Sunday, December 31, 2017

calculus - Are there any known methods to compute series suminftyk=02kbig(suminftyn=0frac(1)n(n2k+1+(2k+1))3big)?



I would like to ask if there are any known methods to compute series like this one ?
k=02k(n=0(1)n(n2k+1+(2k+1))3)


And their names so i can look for them if they exist.
I never studied double sums before that's why i am asking, thanks in advance.



Answer



We have



k=02kn=0(1)n(n2k+1+2k+1)3=k=02k(2k+1)3k=0(122k+3n=1(1)n1(n+2k+12k+1)3)



and



n=1(1)n1(n+a)3=18(ζ(3,12+a2)ζ(3,1+a2)) . (see: Hurwitz zeta function)



It follows




k=0122k+3n=1(1)n1(n+2k+12k+1)3=k=0122k+6(ζ(3,12+2k+12k+2)ζ(3,1+2k+12k+2))



0.05957499...



k=02k(2k+1)3 is divergent because (2k(2k+1)3)k is not a null sequence and



k=0(122k+3n=1(1)n1(n+2k+12k+1)3) is convergent therefore the initial series is divergent.


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