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A160890 a(n) = ((2^b-1)/phi(n))*Sum_{d|n} Moebius(n/d)*d^(b-1) for b = 3. 1

%I

%S 7,21,28,42,42,84,56,84,84,126,84,168,98,168,168,168,126,252,140,252,

%T 224,252,168,336,210,294,252,336,210,504,224,336,336,378,336,504,266,

%U 420,392,504,294,672,308,504,504,504,336,672,392,630,504,588,378,756

%N a(n) = ((2^b-1)/phi(n))*Sum_{d|n} Moebius(n/d)*d^(b-1) for b = 3.

%H Antti Karttunen, <a href="/A160890/b160890.txt">Table of n, a(n) for n = 1..16384</a>

%H J. H. Kwak and J. Lee, <a href="https://doi.org/10.1142/9789812799890_0005">Enumeration of graph coverings, surface branched coverings and related group theory</a>, in Combinatorial and Computational Mathematics (Pohang, 2000), ed. S. Hong et al., World Scientific, Singapore 2001, pp. 97-161. See p. 134.

%t With[{b = 3}, Table[((2^b - 1)/EulerPhi[n]) DivisorSum[n, MoebiusMu[n/#] #^(b - 1) &], {n, 54}]] (* _Michael De Vlieger_, Nov 23 2017 *)

%o (PARI) A160890(n) = ((7/eulerphi(n))*sumdiv(n,d,moebius(n/d)*(d^2))); \\ _Antti Karttunen_, Nov 23 2017

%K nonn

%O 1,1

%A _N. J. A. Sloane_, Nov 19 2009

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Last modified May 31 12:48 EDT 2020. Contains 334748 sequences. (Running on oeis4.)