For p ≥ 2 we consider the problem of, given an n × n matrix A = (aij ) whose diagonal entries vanish, approximating in polynomial time the number Optp(A):=max{Σi,j=1naijxixj:(x1,…,xn)∈ℝ;n∧(Σi=1n∣xi∣p)1/p≤1}.
