Left and Right Inverses of Square Matrix over Field are Equal

Theorem
Let $\Bbb F$ be a field, usually one of the standard number fields $\Q$, $\R$ or $\C$.

Let $n \in \Z_{>0}$ be a (strictly) positive integer.

Let $\map \MM n$ denote the matrix space of order $n$ square matrices over $\Bbb F$.

Let $\mathbf B$ be a left inverse matrix of $\mathbf A$.

Then $\mathbf B$ is also a right inverse matrix of $\mathbf A$.

Similarly, let $\mathbf B$ be a right inverse matrix of $\mathbf A$.

Then $\mathbf B$ is also a right inverse matrix of $\mathbf A$.

Proof
Consider the algebraic structure $\struct {\map \MM {m, n}, +, \circ}$, where:
 * $+$ denotes matrix entrywise addition
 * $\circ$ denotes (conventional) matrix multiplication.

From Ring of Square Matrices over Field is Ring with Unity, $\struct {\map \MM {m, n}, +, \circ}$ is a ring with unity.

Hence a fortiori $\struct {\map \MM {m, n}, +, \circ}$ is a monoid.

The result follows directly from Left Inverse and Right Inverse is Inverse.