Dear Bill, GRAPE always defaults to labelling the vertices of a n-vertex graph as [1..n]. So when you construct an induced subgraph on some k-subset of in this way, the new graph will be on vertex set [1..k].
However, when you define your subgraph "gamma_1_2_6", the original vertex names are preserved, in the ".names" component of the graph record: gap> gamma_1_2_6; rec( adjacencies := [ [ 2 ], [ 1, 3 ], [ 2 ] ], group := Group(()), isGraph := true, names := [ 1, 2, 6 ], order := 3, representatives := [ 1, 2, 3 ], schreierVector := [ -1, -2, -3 ] ) So it *is* possible to extract the information you need from this: gap> e:= UndirectedEdges(gamma_1_2_6); [ [ 1, 2 ], [ 2, 3 ] ] gap> List(e, x-> List(x, y->gamma_1_2_6.names[y]) ); [ [ 1, 2 ], [ 2, 6 ] ] I hope this helps! Best wishes, Robert. ============================== Dr. Robert Bailey Associate Professor, Mathematics Chair, General Science program School of Science and the Environment Grenfell Campus Memorial University of Newfoundland Corner Brook, NL A2H 6P9, Canada Office: AS 3022 Phone: +1 (709) 637-6293 Web: http://www2.grenfell.mun.ca/rbailey/ -----Original Message----- From: Butske, Bill <but...@rose-hulman.edu> Sent: 07 December 2021 21:01 To: forum@gap-system.org Subject: [GAP Forum] Grape UndirectedEdges Question Dear All, I'm having a bit of trouble (at least in my mind) with GRAPE. I'm interested in subgraphs of the following graph and its associated symmetry group which I define through the adjacency matrix as follows: gap>A:=[ [ 0, 1, 1, 1, 1, 0 ], [ 1, 0, 1, 0, 1, 1 ], [ 1, 1, 0, 1, 0, 1 ], [ 1, 0, 1, 0, 1, 1 ], [ 1, 1, 0, 1, 0, 1,], [ 0, 1, 1, 1, 1, 0 ] ];; gap>G:=Group((2,3,4,5),(1,2,6,4));: gap>LoadPackage("grape");; gap>gamma:=Graph(G, [1..6], OnPoints, function(x,y) return A[x][y]=1; end, true );; gap> UndirectedEdges(gamma); [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 1, 5 ], [ 2, 3 ], [ 2, 5 ], [ 2, 6 ], [ 3, 4 ], [ 3, 6 ], [ 4, 5 ], [ 4, 6 ], [ 5, 6 ] ] So far so good. Then I define the induced subgraph determined by the vertices 1,2,6 gap>gamma_1_2_6:=InducedSubgraph(gamma,[1,2,6]);; This should, in my mind anyway, give me the subgraph with the edges [1,2] and [2,6]. However, when I check this with the UndirectedEdges command I get: gap> UndirectedEdges(InducedSubgraph(gamma,[1,2,6])); [ [ 1, 2 ], [ 2, 3 ] ] This bothers me because 3 isn't a specified vertex, furthermore the UndirectedEdges command appears to be order dependent? gap> UndirectedEdges(InducedSubgraph(gamma,[1,6,2])); [ [ 1, 3 ], [ 2, 3 ] ] My expectation is that either version of the command should return [[1,2],[2,6]] and I am very much confused as to why this isn't case. Any insights are appreciated and thanks in advance, Bill Butske _______________________________________________ Forum mailing list Forum@gap-system.org https://mail.gap-system.org/mailman/listinfo/forum _______________________________________________ Forum mailing list Forum@gap-system.org https://mail.gap-system.org/mailman/listinfo/forum
