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%I #27 Sep 08 2022 08:45:38
%S 1,4,26,148,843,4632,25124,133784,703553,3655340,18800886,95819580,
%T 484416675,2431094352,12120072472,60058765072,295959923287,
%U 1450980481036,7079894939166,34393241899772,166390593502701,801877654792696,3850469199935412,18426281811165880
%N Euler transform of powers of 4.
%H Alois P. Heinz, <a href="/A144068/b144068.txt">Table of n, a(n) for n = 0..1000</a>
%H Vaclav Kotesovec, <a href="https://arxiv.org/abs/1509.08708">A method of finding the asymptotics of q-series based on the convolution of generating functions</a>, arXiv:1509.08708 [math.CO], Sep 30 2015, p. 27.
%H N. J. A. Sloane, <a href="/transforms.txt"> Transforms</a>
%F G.f.: Product_{j>0} 1/(1-x^j)^(4^j).
%F a(n) ~ 4^n * exp(2*sqrt(n) - 1/2 + c) / (2 * sqrt(Pi) * n^(3/4)), where c = Sum_{m>=2} 1/(m*(4^(m-1)-1)) = 0.1938490811676466793200632998157568919969827... . - _Vaclav Kotesovec_, Mar 14 2015
%F G.f.: exp(4*Sum_{k>=1} x^k/(k*(1 - 4*x^k))). - _Ilya Gutkovskiy_, Nov 09 2018
%p with(numtheory): etr:= proc(p) local b; b:=proc(n) option remember; `if`(n=0, 1, add(add(d*p(d), d=divisors(j)) *b(n-j), j=1..n)/n) end end: a:=n-> etr(j->4^j)(n): seq(a(n), n=0..40);
%t etr[p_] := Module[{b}, b[n_] := b[n] = If[n == 0, 1, Sum[Sum[d*p[d], {d, Divisors[j]}]*b[n-j], {j, 1, n}]/n]; b]; a[n_] := etr[Function[4^#]][n]; Table[ a[n], {n, 0, 40}] (* _Jean-François Alcover_, Mar 09 2015, after _Alois P. Heinz_ *)
%t CoefficientList[Series[Product[1/(1-x^k)^(4^k), {k, 1, 30}], {x, 0, 30}], x] (* _G. C. Greubel_, Nov 09 2018 *)
%o (PARI) m=30; x='x+O('x^m); Vec(prod(k=1,m,1/(1-x^k)^(4^k))) \\ _G. C. Greubel_, Nov 09 2018
%o (Magma) m:=30; R<x>:=PowerSeriesRing(Integers(), m); Coefficients(R!( (&*[1/(1-x^k)^(4^k): k in [1..m]]) )); // _G. C. Greubel_, Nov 09 2018
%Y 4th column of A144074.
%K nonn
%O 0,2
%A _Alois P. Heinz_, Sep 09 2008
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