Inverse of Inverse of Matrix

Theorem
Let $\mathbf A$ be an invertible matrix.

Then:
 * $\left({\mathbf A^{-1} }\right)^{-1} = \mathbf A$.

That is, an invertible matrix equals the inverse of its inverse.

Proof
By definition of inverse matrix:
 * $\mathbf A^{-1} \mathbf A = \mathbf I$

where $\mathbf I$ is the unit matrix.

Thus the inverse of $\mathbf A^{-1}$ is $\mathbf A$.

Hence the result.