tailieunhanh - Báo cáo toán học: "Matrix Partitions with Finitely Many Obstructions"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: Matrix Partitions with Finitely Many Obstructions. | Matrix Partitions with Finitely Many Obstructions Tomás Feder Pavol Helly and Wing Xiez Submitted Jan 29 2007 Accepted Aug 10 2007 Published Aug 20 2007 Mathematics Subject Classification 05C75 Abstract Each m by m symmetric matrix M over 0 1 defines a partition problem in which an input graph G is to be partitioned into m parts with adjacencies governed by M in the sense that two distinct vertices in possibly equal parts i and j are adjacent if M i j 1 and nonadjacent if M i j 0. The entry implies no restriction. We ask which matrix partition problems admit a characterization by a hnite set of forbidden induced subgraphs. We prove that matrices containing a certain two by two diagonal submatrix S never have such characterizations. We then develop a recursive technique that allows us with some extra effort to verify that matrices without S of size five or less always have a finite forbidden induced subgraph characterization. However we exhibit a six by six matrix without S which cannot be characterized by finitely many induced subgraphs. We also explore the connection between finite forbidden subgraph characterizations and related questions on the descriptive and computational complexity of matrix partition problems. 1 Introduction Many graph partition problems especially those arising from the study of perfect graphs 6 7 16 can be formulated in the following terms. Let M be a symmetric m by m matrix over 0 1 . An M-partition of a graph G is a partition of V G into parts V1 V2 . Vm such that for distinct vertices u 2 V v 2 Vj we have uv 2 E G if M i j 1 and uv 2 E G if M i j 0. Note that we admit i j in particular if M i i 0 the set Vi is independent in G and if M i i 1 it is a clique. Also note that means no restriction. For each fixed matrix M we obtain the M-partition problem - to decide whether or not an input graph G admits an M-partition. For instance for the identity 268 Waverley St. Palo Alto CA 94301 USA tomas@ ySchool of Computing .

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