  Given positive integers m,n,s,t, let z(m,n,s,t) be the maximum number of ones
in a (0,1) matrix of size m-by-n that does not contain an all ones submatrix of
size s-by-t. We find a flexible upper bound on z(m,n,s,t) that implies the
known bounds of Kovari, Sos and Turan, and of Furedi. As a consequence, we find
an upper bound on the spectral radius of a graph of order n without a complete
bipartite subgraph K_{s,t}.
