Left and Right Inverse Mappings Implies Bijection/Proof 1

Proof
Suppose:
 * $\exists g_1: T \to S: g_1 \circ f = I_S$
 * $\exists g_2: T \to S: f \circ g_2 = I_T$

We have that the Identity Mapping is Bijection, thus $I_S$ and $I_T$ are both bijections.

From Injection if Composite is Injection, if $I_S = g_1 \circ f$ is an injection, then so is $f$.

From Surjection if Composite is Surjection, if $I_T = f \circ g_2$ is a surjection, then so is $f$.

So $f$ is both an injection and a surjection and, by definition, therefore also a bijection.