Normaliser

In group theory, the normaliser of a subgroup of a group (mathematics) is the set of all group elements which map the given subgroup to itself by conjugation.

Formally, for H a subgroup of a group G, we define

$N_{G}(H)=\{g\in G:g^{-1}Hg=H\}.\,$

A subgroup of G is normal in G if its normaliser is the whole of G.

The normaliser of the trivial subgroup is the whole group G.