Composition of Functors is Associative
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Theorem
Let $\mathbf A$, $\mathbf B$, $\mathbf C$ and $\mathbf D$ be metacategories.
Let $F: \mathbf A \to \mathbf B$, $G: \mathbf B \to \mathbf C$ and $H: \mathbf C \to \mathbf D$ be functors.
Then composition of functors is associative:
- $H \paren {G F} = \paren {H G} F$
Proof
Let $A$ be an object of $\mathbf A$.
Then, solely by the definition of composite functor:
\(\ds H \paren {G F} A\) | \(=\) | \(\ds H \paren {G F A}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds H \paren {G \paren {F A} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds H G \paren {F A}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {H G} F A\) |
Then, mutatis mutandis, the same proof works for a morphism $f$ of $\mathbf A$ as well.
Hence the result.
$\blacksquare$