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 n≥1, and the explanation of the criterion of Fubini's theorem, see [2] if you need it, I can prove that ζ(3)=∫∞0∞∑n=1sin(√n3+1x)√n3+1e−ixdx.
I believe that such reasoning and calculation is right since our functions fn(x)=sin(√n3+1x)√n3+1e−ix satisfy for each n≥1 that |fn(x)|≤1⋅|e−ix|√n3+1=1√n3+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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