%I A112921
%S A112921 1,1,2,4,4,6,8,10,7,24,10,20,26,26,15,44,19,54,44,44,26,102,38,62,57,96,
%T A112921 40,164,46,104,91,102,91,213,64,128,124,222,77,290,85,212,200,184,100,
%U A112921 388,128,268,199,292,126
%N A112921 Number of nonisomorphic Y-graphs Y(n:i,j,k) on 4n vertices (or nodes) 
               for 1<=i,j,k<n/2.
%C A112921 A Y-graph Y(n:i,j,k) has 4n vertices arranged in four segments of n vertices. 
               Let the vertices be v_{x,y} for x=0,1,2,3 and y in the integers modulo 
               n. The edges are v_{1,y}v_{1,y+i}, v_{2,y}v_{2,y+j}, v_{2,y}v_{2,
               y+k} and v_{0,y}v_{x,y}, where y=0,1,...,n-1 and x=1,2,3 and the 
               subscript addition is performed modulo n.
%D A112921 I. Z. Bouwer, W. W. Chernoff, B. Monson and Z. Starr (Eds.), "Foster's 
               Census", Charles Babbage Research Centre, Winnipeg, 1988.
%D A112921 J. D. Horton and I. Z. Bouwer, Symmetric Y-graphs and H-graphs, J. Comb. 
               Theory B 53 (1991) 114-129
%e A112921 Y(7:1,2,3) is the Coxeter graph, the only (connected) symmetric (vertex- 
               and edge-tansitive) Y-graph of girth 7 or less.
%Y A112921 Cf. A112922, A112923, A112924, A112921, A107452.
%Y A112921 Sequence in context: A023847 A000061 A153176 this_sequence A008133 A022471 
               A081238
%Y A112921 Adjacent sequences: A112918 A112919 A112920 this_sequence A112922 A112923 
               A112924
%K A112921 nonn
%O A112921 3,3
%A A112921 Marko Boben (Marko.Boben(AT)fmf.uni-lj.si), Tomaz Pisanski (Tomaz.Pisanski(AT)fmf.uni-lj.si) 
               and Arjana Zitnik (Arjana.Zitnik(AT)fmf.uni-lj.si), Oct 06 2005