Updates:
Cc: wdjoy...@gmail.com amake...@college.harvard.edu
mario.pe...@gmail.com
Labels: Combinatorics
Comment #1 on issue 3323 by asmeu...@gmail.com: perm_groups alt_or_sym
failure
http://code.google.com/p/sympy/issues/detail?id=3323
This is caused by a failure of the random algorithm is_alt_sym() to
determine that the group is symmetric. Some issues here:
- The correct return value should be None, not False. (that's unrelated to
this failure, though)
- Apparently we need to print the random seed at the top of the doctests as
well.
- The docstring of this function states that it should be called only after
is_alt_sym() returns True. One possible fix would be to remove this check
from alt_or_sym(), and simply state (clearly!) in the docstring that if the
group is not known to be alternating or symmetric, then you will get
meaningless output. This would have two advantages: one, there would be no
unnecessary repeat call of is_alt_sym(), which the user is told to call
before alt_or_sym() anyway. Two, it would fix this doctest, because
AlternatingGroup(20) *is* alternating, so it will always return 'A',
regardless of the random seed.
The downside of course is that if someone does not read the docs and calls
alt_or_sym() on a group that is neither, they will get an essentially wrong
result.
- Whatever we do, the docstring should be updated accordingly.
Aleksandar, David, Mario, and any other interested parties, what do you
think? I didn't look to see how this might apply to the other randomized
algorithms in the combinatorics module.
By the way, David and Aleksandar, I've added your emails here on Google
code so I could CC you. Let me know if you want me to use a different
email.
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