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%I #19 Mar 12 2021 22:24:44
%S 1,-1,0,-1,0,-1,2,0,1,0,0,0,0,1,0,-2,-1,0,0,0,0,-1,0,0,0,2,0,0,0,0,0,
%T 2,0,-1,0,2,0,0,-2,0,-1,0,0,0,0,0,-2,0,0,0,0,0,0,1,0,0,-1,0,0,0,0,2,0,
%U 2,0,1,0,0,0,0,-2,0,0,0,0,0,0,0,2,0,-2,0,0,-1,0,-1,0,0,-2,0,0,0,0,0,0,0,1,0,0,0,0,2,0,0,0
%N Expansion of q^(-1/3)*eta(q)*eta(q^4)*eta(q^5)/eta(q^2) in powers of q.
%C Ramanujan theta functions: f(q) := Product_{k>=1} (1-(-q)^k) (see A121373), phi(q) := theta_3(q) := Sum_{k=-oo..oo} q^(k^2) (A000122), psi(q) := Sum_{k=0..oo} q^(k*(k+1)/2) (A010054), chi(q) := Product_{k>=0} (1+q^(2k+1)) (A000700).
%H G. C. Greubel, <a href="/A123758/b123758.txt">Table of n, a(n) for n = 0..1000</a>
%H Michael Somos, <a href="/A010815/a010815.txt">Introduction to Ramanujan theta functions</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/RamanujanThetaFunctions.html">Ramanujan Theta Functions</a>
%F Expansion of f(-q^5)*psi(-q) in powers of q where f(),psi() are Ramanujan theta functions.
%F Euler transform of period 20 sequence [ -1, 0, -1, -1, -2, 0, -1, -1, -1, -1, -1, -1, -1, 0, -2, -1, -1, 0, -1, -2, ...].
%F Product_{k>0} (1-x^k)*(1+x^(2k))*(1-x^(5k)).
%F a(8n+2) = a(8n+4) = 0.
%t eta[q_] := q^(1/24)*QPochhammer[q]; CoefficientList[Series[q^(-1/3) eta[q] eta[q^4] eta[q^5]/eta[q^2], {q, 0, 100}], q] (* _G. C. Greubel_, Apr 19 2018 *)
%o (PARI) {a(n)=local(A); if(n<0, 0, A=x*O(x^n); polcoeff( eta(x+A)*eta(x^4+A)*eta(x^5+A)/eta(x^2+A), n))}
%o (PARI) {a(n) = local(s, k); if(n<0, 0, n=24*n+8; for(j=1, sqrtint(n\5), if((j^2%6==1)& issquare( (n-5*j^2)/3, &k)& (k%2), s+= (-1)^((j+1)\6+ (k+2)\4))); s)}
%K sign
%O 0,7
%A _Michael Somos_, Oct 12 2006
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