tailieunhanh - Báo cáo toán học: "On the Theory of Pfaffian Orientations"

Tuyển tập các báo cáo nghiên cứu khoa học trên tạp chí toán học quốc tế đề tài: On the Theory of Pfaffian Orientations. | On the Theory of Pfaffian Orientations. I. Perfect Matchings and Permanents. Anna Galluccio Istituto di Analisi dei Sistemi ed Informatica - CNR viale Manzoni 30 00185 Roma ITALY galluccio@ yMartin Loebl Department of Applied Mathematics Charles University Malostranske n. 25 118 00 Praha 1 CZECH REPUBLIC loebl@ Received May 7 1998 Accepted October 28 1998. Abstract Kasteleyn stated that the generating function of the perfect matchings of a graph of genus g may be written as a linear combination of 4g Pfaffians. Here we prove this statement. As a consequence we present a combinatorial way to compute the permanent of a square matrix. Mathematical Reviews Subject Numbers 05B35 05C15 05A15 Supported by NATO-CNR Fellowship y Supported by DONET GACR 0194 and GAUK 194 1 THE ELECTRONIC .JOURNAL OF COmBINATORICS 6 1999 R6 1 1 Introduction The theory of Pfaffian orientations of graphs has been introduced by Kasteleyn 7 6 5 in early sixties to solve some enumeration problems arising from statistical physics 4 10 . He proved fundamental results in the planar case and extended his approach to toroidal grids 5 6 7 . The case of general toroidal graphs was also considered in an unpublished manuscript by Barahona 1 . In the present paper we extend the method proposed by Kasteleyn and we prove that the generating function of the perfect matchings of a graph of genus g may be obtained as a linear combination of 4g Pfaffians. As a consequence we provide a new technique to compute permanents of square matrices which completes the scheme proposed by Polya in 9 . A graph is a pair G V E where V is a set of vertices and E is a set of unordered pairs of elements of V called edges. In this paper we shall consider only graphs with hnite number of vertices. If e xy is an edge then the vertices X y are called endvertices of e. We associate with each edge e of G a variable xe and we let x xe e 2 E . For each M c E let x M denote the product of the variables of .

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