Hi folks,
I believe the following is a bug in the implementation of
incidence_matrix() in the module sage/graphs/generic_graph.py. Say I
define an undirected graph as follows:
[mv...@sage sage-4.3.3]$ ./sage
----------------------------------------------------------------------
| Sage Version 4.3.3, Release Date: 2010-02-21 |
| Type notebook() for the GUI, and license() for information. |
----------------------------------------------------------------------
sage: G = Graph({1: [4], 2: [4], 3: [4], 4: [1, 2, 3]})
sage: G.is_directed()
False
By definition of incidence matrix (see [1,2] or a text on graph
theory), the following incidence matrix for G should not have -1 in
any entries at all.
sage: G.incidence_matrix()
[-1 0 0]
[ 0 -1 0]
[ 0 0 -1]
[ 1 1 1]
Can someone confirm this?
[1] http://en.wikipedia.org/wiki/Incidence_matrix
[2] http://mathworld.wolfram.com/IncidenceMatrix.html
--
Regards
Minh Van Nguyen
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