TAILIEUCHUNG - Báo cáo toán học: "Semidefinite functions on categories"

Tuyển tập các báo cáo nghiên cứu khoa học ngành toán học tạp chí toán học quốc tế đề tài: Semidefinite functions on categories. | Semidefinite functions on categories László Lovasz Institute of Mathematics Eotvos Lorand University Budapest Hungary Alexander Schrijver CWI and University of Amsterdam The Netherlands Submitted Oct 3 2008 Accepted May 25 2009 Published May 31 2009 Mathematics Subject Classification 05C50 Dedicated to Anders Bjorner on his 60th birthday. Abstract Freedman Lovasz and Schrijver characterized graph parameters that can be represented as the weighted number of homomorphisms into a fixed graph. Several extensions of this result have been proved. We use the framework of categories to prove a general theorem of this kind. Similarly as previous resuts the characterization uses certain infinite matrices called connection matrices which are required to be positive semidefinite. 1 Introduction For two finite graphs F and G let hom F G denote the number of homomorphisms F G. The definition can be extended to weighted graphs. In 7 graph parameters of the form hom - G defined on finite multigraphs were characterized where G is a fixed weighted graph. Several variants of this result have been obtained characterizing graph parameters hom - G where all nodeweights of G are 1 16 such graph parameters defined on simple graphs 13 etc. These characterizations involve certain infinite matrices called connection matrices which are required to be positive semidefinite and to satisfy a condition on their rank. The results can be extended to directed graphs hypergraphs etc. The goal of this paper is to use the framework of categories to prove a general theorem of this kind. Let C be a category. We need to assume that it satisfies a number of natural conditions C1-C4 below but for the statement of the main theorem we only need that it is locally finite it has pullbacks and it contains a terminal object t. In particular every Research sponsored by OTKA Grant No. 67867. the electronic journal of combinatorics 16 2 2009 R14 1 two objects a and b have a direct product a X b. We denote by C a b .

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