Hello everybody,
I have been playing with groups recently, and fought very hard with my code
before I noticed the usual bug:
sage: a == b
True
sage: hash(a) == hash(b)
False
Which has (among others) the following consequence:
sage: G = groups.presentation.Cyclic(4)
sage: G.cayley_graph().vertices()
[1, a, a^2, a^-2, a^3, a^-1]
To me, the problem comes from the "default" implementation of __hash__ that
can
be found in sage.structure.element:
def __hash__(self):
return hash(str(self))
Of course, with such a definition it is very likely that "__hash__" and
"__eq__"
will never agree. As I expect that removing this dangerous default
implementation may break a *lot* of things, I created a ticket
(needs_review)
that replaces it with "return 0".
http://trac.sagemath.org/ticket/19016
Nathann
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