OFFSET
0,7
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of f(-q^5)*psi(-q) in powers of q where f(),psi() are Ramanujan theta functions.
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, ...].
Product_{k>0} (1-x^k)*(1+x^(2k))*(1-x^(5k)).
a(8n+2) = a(8n+4) = 0.
MATHEMATICA
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 *)
PROG
(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))}
(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)}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 12 2006
STATUS
approved