Monday, 20 April 2015

sequences and series - What's the closed form of this :sum+inftyn=1frac(1)nphi(n)n



I have checked some links related the below sum which is related to The Euler totient function to check if it has any known closed form but i don't find anything then my question here is :





Question:
What is the closed form of this :+n=1(1)nϕ(n)n , where ϕ(n) is Euler totient function ?





Answer



As stated by reuns in the comments, for any s with a large enough real part we have



n1φ(n)ns=p(1+φ(p)ps+φ(p2)p2s+φ(p3)p3s+)=pps1psp
by Euler's product, hence
n1φ(n)ns=p11ps11ps1=ζ(s1)ζ(s)
n1n oddφ(n)ns=p>211ps11ps1=ζ(s1)ζ(s)2s22s1
n1(1)nφ(n)ns=ζ(s1)ζ(s)(122s22s1)=ζ(s1)ζ(s)2s32s1
but the series in the LHS is convergent only for Re(s)>2.



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