/* this disappears when adding a Integer() cast
sage: n = 20 sage: k = 3 sage: g = DiGraph() sage: g.add_edges( (Integer(i),Integer(Mod(i+j,n))) for i in range(n) for j in range(1,k+1) ) sage: k == g.edge_connectivity() True */ Nathann On 25 July 2010 15:27, Nathann Cohen <nathann.co...@gmail.com> wrote: > Hello everybody !!! > > I was trying to write the following doctest, and noticed something scary : > > We build a directed circulant graph on n vertices by linking the i th > vertex to i+1, i+2, ... , i+k, thus ensuring our graph is k-connected. > Then, by Edmond's theorem, we know this graph has `k` edge-disjoint > spanning arborescences > > sage: n = 20 > sage: k = 3 > sage: g = DiGraph() > sage: g.add_edges( (i,Mod(i+j,n)) for i in range(n) for j in range(1,k+1) ) > sage: k == g.edge_connectivity() > False > > This should be k, but it is not. Not *that* bad. But it gets worse : > > sage: g.strongly_connected_components() > [[0], [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]] > > Two different "0" ? Then I thought it was because of the Mod, and > maybe some zeroes where regular ones, while other were zeroes of > Z/20Z.... But then : > > sage: g > Digraph on 21 vertices > sage: len(g.vertices()) > 20 > sage: g.vertices() > [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] > > That's were I got really scared :-D > > Robert ? Could you please tell me "oh yeah, I know where it comes > from, that's just a typo" ? :-D > > Thankssssssssssss !!! > > Nathann > -- To post to this group, send an email to sage-devel@googlegroups.com To unsubscribe from this group, send an email to sage-devel+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/sage-devel URL: http://www.sagemath.org
