# Issue with symbolic summation [duplicate]

Here is the sum:

$$\sum_{j=0}^{n-1}r^{a+b}\cos^a\left(\frac{2\pi j}{n}+t\right)\sin^b\left(\frac{2\pi j}{n}+t\right)$$

Code:

F[n_, a_, b_, r_, t_] :=
Sum[r^(a + b)*Cos[2*Pi/n*j + t]^a*Sin[2*Pi/n*j + t]^b, {j, 0, n - 1}]


I do not expect it to have a simple formula for general n, but Mathematica gives this one:

F[n, 2, 2, r, t]

(n r^4)/8


The issue here is that the generic formula is incorrect. For example, the correct expression for n=4:

F[4, 2, 2, r, t]

4 r^4 Cos[t]^2 Sin[t]^2


Why is an obviously wrong result generated in symbolic summation? Is it a bug or feature?

Would appreciate any help.

• It looks to be a generically correct result: With[{a = 2, b = 2}, Table[FullSimplify[Sum[r^(a + b) Cos[2 π/n j + t]^a Sin[2 π/n j + t]^b, {j, 0, n - 1}]] == n r^4/8, {n, 20}]] – J. M.'s technical difficulties Jul 28 '16 at 4:05
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• I am pretty sure that Sum[...] in LHS was reduced to incorrect formula first. So that this example just proves the issue. – Pavel Holoborodko Jul 28 '16 at 6:53
• It's a correct result, except possibly on a set of zero measure. That's the meaning of "generically correct". Your case of interest happens to be the case where the formula does not work. – J. M.'s technical difficulties Jul 28 '16 at 6:58
• Actually it doesn't work in all cases when a and b are both even. – Pavel Holoborodko Jul 28 '16 at 6:59

I believe this will cast some light on the problem.

F[n_, a_, b_, r_, t_] :=
r^(a + b)*
Sum[Cos[2*Pi/n*j + t]^a*Sin[2*Pi/n*j + t]^b, {j, 0, n - 1},
GenerateConditions -> True]

F[n, 2, 2, r, t]


ConditionalExpression[(n r^4)/8, n ∈ Integers && 4/n ∉ Integers && n >= 1]

This tells us the general case (n r^4)/8 won't be valid for n = 4 as indeed it is not. In fact, the general expression fails for n = 1, 2 & 4 and is valid for all other positive integers.

• Absolutely, thank you! – Pavel Holoborodko Jul 28 '16 at 7:09