Comment #16 on issue 3128 by [email protected]: Sum and Product
manipulations
http://code.google.com/p/sympy/issues/detail?id=3128
When the same variable is used in more than one limit, generally reordering
will
change the summation. For example Sum(x, (x, 1, 3), (x, 1, 9)) evaluates to
54 while
Sum(x, (x, 1, 9), (x, 1, 3)) evaluates to 135. So, should we skip the
reordering of
Sum/Products where the same variable is repeated in limits or return the
same
Sum/Product without any changes if repetition is present?
I searched in mathematica and few other CASs and it seems like they do not
provide
reordering of variables in Sum. Actually they give a computed result for
Sum as
opposed to SymPy doing it on user request. So reordering of a variable
doesn't really
apply to them.
If we skip changing orders when repetition is present, I think there will
be no harm
in using the interface I proposed. Comments will be appreciated.
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