Bug

Robert Israel — Mon, 07 Feb 2011 03:49:51 Z

The bug seems to be in UnderlyingGraph.

> with(GraphTheory):
G:= Digraph({[1,2],[2,4],[4,8]});

G := `Graph 1: a directed unweighted graph with 4 vertices and 3 arc(s)`

> U:= UnderlyingGraph(G);

U := `Graph 2: an undirected unweighted graph with 4 vertices and 2 edge(s)`

The arc [4,8] of the digraph does not produce an edge in the underlying graph.
This is triggered by the fact that the name of the third vertex is 4. It would not happen with Digraph({[1,2],[2,four],[four,8]}). The problem is a confusion between names of vertices and numbers of vertices: in line 9 of UnderlyingGraph

if V[i] <> j then

V[i] is the name of a vertex, and j is the number of a vertex.

I'm submitting an SCR.

A work-around is to use names rather than numbers to indicate the vertices.

thanks

djordan99 — Mon, 07 Feb 2011 04:13:02 Z

Thank you.

MaplePrimes - answers and comments on Question, Why is this graph not a network?

Bug

thanks