Left and Right Inverses of Mapping are Inverse Mapping/Proof 3

Theorem
Let $f: S \to T$ be a mapping such that:


 * $(1): \quad \exists g_1: T \to S: g_1 \circ f = I_S$
 * $(2): \quad \exists g_2: T \to S: f \circ g_2 = I_T$

Then:
 * $g_1 = g_2 = f^{-1}$

where $f^{-1}$ is the inverse of $f$.

Proof
Because of Associativity of composition of mappings, brackets do not need to be used.

Thus $g_1 = g_2$.

Now suppose there exists $g_3: T \to S: g_3 \circ f = I_S$.

By the same argument as above, $g_3 = g_2$.

This means that $g_1 (= g_3)$ is the only left inverse of $f$.

Similarly, suppose there exists $g_4: T \to S: f \circ g_4 = I_T$.

By the same argument as above, $g_4 = g_1$.

This means that $g_2 (= g_4)$ is the only right inverse of $f$.

So $g_1 = g_2 = g_3 = g_4$ are all the same.

By Bijection Composite with Inverse, it follows that this unique mapping is the inverse $f^{-1}$.