Hello group,
I think this is probably an embarrassing bug:
def IncidenceStructureFromMatrix(M, name=None):
"""
Builds and incidence structure from a matrix.
INPUT:
- ``M`` -- a binary matrix. Creates a set of "points" from the rows and a
set of "blocks" from the columns.
EXAMPLES::
sage: from sage.combinat.designs.block_design import BlockDesign
sage: BD1 =
BlockDesign(7,[[0,1,2],[0,3,4],[0,5,6],[1,3,5],[1,4,6],[2,3,6],[2,4,5]])
sage: M = BD1.incidence_matrix()
sage: BD2 = IncidenceStructureFromMatrix(M)
sage: BD1 == BD2
True
"""
nm = name
v = len(M.rows())
b = len(M.columns())
#points = range(v)
blocks = []
for i in range(b):
B = []
for j in range(v):
if M[i, j] != 0:
B.append(j)
blocks.append(B)
return IncidenceStructure(range(v), blocks, name=nm)
The problem is with the indexing "if M[i, j] != 0". Of course, this will not be
a problem for square 2-designs like the Hadamard design or
ProjectivePlaneDesign (thanks to Fisher Type counting), but this should cause
problem for designs that are not square. I was bitten by this bug when I tried
to implement Hadamard 3-design from a Hadamard matrix.
I will open a ticket if somebody does a sanity check for me.
With Sincere Regards,
Kannappan.
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