Centralizer in Subgroup is Intersection

Theorem
Let $G$ be a group.

Let $H$ be a subgroup of $G$.

Then:


 * $\forall x \in G: C_H \left({x}\right) = C_G \left({x}\right) \cap H$

That is, the centralizer of an element in a subgroup is the intersection of that subgroup with the centralizer of the element in the group.

Proof
It is clear that:
 * $g \in C_H \left({x}\right) \iff g \in C_G \left({x}\right) \land g \in H$

The result follows by definition of set intersection.