Linear Programming Approaches to Semidefinite Programming Problems

Kartik Krishnan Sivaramakrishnan

PhD Thesis, August 2002

Mathematical Sciences

Rensselaer Polytechnic Institute

Advisor: John Mitchell

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Semidefinite Programming (SDP) has been one of the most exciting and active areas in optimization lately. This tremendous activity was spurred by the discovery of efficient interior point algorithms for solving SDP problems, and important appl ications of the SDP in developing approximation algorithms for various combinatorial optimizatio n problems. However these applications require effective techniques, to solve SDP's arising as relaxations of these problems. Although interior point methods are a great theoretical tool, and offer polynomi al time complexity, they are fairly limited in the size of problems they can handle. The thesis inve stigates linear programming approaches to solving SDP's. This potentially allows one to s olve large problems approximately and quickly, using state of the art linear solvers. The LP approac h is also incorporated in a branch and cut approach to solving integer programming problems.

One of the various characterizations of the positive semidefiniteness constraint leads to a semi-infinite LP formulation for the SDP. We formulate the dual SDP as a semi-infinite LP, and discuss the issue of its discretization in detail. Using the notions of nondegeneracy and basic feasible solutions developed in the context of semi-infinite linear programming, we recover a theorem due to Pataki on the rank of extreme matrices in SDP, which in turn implies that not more than $O(\sqrt{k})$ ($k$ is the number of constraints in the SDP) constraints are required in the LP relaxations. To generate these constraints we use the spectra l bundle approach due to Helmberg and Rendl. This scheme recasts any SDP with a bounded feasible set as a n eigenvalue optimization problem. These are convex but nonsmooth problems that can be tacked by bundle me thods for nondifferentiable optimization. The LP approach provides an upper bound, and can also be utilized to generate a primal feasible matrix $X$ for the spectral bundle app roach. We demonstrate the efficiency of the approach on two combinatorial optimization problems namely maxcut and minbisection.

The semi-infinite LP formulation of the SDP, together with its polynomial time s eparation oracle leads naturally to a cutting plane LP approach for the SDP. We investigate such an approach. However to make the resulting method competitive with interior point methods for the SDP, several refinements are necessary. In particular the cutting plane method requires solving the LP relaxations approximately using an interior point method, since this results in better cutting planes. We experiment with various separation ora cles generating deep cuts, and techniques to restart the LP relaxations with strictly feasible s tarting points. We also relate these cutting planes to the geometry of the SDP cone. We then tes t the approach on the maxcut SDP.

Finally we incorporate the cutting plane LP approach to the SDP in an SDP approa ch for the maxcut problem. In particular, we formulate the dual of the well known SDP r elaxation for maxcut as a semi-infinite LP, and solve it using an interior point cutting plane scheme. We then add cutting planes, which are facets of the maxcut polytope, to the primal probl em in order to tighten the SDP relaxations. The approach resembles a SDP cutting plane scheme f or the maxcut problem; in reality it uses an LP subroutine to solving the SDP within an overal l LP cutting plane approach for integer programming. This overcomes another shortc oming of an SDP cutting plane scheme, where there is no convincing warm start strategy, that allows one to qui ckly reoptimize a slightly perturbed version of the original SDP, after the addition of cutting planes. We present computational results on a variety of maxcut problems.

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Download the thesis, postscript or pdf or gzipped postscript.