wolffd@0: function b = positive_semidefinite(M)
wolffd@0: %
wolffd@0: % Return true iff v M v' >= 0 for any vector v.
wolffd@0: % We do this by checking that all the eigenvalues are non-negative.
wolffd@0: 
wolffd@0: E = eig(M);
wolffd@0: if length(find(E>=0)) == length(E)
wolffd@0:   b = 1;
wolffd@0: else
wolffd@0:   b = 0;
wolffd@0: end