1)Let D be a 2−(v,k,λ) design with b blocks and r blocks through every point. Let B be any block. How to show that the number of blocks that meet B is at least $k(r−1)^2/[(k−1)(λ−1)+(r−1)]$
2) How to show that if v<=k+t then a t design is trivial?
3)Is a complement of a t design is also a design?