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 A319455 Expansion of Product_{k>=1} 1/((1 - x^k)*(1 - x^(2*k)))^2. 1
 1, 2, 7, 14, 35, 66, 140, 252, 485, 840, 1512, 2534, 4347, 7084, 11705, 18622, 29862, 46522, 72779, 111310, 170534, 256586, 386101, 572488, 848050, 1240974, 1812979, 2621486, 3782669, 5410360, 7720237, 10932740, 15443120, 21669546, 30327570, 42196022, 58555543, 80832850 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Convolution inverse of A002171. Self-convolution of A002513. Convolution of A000041 and A029862. Euler transform of period 2 sequence [2, 4, ...]. LINKS FORMULA G.f.: Product_{k>=1} (1 + x^k)^2/(1 - x^(2*k))^4. G.f.: exp(2*Sum_{k>=1} (4*sigma(k) - sigma(2*k))*x^k/k). MAPLE a:=series(mul(1/((1-x^k)*(1-x^(2*k)))^2, k=1..55), x=0, 38): seq(coeff(a, x, n), n=0..37); # Paolo P. Lava, Apr 02 2019 MATHEMATICA nmax = 37; CoefficientList[Series[Product[1/((1 - x^k)*(1 - x^(2*k)))^2, {k, 1, nmax}], {x, 0, nmax}], x] nmax = 37; CoefficientList[Series[1/(QPochhammer[x] QPochhammer[x^2])^2, {x, 0, nmax}], x] nmax = 37; CoefficientList[Series[Exp[2 Sum[(4 DivisorSigma[1, k] - DivisorSigma[1, 2 k]) x^k/k, {k, 1, nmax}]], {x, 0, nmax}], x] PROG (PARI) seq(n)={Vec(exp(2*sum(k=1, n, (4*sigma(k) - sigma(2*k))*x^k/k) + O(x*x^n)))} \\ Andrew Howroyd, Sep 19 2018 CROSSREFS Cf. A000041, A001934, A001936, A002171, A002513, A029862. Sequence in context: A000147 A128902 A227213 * A060552 A274868 A191396 Adjacent sequences:  A319452 A319453 A319454 * A319456 A319457 A319458 KEYWORD nonn AUTHOR Ilya Gutkovskiy, Sep 19 2018 STATUS approved

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Last modified January 28 13:13 EST 2020. Contains 331321 sequences. (Running on oeis4.)