#13225: Check if lattices are upper/lower-semimodular and modular
--------------------------------------------------------+-------------------
Reporter: csar | Owner:
sage-combinat
Type: enhancement | Status:
needs_review
Priority: major | Milestone:
sage-5.6
Component: combinatorics | Resolution:
Keywords: sd40 | Work issues:
Report Upstream: N/A | Reviewers:
Authors: Alex Csar, Kevin Dilks, Sangwook Kim | Merged in:
Dependencies: | Stopgaps:
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Comment (by csar):
The issue in hasse_diagram was that taking the dual of the lattice somehow
created a situation where the dual poset would attempt to take a join, but
joins wouldn't be defined.
For example,
{{{
L = LatticePoset({0:[1,2,3],1:[4],2:[4],3:[4]})
P=L.dual()
x=P.random_element()
y=P.random_element()
P.join(x,y)
}}}
produces
{{{
/Applications/sage-5.3/local/lib/python2.7/site-
packages/sage/combinat/posets/lattices.pyc in join(self, x, y)
268 """
269 i, j = map(self._element_to_vertex,(x,y))
--> 270 return
self._vertex_to_element(self._hasse_diagram._join[i,j])
271
272
####################################################################################
AttributeError: 'HasseDiagram' object has no attribute '_join'
}}}
I must admit I no longer see why the code currently in sage fails to
create {{{ _join }}} when it's needed. (I realise I produced the example
using 5.3, not 5.4. I don't have 5.4 to hand at the moment.)
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13225#comment:5>
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