On t -designs

Introduction and preliminaries An incidence structure is a triple S=(X, <J,9 S} where X and JL are disjoint sets andc^ci^fX^?. Elements x^X are called points and elements A^JL are called blocks of S. A point x and a block ^4 are incident iff (#, ^4)ec?. For any block A, (A) will denote the set of points incident with A