In this paper we consider the constant rank unconstrained quadratic 0–1 optimization problem, CR-QP01 for short. This problem consists in minimizing the quadratic function <x, Ax> + <c, x> over the set {0,1}n where c is a vector in ℝn and A is a symmetric real n × n matrix of constant rank r. We first present a pseudo-polynomial algorithm for solving the problem CR-QP01, which is known to be NP-hard already for r = 1. We then derive two new classes of special cases of the CR-QP01 which can be solved in polynomial time. These classes result from further restrictions on the matrix A. Finally we compare our algorithm with the algorithm of Allemand et al. for the CR-QP01 with negative semidefinite A and extend the range of applicability of the latter algorithm. It turns out that neither of the two algorithms dominates the other with respect to the class of instances which can be solved in polynomial time.
