Restriction of Associative Operation is Associative

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Let $\struct {S, \circ}$ be an algebraic structure.

Let $T \subseteq S$.

Let $\circ_T$ denote the restriction of $\circ$ to $T$.

Let the operation $\circ$ be associative on $\struct {S, \circ}$.

Then $\circ_T$ is associative on $\struct {T, \circ_T}$.


\(\ds T\) \(\subseteq\) \(\ds S\)
\(\ds \leadsto \ \ \) \(\ds \forall a, b, c \in T: \, \) \(\ds a, b, c\) \(\in\) \(\ds S\) Definition of Subset
\(\ds \leadsto \ \ \) \(\ds a \mathop {\circ_T} \paren {b \mathop {\circ_T} c}\) \(=\) \(\ds a \circ \paren {b \circ c}\)
\(\ds \) \(=\) \(\ds \paren {a \circ b} \circ c\) as $\circ$ is associative
\(\ds \) \(=\) \(\ds \paren {a \mathop {\circ_T} b} \mathop {\circ_T} c\)