Balas formulation for the union of polytopes is optimal
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A celebrated theorem of Balas gives a linear mixed-integer formulation for the union of two nonempty polytopes whose relaxation gives the convex hull of this union. The number of inequalities in Balas formulation is linear in the number of inequalities that describe the two polytopes and the number of variables is doubled. In this paper we show that this is best possible: in every dimension there exist two nonempty polytopes such that if a formulation for the convex hull of their union has a number of inequalities that is polynomial in the number of inequalities that describe the two polytopes, then the number of additional variables is at least linear in the dimension of the polytopes. We then show that this result essentially carries over if one wants to approximate the convex hull of the union of two polytopes and also in the more restrictive setting of lift-and-project.
Mathematics Subject ClassificationPrimary 90C11 Secondary 52B05
We are grateful to Volker Kaibel and Stefan Weltge for helpful discussions and suggestions regarding the present work. We thank two anonymous referees for their suggestions and pointers to relevant papers from the literature.
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