PPROJ

Projection onto a polyhedron that exploits sparsity. An algorithm is developed for projecting a point onto a polyhedron. The algorithm solves a dual version of the projection problem and then uses the relationship between the primal and dual to recover the projection. The techniques in the paper exploit sparsity. Sparse reconstruction by separable approximation (SpaRSA) is used to approximately identify active constraints in the polyhedron, and the dual active set algorithm (DASA) is used to compute a high precision solution. A linear convergence result is established for SpaRSA that does not require the strong concavity of the dual to the projection problem, and an earlier R-linear convergence rate is strengthened to a Q-linear convergence property. An algorithmic framework is developed for combining SpaRSA with an asymptotically preferred algorithm such as DASA. It is shown that only the preferred algorithm is executed asymptotically. Numerical results are given using the polyhedra associated with the Netlib LP test set. A comparison is made to the interior point method contained in the general purpose open source software package IPOPT for nonlinear optimization, and to the commercial package CPLEX, which contains an implementation of the barrier method that is targeted to problems with the structure of the polyhedral projection problem.