Restriction of Composition is Composition of Restriction
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Theorem
Let $X, Y, Z$ be sets.
Let $f: X \to Y$ and $g: Y \to Z$ be mappings.
Let $S \subseteq X$.
Then:
- $\paren {g \circ f} \restriction S = g \circ \paren {f \restriction S}$
Proof
By definitions of composition of mappings and restriction of mapping:
- $\paren {g \circ f} \restriction S: S \to Z$ and $g \circ \paren {f \restriction S}: S \to Z$
Let $s \in S$.
By definition of restriction of mapping:
- $\map {\paren {\paren {g \circ f} \restriction S} } s = \map {\paren {g \circ f} } s$
Thus
\(\ds \map {\paren {g \circ \paren {f \restriction S} } } s\) | \(=\) | \(\ds \map g {\map {\paren {f \restriction S} } s}\) | Definition of Composition of Mappings | |||||||||||
\(\ds \) | \(=\) | \(\ds \map g {\map f s}\) | Definition of Restriction of Mapping | |||||||||||
\(\ds \) | \(=\) | \(\ds \map {\paren {\paren {g \circ f} \restriction S} } s\) | Definition of Composition of Mappings |
$\blacksquare$
Sources
- Mizar article RELAT_1:83