OFFSET
0,8
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
Euler transform of period 20 sequence [ -1, -1, -1, -1, -2, -1, -1, -1, -1, -1, -1, -1, -1, -1, -2, -1, -1, -1, -1, -2, ...].
Product_{k>0} (1-x^k)*(1-x^(5k))*(1+x^(10k)).
a(8n+3) = a(8n+5) = 0.
Expansion of q^(-2/3) * eta(q) * eta(q^5) * eta(q^20)/ eta(q^10) in powers of q.
MATHEMATICA
eta[q_]:= q^(1/24)*QPochhammer[q]; a[n_]:= SeriesCoefficient[q^(-2/3)* eta[q]*eta[q^5]*eta[q^20]/eta[q^10], {q, 0, n}]; Table[a[n], {n, 0, 50}] (* G. C. Greubel, Mar 08 2018 *)
PROG
(PARI) {a(n)=local(A); if(n<0, 0, A=x*O(x^n); polcoeff( eta(x+A)*eta(x^5+A)*eta(x^20+A)/eta(x^10+A), n))}
(PARI) {a(n) = local(s, k); if(n<0, 0, n=24*n+16; forstep(k=1, sqrtint(n\15), 2, if(issquare(n-15*k^2, &j)& (j^2%6==1), s+= (-1)^((j+1)\6+ (k+2)\4))); s)}
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 12 2006
STATUS
approved