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
∑n≥1φ(n)ns=∏p(1+φ(p)ps+φ(p2)p2s+φ(p3)p3s+…)=∏pps−1ps−p
by Euler's product, hence
∑n≥1φ(n)ns=∏p1−1ps1−1ps−1=ζ(s−1)ζ(s)
∑n≥1n oddφ(n)ns=∏p>21−1ps1−1ps−1=ζ(s−1)ζ(s)⋅2s−22s−1
∑n≥1(−1)nφ(n)ns=ζ(s−1)ζ(s)(1−2⋅2s−22s−1)=−ζ(s−1)ζ(s)⋅2s−32s−1
but the series in the LHS is convergent only for Re(s)>2.
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