Let \[A=\begin{bmatrix}
a_0 & a_1 & \dots & a_{n-2} &a_{n-1} \\
a_{n-1} & a_0 & \dots & a_{n-3} & a_{n-2} \\
a_{n-2} & a_{n-1} & \dots & a_{n-4} & a_{n-3} \\
\vdots & \vdots & \dots & \vdots & \vdots \\
a_{2} & a_3 & \dots & a_{0} & a_{1}\\
a_{1} & a_2 & \dots & a_{n-1} & a_{0}
\end{bmatrix}\] be a complex $n \times n$ matrix.
Such a matrix is called circulant matrix.
Then prove that the determinant of the circulant matrix $A$ is given by
\[\det(A)=\prod_{k=0}^{n-1}(a_0+a_1\zeta^k+a_2 \zeta^{2k}+\cdots+a_{n-1}\zeta^{k(n-1)}),\] where $\zeta=e^{2 \pi i/n}$ is a primitive $n$-th root of unity.

Proof.

Let $\omega$ be any $n$-th root of unity.
Consider the vector
\[\mathbf{v}=\begin{bmatrix}
1 \\
\omega \\
\omega^2 \\
\vdots \\
\omega^{n-1}
\end{bmatrix}.\] We show that the vector $\mathbf{v}$ is an eigenvector of $A$.
We compute
\begin{align*}
A\mathbf{v}=\
\begin{bmatrix}
a_0 & a_1 & \dots & a_{n-2} &a_{n-1} \\
a_{n-1} & a_0 & \dots & a_{n-3} & a_{n-2} \\
a_{n-2} & a_{n-1} & \dots & a_{n-4} & a_{n-3} \\
\vdots & \vdots & \dots & \vdots & \vdots \\
a_{2} & a_3 & \dots & a_{0} & a_{1}\\
a_{1} & a_2 & \dots & a_{n-1} & a_{0}
\end{bmatrix}
\begin{bmatrix}
1 \\
\omega \\
\omega^2 \\
\vdots \\
\omega^{n-1}\\
\end{bmatrix}.
\end{align*}

The first entry of the vector $A\mathbf{v}$ is
\[a_0+a_1\omega+a_2\omega^2+\cdots a_{n-2}\omega^{n-2}+a_{n-1}\omega^{n-1}=:\lambda.\] We define $\lambda$ to be this number.

The second entry is
\begin{align*}
&a_{n-1}+a_0\omega+\cdots+a_{n-3}\omega^{n-2}+a_{n-2}\omega^{n-1}\\
&=(a_{n-1}\omega^{n-1}+a_0+\cdots+a_{n-3}\omega^{n-3}+a_{n-2}\omega^{n-2})\omega \\
&=(a_0+\cdots+a_{n-3}\omega^{n-3}+a_{n-2}\omega^{n-2}+a_{n-1}\omega^{n-1})\omega \\
&=\lambda \omega
\end{align*}

Similarly the $i$-th entry of the vector $A\mathbf{v}$ is
\begin{align*}
&a_{n-i+1}+a_{n-i+2}\omega +\cdots+ a_{n-i}\omega^{n-1}\\
&= (a_{n-i+1}\omega^{n-i+1}+a_{n-i+2}\omega^{n-i+2} +\cdots+ a_{n-i}\omega^{n-i})\omega^{i-1}\\
&=\lambda \omega^{i-1} .
\end{align*}
Therefore we obtain
\[A\mathbf{v}=\begin{bmatrix}
\lambda \\
\lambda\omega \\
\lambda \omega^2 \\
\vdots \\
\lambda\omega^{n-1}
\end{bmatrix}=\lambda \mathbf{v}.\]

Since $\mathbf{v}$ is a nonzero vector, it follows that $\lambda$ is an eigenvalue of $A$ and $\mathbf{v}$ is an eigenvector corresponding to $\lambda$.

The above argument holds for any $n$-th root of unity $\omega$.
We take $\omega=\zeta^k$, where $k$ runs from $0$ to $n-1$.
It follows that the vector
\[\mathbf{v}_k:=\begin{bmatrix}
1 \\
\zeta^k \\
\zeta^{2k} \\
\vdots \\
\zeta^{k(n-1)}
\end{bmatrix}\] is eigenvector corresponding to the eigenvalue
\[\lambda_k:=a_0+a_1\zeta^k+a_2\zeta^{2k}+\cdots a_{n-2}\zeta^{k(n-2)}+a_{n-1}\zeta^{k(n-1)}\] for each $k=0,1, \dots, n-1$.

We claim that the vectors $\mathbf{v}_k$ are linearly independent.

To see this, form a matrix whose column vectors are these vectors. That is, we consider
\[B=\begin{bmatrix}
1& 1 & 1 & \dots & 1 &1 \\
1&\zeta & \zeta^{2} & \dots & \zeta^{n-2} & \zeta^{n-1} \\
1&\zeta^{2} & \zeta^{4} & \dots & \zeta^{2(n-2)} & \zeta^{2(n-1)} \\
1& \zeta^{3} & \zeta^{6} & \dots & \zeta^{3(n-2)} & \zeta^{3(n-1)} \\
\vdots& \vdots & \vdots & \dots & \vdots & \vdots \\
1& \zeta^{n-1} & \zeta^{2(n-1)} & \dots & \zeta^{(n-1)(n-2)} &\zeta^{(n-1)(n-1)}
\end{bmatrix}.\]

This is the Vandermonde matrix and its determinant is
\[\det(B)=\prod_{i < j }(\zeta^j-\zeta^i)\neq 0.\] Thus, the matrix $B$ is nonsingular, hence its column vectors are linearly independent. It follows that $\lambda_k$, $k=0, 1, \dots, n$ are all the eigenvalues of $A$. Since the determinant is the product of all the eigenvalues of $A$, we have \begin{align*} \det(A)&=\prod_{k=0}^{n-1}\lambda_k\\ &=\prod_{k=0}^{n-1}(a_0+a_1\zeta^k+a_2 \zeta^{2k}+\cdots+a_{n-1}\zeta^{k(n-1)}), \end{align*} as required. This completes the proof.

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