}$, then $a*b\in{G}$.
2. Associativity: $a*(b*c)=(a*b)*C$ for all $a,b,c\in{G}$.
3. There is an element $e\in{G}$ (called the identity element) such that $a*e=a=e*a$ for every $a\in{G}$.
4. For every $a\in{G}$, there is an element $d\in{G}$ (called the inverse of $a$) such that $a*d=e$ and $d*a=e$.

A group is said to be abelian if it also satisfies this axiom:

1. Commutativity: $a*b=b*a$ for all $a,b\in{G}$.

• You have to check that the set is closed under multiplication. Then check associativity, check whether there is an identity element ($1$) and whether each element in your set has an inverse element. Using Moivre's formula, this is almost trivial. – Václav Mordvinov Mar 10 '18 at 14:33
• If you have the fact available, this is a little easier (i.e., requires no invocation of other trig identities) if you know that we can also write $z = e^{2 \pi i / n}$. Either way, this is just a matter of checking the three group axioms. (In fact, associativity comes for free, because multiplication in $\Bbb C$ is associative.) – Travis Willse Mar 10 '18 at 14:33

More generally, if $C$ is a group and $z \in C$ with $z^n=1$, where $1$ is the identity of $C$, then $G=\{1,z,z^2,\dots,z^{n-1}\}$ is a group (actually, a subgroup of $C$).

Indeed:

• Closure: $z^j z^k = z^{j+k} = z^r \in G$, where $r = (j+k) \bmod n$.

• Associativity: comes from $C$.

• Identity: $1 \in G$ by definition, and $1$ is the identity of $C$.

• Inverse: $z^j z^{n-j} =1$.

• Commutativity: $z^j z^k = z^{j+k} = z^{k+j} = z^k z^j$.

The set $G$ is the set$$\left\{\cos\left(\frac{2k\pi}n\right)+i\sin\left(\frac{2k\pi}n\right)\,\middle|\,k\in\{0,1,\ldots,n-1\}\right\},$$since $z^k=\cos\left(\frac{2k\pi}n\right)+i\sin\left(\frac{2k\pi}n\right)$. On the other hand\begin{multline}\left(\cos\left(\frac{2k\pi}n\right)+i\sin\left(\frac{2k\pi}n\right)\right)\left(\cos\left(\frac{2k'\pi}n\right)+i\sin\left(\frac{2k'\pi}n\right)\right)=\\=\cos\left(\frac{2(k+k')\pi}n\right)+i\sin\left(\frac{2(k+k')\pi}n\right).\end{multline}Can you take it from here?

Let $$\mathbb{Z}_n=\{0,1,2,\ldots,n-1\}$$ and $$G$$ be the set of all $$n$$-th roots of unity. Mathematically, $$$$G=\left\{w_k=\exp\left(i \frac{2k\pi}{n}\right)\bigg| ~k\in \mathbb{Z}_n\right\}.$$$$

Closure: Take $$k,~l\in \mathbb{Z}_n$$. $$\begin{eqnarray} w_k w_l&=&\exp\left(i \frac{2k\pi}{n}\right)\exp\left(i \frac{2l\pi}{n}\right)\nonumber\\ &=&\exp\left(i \frac{2(k+l)\pi}{n}\right) \end{eqnarray}$$ By division algorithm, $$k+l=qn+r$$ with $$q\in \mathbb{Z}$$ and $$0\leq r. $$\begin{eqnarray} w_k w_l&=&\exp\left(i \frac{2(qn+r)\pi}{n}\right)\\ &=&\exp\left(2q\pi i\right)\exp\left(i \frac{2r\pi}{n}\right)\\ &=&w_r\\ &=&w_{(k+l)\text{mod}~n} \end{eqnarray}$$

Associativity: Associativity follows from $$\mathbb{C}$$ since $$G\subset \mathbb{C}$$. Or take $$k,~l,~m\in \mathbb{Z}_n$$. $$\begin{eqnarray} w_k (w_l w_m)&=&w_k w_{(l+m)\text{mod}~n}\\ &=&w_{\left(k+(l+m)\text{mod}~n\right)\text{mod}~n}\\ &=&w_{\left((k+l)+m\right)\text{mod}~n}\\ &=&w_{\left((k+l)\text{mod}~n+m\right)\text{mod}~n}\\ &=&w_{(k+l)\text{mod}~n}w_m\\ &=&(w_kw_l)w_m \end{eqnarray}$$

Identity: Take $$k \in \mathbb{Z}_n$$. $$\begin{eqnarray} w_k w_0&=&w_{(k+0) \text{mod}~n}\\ &=&w_{k~\text{mod}~n}\\ &=&w_k \end{eqnarray}$$ Similarly, for all $$k\in \mathbb{Z}_n$$, $$w_0 w_k=w_k$$. Therefore, $$w_0=1$$ is the identity element.

Inverse: If $$k=0$$, then $$$$w_0 w_0=1.$$$$ Now consider $$k \in \mathbb{Z}_n$$ with $$k\neq 0$$. Since $$1\leq k , then $$n-k\in \mathbb{Z}_n$$, and $$\begin{eqnarray} w_k w_{n-k}&=&w_{n~\text{mod}~n}\\ &=&w_0\\ &=&1 \end{eqnarray}$$ Therefore, the inverse of $$w_k$$ is $$w_{n-k}$$.

Moreover, since for all $$k, l \in \mathbb{Z}_n$$, $$$$w_k w_l=w_l w_k$$$$ this group is commutative or, abelian.

Further, since for all $$k\in \mathbb{Z}_n$$, $$w_k=w_1^k$$, $$$$G=\langle w_1 \rangle,$$$$ in other words, $$G$$ is cyclic.

$$G=\{1,z,z^2,...,z^{n-1}\}$$ Consists of the $n_{th}$ roots of unity.

Note that this is like rotating around the unit circle at an equal angles of $\frac {2\pi }{n}$

It is closed under multiplication and every element has an inverse, and it has an identity element and the associativity is there.