Bijection iff Left and Right Inverse/Corollary

Corollary to Bijection iff Left and Right Inverse
Let $f: S \to T$ and $g: T \to S$ be mappings such that:


 * $g \circ f = I_S$
 * $f \circ g = I_T$

Then both $f$ and $g$ are bijections.

Proof
Suppose we have such mappings $f$ and $g$ with the given properties.

From the main result, we have that $f$ is a bijection, by considering $g = g_1$ and $g = g_2$.

It directly follows that by setting $g = f, f = g_1, f = g_2$, the same result can be used the other way about.