Thanks Dima for the suggestion. I'm not sure that this does what I need: the associated directed graph has each edge directed, which is not what I was needing, and the faces of the directed graph look identical to those of the original -- in particular, the edges of each face are tuples (x,y) where both (x,y) and (y,x) appear on different faces (as I need to always hold) regardless of the orientation of the edges.
I will keep on experimenting. My polyhedra are small and few enough that check the orientations of each is not very time-consuming. John On Monday, June 19, 2023 at 12:08:18 PM UTC+1 Dima Pasechnik wrote: > On Mon, Jun 19, 2023 at 11:18 AM John Cremona <john.c...@gmail.com> wrote: > > > > I have some quite small graphs which are polyhedral, each is the > 1-skeleton of a (connected convex) polyhedron such as a cube, tetrahedron, > etc, constructed from a list of edge pairs. > > > > I can get the faces of one of these, say G, via G.faces(). This returns > a list of lists of vertices, each one being a list of the vertices of one > face in some cyclic order. So each face in the list has an implied > orientation. OK so far. > > > > What I want is for the orientations of the different faces to be > coherent, coming from a single orientation of the surface. Expliticly that > means that each edge xy, which will appear in exactly two faces, appears > once in each direction, i.e. x before y (cyclically) in one and the other > way round in the other. > > > > It is possible that G.faces() is already returning such a globally > oriented list of faces, as experimentation suggests, but I need to be > certain -- otherwise I can write code to check and if necessary reverse > some faces. I would prefer not to have to though. > > > > The docstrong of G.faces() does not make this clear (at least not to me). > > You can call G.strong_orientation() > to get an orientation of the edge of your graph, and use it. > > HTH > Dima > > > > > -- > > 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...@googlegroups.com. > > To view this discussion on the web visit > https://groups.google.com/d/msgid/sage-support/CAD0p0K5e9ch3tgrM2RAHxQieVOqG%2BEX37fyWj7BwF0ns-NTPpA%40mail.gmail.com > . > -- 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/08ace3c1-1e96-427a-97a1-53605ca42742n%40googlegroups.com.
