Comment #8 on issue 3175 by [email protected]: Sum with reversed limits
gives wrong answer
http://code.google.com/p/sympy/issues/detail?id=3175
There is debate over what these "reversed" sums should return (see the pull
request). There are four options:
- summation(n, (n, 4, 0)) should give 0.
- summation(n, (n, 4, 0)) should give the same thing as summation(n, (n, 0,
4)).
- summation(n, (n, 4, 0)) should give the same thing as -summation(n, (n,
0, 4)).
- summation(n, (n, 4, 0)) should raise ValueError.
One thing is clear, though: doit() and evalf() should agree.
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