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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
 7, 21, 28, 42, 42, 84, 56, 84, 84, 126, 84, 168, 98, 168, 168, 168, 126, 252, 140, 252, 224, 252, 168, 336, 210, 294, 252, 336, 210, 504, 224, 336, 336, 378, 336, 504, 266, 420, 392, 504, 294, 672, 308, 504, 504, 504, 336, 672, 392, 630, 504, 588, 378, 756 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Antti Karttunen, Table of n, a(n) for n = 1..16384 J. H. Kwak and J. Lee, Enumeration of graph coverings, surface branched coverings and related group theory, in Combinatorial and Computational Mathematics (Pohang, 2000), ed. S. Hong et al., World Scientific, Singapore 2001, pp. 97-161. See p. 134. MATHEMATICA With[{b = 3}, Table[((2^b - 1)/EulerPhi[n]) DivisorSum[n, MoebiusMu[n/#] #^(b - 1) &], {n, 54}]] (* Michael De Vlieger, Nov 23 2017 *) PROG (PARI) A160890(n) = ((7/eulerphi(n))*sumdiv(n, d, moebius(n/d)*(d^2))); \\ Antti Karttunen, Nov 23 2017 CROSSREFS Sequence in context: A219036 A063469 A155131 * A319527 A297178 A325553 Adjacent sequences:  A160887 A160888 A160889 * A160891 A160892 A160893 KEYWORD nonn AUTHOR N. J. A. Sloane, Nov 19 2009 STATUS approved

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Last modified March 31 19:37 EDT 2020. Contains 333151 sequences. (Running on oeis4.)