Identity of Group is in Center

Theorem
Let $G$ be a group.

Let $e$ be the identity of $G$.

Then $e$ is in the center of $G$:
 * $e \in \map Z G$

Proof
From Center is Intersection of Centralizers:


 * $\ds \map Z G = \bigcap_{g \mathop \in G} \map {C_G} g$

where $\map {C_G} g$ denotes the centralizer of $g$.

From Centralizer of Group Element is Subgroup, each of $\map {C_G} g$ is a subgroup of $G$.

From Identity of Subgroup:
 * $\forall g \in G: e \in \map {C_G} g$

Hence by definition of set intersection:
 * $e \in \ds \bigcap_{g \mathop \in G} \map {C_G} g$

whence the result.