tailieunhanh - Báo cáo toán học: "Some non-normal Cayley digraphs of the generalized quaternion group of certain orders"

Tuyển tập các báo cáo nghiên cứu khoa học ngành toán học tạp chí toán học quốc tế đề tài: Some non-normal Cayley digraphs of the generalized quaternion group of certain orders | Some non-normal Cayley digraphs of the generalized quaternion group of certain orders Edward Dobson Department of Mathematics and Statistics PO Drawer MA Mississippi State MS 39762 . dobson@ Submitted Mar 10 2003 Accepted Jul 30 2003 Published Sep 8 2003 MR Subject Classifications 05C25 20B25 Abstract We show that an action of SL 2 p p 7 an odd prime such that 4 p 1 has exactly two orbital digraphs Pl r2 such that Aut Pj admits a complete block system B of p 1 blocks of size 2 i 1 2 with the following properties the action of Aut Pj on the blocks of B is nonsolvable doubly-transitive but not a symmetric group and the subgroup of Aut Pj that fixes each block of B set-wise is semiregular of order 2. If p 2k 1 7 is a Mersenne prime these digraphs are also Cayley digraphs of the generalized quaternion group of order 2k 1. In this case these digraphs are non-normal Cayley digraphs of the generalized quaternion group of order 2k 1. There are a variety of problems on vertex-transitive digraphs where a natural approach is to proceed by induction on the number of not necessarily distinct prime factors of the order of the graph. For example the Cayley isomorphism problem see 6 is one such problem as well as determining the full automorphism group of a vertex-transitive digraph r. Many such arguments begin by finding a complete block system B of Aut r . Ideally one would then apply the induction hypothesis to the groups Aut r B and fixAut r B b where Aut r B is the permutation group induced by the action of Aut r on B and fixAut r B is the subgroup of Aut r that fixes each block of B set-wise and B E B. Unfortunately neither Aut r B nor fixAut r B y need be the automorphism group of a digraph. In fact there are examples of vertex-transitive graphs where Aut r B is a doubly-transitive nonsolvable group that is not a symmetric group see 7 as well as examples of vertex-transitive graphs where fixAut r B b is a doubly-transitive nonsolvable group that is not

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