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Computational and Applied Mathematics, Rice University, Houston, Texas 77005
Lovász and Schrijver (1991) described a semidefinite operator for generating strong valid inequalities for the 0-1 vectors in a prescribed polyhedron. Among their results, they showed that n iterations of the operator are sufficient to generate the convex hull of 0-1 vectors contained in a polyhedron in n-space. We give a simple example, having Chvátal rank 1, that meets this worst case bound of n. We describe another example requiring n iterations even when combining the semidefinite and Gomory-Chvátal operators. This second example is used to show that the standard linear programming relaxation of a k-city traveling salesman problem requires at least
Computational and Applied Mathematics, Rice University, Houston, Texas 77005
k/8
iterations of the combined operator; this bound is best possible, up to a constant factor, as k+1 iterations suffice.
History: Received: October 27, 1999;
revision received: August 14, 2000;
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