Inverse matrices

$$A \in \mathbb{R}^{n\times n}$$ is invertible or nonsingular if $$\det A \neq 0$$, which is equivalent to the following conditions:
 * Columns of $$A$$ are a basis for $$\mathbb{R}^n$$
 * Rows of $$A$$ are also a basis for $$\mathbb{R}^n$$
 * $$y=Ax$$ has a unique solution $$x$$ for every $$y \in \mathbb{R}^n$$
 * A has a (left and right) inverse denoted $$A^{-1} \in \mathbb{R}^{n\times n}$$ with $$AA^{-1} = A^{-1}A = I$$
 * \mathcal{N}(A) = \{0\}
 * \mathcal{R}(A) = \mathbb{R}^n
 * $$\detA^TA=\det AA^T \neq 0$$