#### About the EventGiven a matrix A with entries in the set {1,-1}, the smallest possible rank of a real matrix B
whose nonzero entries have the signs as prescribed by A, is called the sign-rank of A.
It seems hard to find good lower bounds for the sign-rank of a matrix. I will explain a result of
Forster that gives a lower bound for the sign-rank of A in terms of its spectral norm. In particular,
the sign-rank of an n x n Hademard matrix is at last sqrt(n). I will also discuss the application
of this result to communication complexity. |