OFFSET
0,2
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 psi(x) * f(-x)^5 = phi(-x)^2 * f(-x^2)^4 in powers of x where phi(), psi(), f() are Ramanujan theta functions.
Euler transform of period 2 sequence [ -4, -6, ...].
G.f.: Product_{k>0} (1 - x^k)^4 * (1 - x^(2*k))^2.
a(4*n + 2) = a(8*n + 7) = 0. a(n) = A230277(3*n + 1).
EXAMPLE
G.f. = 1 - 4*x + 16*x^3 - 10*x^4 - 16*x^5 + 39*x^8 - 32*x^11 + ...
G.f. = q - 4*q^4 + 16*q^10 - 10*q^13 - 16*q^16 + 39*q^25 - 32*q^34 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ x]^4 QPochhammer[ x^2]^2, {x, 0, n}];
PROG
(PARI) {a(n) = local(A); if( n<0, 0, A = x * O(x^n); polcoeff( eta(x + A)^4 * eta(x^2 + A)^2, n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Oct 15 2013
STATUS
approved