<s>[INST] Complex Matrix is Orthogonal if and only if... Let D be a 2x2 matrix with entries in the complex numbers. Prove that D is orthogonal if and only if, it is of the form:
\begin{pmatrix}
a & -b\\ 
b & a
\end{pmatrix}
or
\begin{pmatrix}
a & b\\ 
b & -a
\end{pmatrix}
Proof. I've already proved that if D is equal to those forms then, it implies that D is an orthogonal matrix. But how can I prove this? If D is orthogonal, then it must be of the form:
\begin{pmatrix}
a & -b\\ 
b & a
\end{pmatrix}
or
\begin{pmatrix}
a & b\\ 
b & -a
\end{pmatrix}
Update: a and b must satisfy that $a^2+b^2=1$
 [/INST] To Prove: If A is a 2x2 orthogonal matrix, A is of the form given in the question.
Proof:
For any general non-singular matrix, $A= \begin{pmatrix}a & b \\ 
c & d\end{pmatrix}$,
$\begin{equation}\det(A) = ad-bc \end{equation}$ and
$A^T= \begin{pmatrix}a & c \\ 
b & d\end{pmatrix}$.
Using $ \begin{align}A A^T = I \end{align} $, we get $\begin{equation}a^2 + b^2 = c^2 + d^2 = 1\end{equation}$ and $\begin{equation}ac=-bd\end{equation}$.
Using $\begin{align} A^T = A^{-1} \end{align}$, we get $\begin{equation}ad-bc = \frac{d}{a} = \frac{a}{d} = \frac{c}{-b} = \frac{-b}{c}\end{equation}$.
On solving $\begin{equation}d^2 = a^2 \end{equation}$  and $\begin{equation} b^2 = c^2 \end{equation}$ (obtained from above equation) and satisfying in $\begin{equation}ac=-bd\end{equation}$.
You get the required matrices.
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