Generalised classes in groups and association schemes: Duals of results on characters and sharpness

A result of Strunkov on generalised conjugacy classes of groups is most conveniently expressed in terms of the P-matrix of an association scheme. This result is dual to a result of Blichfeldt on permutation characters, which has been shown by Cameron and Kiyota to hold for arbitrary characters and led to the definition of sharp characters. We show that the result carries over to arbitrary (commutative) association scheme classes, and introduce the idea of a sharp class in an association scheme.