Conjugacy class

If g₁ and g₂ are two elements of a group G, they are called conjugate if there exists an element g₃ in G such that:

g₃g₁g₃^-1 = g₂.

Conjugacy is an equivalence relation and therefore partitions G into equivalence classes: every element of the group belongs to precisely one conjugacy class; the classes Cl(g₁) and Cl(g₂) are equal if and only if g₁ and g₂ are conjugate, and disjoint otherwise.

The equivalence class that contains the element g₁ in G is

Cl(g₁) = { g₃g₁g₃^-1| g₃ ∈ G}

and is called the conjugacy class of g₁. The class number of G is the number of conjugacy classes.

For abelian groups the concept is trivial, since each class is a set of one element.