Centralizer in Subgroup is Intersection

Theorem

Let $G$ be a group.

Let $H \le G$. 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 $g \in C_H \left({x}\right) \iff g \in C_G \left({x}\right) \land g \in H$.

Thus by definition of set intersection the result follows.

$\blacksquare$

Sources

• John F. Humphreys: A Course in Group Theory (1996): $\S 10$: Proposition $10.19, \ 10.25$