Illustration in this ask.sagemath.org question <https://ask.sagemath.org/question/79427/the-acyclic-orientations-function-behaves-unexpectedly/> .
HTH, Le jeudi 3 octobre 2024 à 13:06:03 UTC+2, Gordon a écrit : > This is not exactly a bug, but definitely a "gotcha" which will catch many > people unawares but could easily be fixed. > > It is related to the two functions "orientations" and > "acyclic_orientations" which are meant to return iterators to all > orientations and to the acyclic orientations respectively. > > The confusion arises from the fact that these two functions return their > results in different ways, one of which is intuitive, but the other very > counterintuitive. > > One way of viewing an "orientation" is as an undirected graph with some > indication on each edge (u,v) as to whether it is directed from u to v, or > v to u. > > Another way of viewing an "orientation" is that it is a directed graph, > where each edge has been replaced either with the arc u->v or v->u. > > The command "g.orientations()" returns an iterator which produces a > sequence of directed graphs, each being a different orientation of the > original graph. This is natural and unambiguous. > > The command "g.acyclic_orientations()" also returns an iterator that > produces a sequence of directed graphs, but it just produces the SAME > directed graph multiple times. The information about the orientation is > hidden in labels on the edges, so that (u,v,0) means an arc from u to v and > (u,v,1) means an arc from v to u. > > So "acyclic_orientations" is mixing up the two representations - it is > returning digraphs, but also labelling the edges. > > My suggestion is that the output from "acyclic_orientations" should be > changed so that it returns digraphs with unlabelled edges. > > In this fashion, the orientation information is contained directly in the > digraph and the output is then consistent with "orientations()". > > Thanks > > Gordon > > > > > -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/sage-support/f26573cc-d62a-4811-b4bb-54f6c4c6fca5n%40googlegroups.com.
