Identity Mapping is Right Identity/Proof 2
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Theorem
Let $S$ and $T$ be sets.
Let $f: S \to T$ be a mapping.
Then:
- $f \circ I_S = f$
where $I_S$ is the identity mapping on $S$, and $\circ$ signifies composition of mappings.
Proof
By definition, a mapping is also a relation.
Also by definition, the identity mapping is the same as the diagonal relation.
Thus Diagonal Relation is Right Identity can be applied directly.
$\blacksquare$