Computing Multi-Homogeneous Bezout Numbers is Hard

The multi-homogeneous Bézout number is a bound for the number of solutions of a system of multi-homogeneous polynomial equations, in a suitable product of projective spaces. Given an arbitrary, not necessarily multi-homogeneous system, one can ask for the optimal multi-homogenization that would minimize the Bézout number. In this paper, it is proved that the problem of computing, or even estimating the optimal multi-homogeneous Bézout number is actually NP-hard. In terms of approximation theory for combinatorial optimization, the problem of computing the best multi-homogeneous structure does not belong to APX, unless P = NP. Moreover, polynomial time algorithms for estimating the minimal multi-homogeneous Bézout number up to a fixed factor cannot exist even in a randomized setting, unless BPP ⊇ NP.