We prove that the subset sum problem has a polynomial time computable certificate of infeasibility for all $a$ weight vectors with density at most $1/(2n)$ and for almost all integer right hand sides. The certificate is branching on a hyperplane, i.e. by a methodology dual to the one explored by Lagarias and Odlyzko; Frieze; Furst and Kannan; and Coster et. al. The proof has two ingredients. We first prove that a vector that is near parallel to $a$ is a suitable branching direction, regardless of the density. Then we show that for a low density $a$ such a near parallel vector can be computed using diophantine approximation, via a methodology introduced by Frank and Tardos. We also show that there is a small number of long intervals whose disjoint union covers the integer right hand sides, for which the infeasibility is proven by branching on the above hyperplane.
Technical Report 2008-03, Department of Statistics and Operations Research, UNC Chapel Hill