TAILIEUCHUNG - Báo cáo toán học: "Extension of Strongly Regular Graphs"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: Extension of Strongly Regular Graphs. | Extension of Strongly Regular Graphs Ralucca Gera Department of Applied Mathematics Naval Postgraduate School Monterey CA 93943 email rgera@ phone 831 656-2206 fax 831 656-2355 and Jian Shen Department of Mathematics Texas State University San Marcos TX 78666 email js48@ phone 512 245-3740 Submitted Sep 30 2007 Accepted Feb 2 2008 Published Feb 11 2008 Mathematics Subject Classification 05C75 Abstract The Friendship Theorem states that if any two people in a party have exactly one common friend then there exists a politician who is a friend of everybody. In this paper we generalize the Friendship Theorem. Let A be any nonnegative integer and p be any positive integer. Suppose each pair of friends have exactly A common friends and each pair of strangers have exactly p common friends in a party. The corresponding graph is a generalization of strongly regular graphs obtained by relaxing the regularity property on vertex degrees. We prove that either everyone has exactly the same number of friends or there exists a politician who is a friend of everybody. As an immediate consequence this implies a recent conjecture by Limaye et. al. Key Words strongly regular graph Friendship Theorem 1 Introduction and Motivation In this paper all graphs G V G E G are simple. The neighborhood of a vertex v 2 V G is N v u u v 2 E G g. The join denoted G1 V G2 of two graphs G1 and G2 is the graph with vertex set V G1 u V G2 and edge set E G1 u E G2 u u v u 2 V G1 and v 2 V G2 g. Also the disjoint union denoted by G1 G2 of two graphs Research supported by the Research Initiation Program Grant at the Naval Postgraduate School. THE ELECTRONIC JOURNAL OF COMBINATORICS 15 2008 N3 1 G1 and G2 is the graph obtained from G1 and G2 with V G1 G2 V G1 u V G2 and E G1 G2 E G1 u E G2 . We use the notation d u N u I and ố u v N u N v I to denote the number of neighbors of u the degree of u and the number of common neighbors of u and v respectively. An n-vertex graph is called .

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