Tuesday, 7 January 2020

sequences and series - Concerning the sum sumin=1nftysinnx



I recently came across this question and I posted an answer. It has been pointed out that my answer is incorrect. I cannot work out what is wrong with my reasoning. The answer I gave corresponds with the Abel and Cesaro sum, so perhaps is not the usual summation operator? Am I correct in asserting that if x is in the upper half-plane, i.e., I[x]>0, then |eix|<1 and consequently
n=1einx=eix1eix,
or is my argument flawed? Any help would be appreciated.



Answer



Because you assume that x is not real, the imaginary part of einx is not sinnx.



Also, when computing the conjugate if 1eix, you don't get 1eix when x is non-real, but rather 1eiˉx


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