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Expressing Combinatorial Problems by Systems of Polynomial Equations and Hilbert's Nullstellensatz

Published online by Cambridge University Press:  01 July 2009

J. A. LOERA ,
J. LEE ,
S. MARGULIES  and
S. ONN
J. A. LOERA
Affiliation:
Department of Mathematics, University of California, Davis, California, USA (e-mail: deloera@math.ucdavis.edu)
J. LEE
Affiliation:
IBM T. J. Watson Research Center, Yorktown Heights, New York, USA (e-mail: jonlee@us.ibm.com)
S. MARGULIES
Affiliation:
Department of Computer Science, University of California, Davis, California, USA (e-mail: smargulies@ucdavis.edu)
S. ONN
Affiliation:
Davidson Faculty of IE & M, Technion – Israel Institute of Technology, Haifa, Israel (e-mail: onn@ie.technion.ac.il)
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Abstract

Systems of polynomial equations over the complex or real numbers can be used to model combinatorial problems. In this way, a combinatorial problem is feasible (e.g., a graph is 3-colourable, Hamiltonian, etc.) if and only if a related system of polynomial equations has a solution.

For an infeasible polynomial system, the (complex) Hilbert Nullstellensatz gives a certificate that the associated combinatorial problem is infeasible. Thus, unless P = NP, there must exist an infinite sequence of infeasible instances of each hard combinatorial problem for which the minimum degree of a Hilbert Nullstellensatz certificate of the associated polynomial system grows.

In the first part of the paper, we show that the minimum degree of a Nullstellensatz certificate for the non-existence of a stable set of size greater than the stability number of the graph is the stability number of the graph. Moreover, such a certificate contains at least one term per stable set of G. In contrast, for non-3-colourability, we proved that the minimum degree of a Nullstellensatz certificate is at least four. Our efforts so far have only yielded graphs with Nullstellensatz certificates of precisely that degree.

In the second part of this paper, for the purpose of computation, we construct new polynomial encodings for the problems of finding in a graph its longest cycle, the largest planar subgraph, the edge-chromatic number, or the largest k-colourable subgraph. We include some applications to graph theory.

Type
Paper
Information
Combinatorics, Probability and Computing , Volume 18 , Issue 4 , July 2009 , pp. 551 - 582
Copyright
Copyright © Cambridge University Press 2009

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