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 A309729 Expansion of Sum_{k>=1} x^k/(1 - x^k - 2*x^(2*k)). 1
 1, 2, 4, 7, 12, 26, 44, 92, 175, 354, 684, 1396, 2732, 5506, 10938, 21937, 43692, 87578, 174764, 349884, 699098, 1398786, 2796204, 5593886, 11184823, 22372354, 44739418, 89483996, 178956972, 357925242, 715827884, 1431677702, 2863312218, 5726666754, 11453246178, 22906581193 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Inverse Moebius transform of Jacobsthal numbers (A001045). LINKS FORMULA G.f.: Sum_{k>=1} A001045(k) * x^k/(1 - x^k). a(n) = (1/3) * Sum_{d|n} (2^d - (-1)^d). MAPLE seq(add(2^d-(-1)^d, d=numtheory:-divisors(n))/3, n=1..50); # Robert Israel, Aug 14 2019 MATHEMATICA nmax = 36; CoefficientList[Series[Sum[x^k/(1 - x^k - 2 x^(2 k)), {k, 1, nmax}], {x, 0, nmax}], x] // Rest Table[(1/3) Sum[(2^d - (-1)^d), {d, Divisors[n]}], {n, 1, 36}] PROG (PARI) a(n)={sumdiv(n, d, 2^d - (-1)^d)/3} \\ Andrew Howroyd, Aug 14 2019 (Python) n = 1 while n <= 36:     s, d = 0, 1     while d <= n:         if n%d == 0:             s = s+2**d-(-1)**d         d = d+1     print(n, s//3) n = n+1 # A.H.M. Smeets, Aug 14 2019 CROSSREFS Cf. A001045, A007435, A055895, A100107, A104723, A256281. Sequence in context: A332338 A332836 A328129 * A027945 A079800 A217595 Adjacent sequences:  A309726 A309727 A309728 * A309730 A309731 A309732 KEYWORD nonn AUTHOR Ilya Gutkovskiy, Aug 14 2019 STATUS approved

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Last modified August 11 15:12 EDT 2020. Contains 336428 sequences. (Running on oeis4.)