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 d) in Γ0(N). The diamond operators form an abelian group and commute with the Hecke operators.
Diamond operator Wikipedia
In Unicode, the diamond operator is represented by the character U+22C4 ⋄ DIAMOND OPERATOR.