%I #9 Dec 05 2017 04:07:25

%S 1,2,3,6,10,16,25,38,57,84,121,172,243,338,465,636,862,1158,1546,2050,

%T 2701,3540,4613,5980,7719,9916,12682,16158,20506,25926,32667,41022,

%U 51348,64080,79730,98922,122407,151068,185968,228384,279816,342052

%N Expansion of q^(-3/4) * eta(q^2)^2 * eta(q^20) / (eta(q)^2 * eta(q^4)) in powers of q.

%H G. C. Greubel, <a href="/A146163/b146163.txt">Table of n, a(n) for n = 0..1000</a>

%F Euler transform of period 20 sequence [ 2, 0, 2, 1, 2, 0, 2, 1, 2, 0, 2, 1, 2, 0, 2, 1, 2, 0, 2, 0, ...].

%F a(n) ~ exp(2*Pi*sqrt(n/5) / (4*5^(3/4)*n^(3/4)). - _Vaclav Kotesovec_, Jul 11 2016

%F a(n) = A146162(4*n + 3).

%e q^3 + 2*q^7 + 3*q^11 + 6*q^15 + 10*q^19 + 16*q^23 + 25*q^27 + 38*q^31 + ...

%t nmax = 50; CoefficientList[Series[Product[(1+x^k)^2 * (1-x^(20*k)) / (1-x^(4*k)), {k, 1, nmax}], {x, 0, nmax}], x] (* _Vaclav Kotesovec_, Jul 11 2016 *)

%t a[n_]:= SeriesCoefficient[QPochhammer[-q, q]^2*QPochhammer[q^20, q^20]/(QPochhammer[q^4, q^4]), {q, 0, n}]; Table[a[n], {n, 0, 50}] (* _G. C. Greubel_, Dec 05 2017 *)

%o (PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x^2 + A)^2 * eta(x^20 + A) / (eta(x + A)^2 * eta(x^4 + A)), n))}

%K nonn

%O 0,2

%A _Michael Somos_, Oct 27 2008