Is the identity matrix the only matrix which is its own inverse? I just gave a proof for this question. Here's my follow up question: Let $A \in \ \mathbb{M}_n(\mathbb{F})$ where F is a field and there exists $n\in N$  where $A^n$= I. In the case where n=1,2, $A^1$=I and $A^2$=I. 
Here's my question: In general, if A is it's own inverse, then does it necessarily follow A=I? In other words, is I the only matrix which is it's own inverse? My gut reaction is to say no, but it would probably be fairly tedious to construct a matrix multiplication formula which produces the subset $S\subset \mathbb{M}_n(\mathbb{R})$ where S = {A | AA =I }. Is there such a subset in general? We know the set's nonempty since $I\in S$. Are there any others?      
 A: You are looking for involutory matrices. To answer the question: no, there are other matrices that are their own inverses.
A: If $A^2=I$, then $(A-I)(A+I)=0$. Suppose that $J$ is the Jordan Canonical Form of $A$, then since $A=SJS^{-1}$, we have
$$
\begin{align}
0
&=S^{-1}(A^2-I)S\\
&=S^{-1}AAS-S^{-1}IS\\
&=S^{-1}ASS^{-1}AS-I\\
&=J^2-I
\end{align}
$$
Thus, $J$ must be a diagonal matrix all of whose diagonal elements are $+1$ and $-1$.
Therefore, $A^2=I$ if and only if $A$ is diagonalizable and has eigenvalues $+1$ and $-1$.
Easy examples are diagonal matrices whose diagonal elements are $+1$ and $-1$, but any similar matrices will also work.
A: $$\begin{pmatrix}\cos\theta & \sin\theta \\ \sin\theta &-\cos\theta\end{pmatrix}^{-1}=\begin{pmatrix}\cos\theta & \sin\theta \\ \sin\theta &-\cos\theta\end{pmatrix}$$
for any $\theta\in\mathbb{R}$.
A: Well, you can actually create examples of such matrices very easily. All you need is a linear transformation which is it's own inverse. Just choose a basis and swap some extries (make sure to do disjoint swaps), for example, say $T : \mathbb{R}^2 \to \mathbb{R}^2$, such that $T(e_1) = e_2, T(e_2) = e_1$, the corresponding matrix will be a $2 \times 2$ matrix with $1$s in bottom left and top right entries and zeroes elsewhere.
A harder question would be does there exist a basis with respect to which this linear transformation is a diagonal matrix all of whose diagonal elements are $+1$ and $−1$. The answer is yes, since then such a involutory matrix has eigenvalues $+1$ and $-1$.
