Diamond operator

In number theory, the diamond operators 〈d〉 are operators acting on the space of modular forms for the group Γ1(N), given by the action of a matrix ($a b c δ$) in Γ0(N) where δ ≈ d mod N. The diamond operators form an abelian group and commute with the Hecke operators.

Unicode
In Unicode, the diamond operator is represented by the character.