Thursday, 15 September 2016

real analysis - On a limit involving sumin=1nftyfracsin(sqrtn3+1x)sqrtn3+1

Combining the identity (1) from [1], I am saying the specialization an=n3+1 and bn=i (here i denotes the imaginary unit, thus i2=1) for integers n1, and the explanation of the criterion of Fubini's theorem, see [2] if you need it, I can prove that ζ(3)=0n=1sin(n3+1x)n3+1eixdx.
I believe that such reasoning and calculation is right since our functions fn(x)=sin(n3+1x)n3+1eix satisfy for each n1 that |fn(x)|1|eix|n3+1=1n3+1, and we conclude using the comparison test for series.




Question. I was wondering about questions involving this function f(x):=n=1sin(n3+1x)n3+1 defined on [0,) that I know how solve or I don't know how solve those.




I know that using the Weierstrass M-test I can to prove that f(x) is continuous on [0,), but how to prove that there no exists (as I suspect) lim



Many thanks.




Feel free, if you prefer, add hints for some of previous question, instead of a full answer.





[1] See the answer by D'Aurizio for Cantarini's lemma, identity (1) from: Find the closed form for \int_{0}^{\infty}\cos{x}\ln\left({1+e^{-x}\over 1-e^{-x}}\right)dx=\sum_{n=0}^{\infty}{1\over n^2+(n+1)^2}.




[2] See the second paragraph of the answer by Eldredge: When can a sum and integral be interchanged?

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