Comment #10 on issue 3175 by anders.kaseorg: Sum with reversed limits gives
wrong answer
http://code.google.com/p/sympy/issues/detail?id=3175
The third option is written incorrectly; it should be:
- summation(n, (n, 4, 0)) should give the same thing as -summation(n, (n,
1, 3)).
(Briefly, this is what naturally falls out of the identity summation(f(n),
(n, a, b)) = f(a) + summation(f(n), (n, a+1, b)), and it entails the empty
sum summation(f(n), (n, 1, 0)) = 0 as corollary. I have a more
comprehensive justification and citations on the pull request.)
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