Center (group theory)
The center is a normal subgroup, Z(G) ⊲ G. As a subgroup, it is always characteristic, but is not necessarily fully characteristic. The quotient group, G / Z(G), is isomorphic to the inner automorphism group, Inn(G).
Consider the map, f: G → Aut(G), from G to the automorphism group of G defined by f(g) = ϕg, where ϕg is the automorphism of G defined by
The function, f is a group homomorphism, and its kernel is precisely the center of G, and its image is called the inner automorphism group of G, denoted Inn(G). By the first isomorphism theorem we get,
Quotienting out by the center of a group yields a sequence of groups called the upper central series:
The kernel of the map G → Gi is the ith center of G (second center, third center, etc.) and is denoted Zi(G). Concretely, the (i + 1)-st center are the terms that commute with all elements up to an element of the ith center. Following this definition, one can define the 0th center of a group to be the identity subgroup. This can be continued to transfinite ordinals by transfinite induction; the union of all the higher centers is called the hypercenter.[note 1]
stabilizes at i (equivalently, Zi(G) = Zi+1(G)) if and only if Gi is centerless.