Intersection of Subalgebras is Subalgebra
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Theorem
Let $K$ be a field.
Let $\struct {A, +, \cdot, \circ}_K$ be an algebra over $K$.
Let $\family {A_\alpha}_{\alpha \mathop \in I}$ be an $I$-indexed family of subalgebras of $A$.
Let:
- $\ds B = \bigcap_{\alpha \mathop \in I} A_\alpha$
Then $B$ is a subalgebra of $A$.
Proof
From Set of Linear Subspaces is Closed under Intersection, $\struct {B, +, \cdot}_K$ is a vector subspace of $\struct {A, +, \cdot}_K$.
Now let $x, y \in B$.
That is, $x, y \in A_\alpha$ for each $\alpha \in I$.
Since $A_\alpha$ is a subalgebras of $A$ for each $\alpha \in I$, we have:
- $x y \in A_\alpha$ for each $\alpha \in I$.
So $x y \in B$.
So $B$ is a subalgebra of $A$.
$\blacksquare$