OFFSET
0,1
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 G(x) * f(-x^2) where G() is the g.f. of A003114.
Expansion of f(-x^13, -x^17) + x * f(-x^7, -x^23) in powers of x.
Euler transform of period 10 sequence [ 1, -1, 0, 0, 0, 0, 0, -1, 1, -1, ...].
G.f.: Sum_{k} (-1)^k * x^(15*k^2) * (x^(2*k) + x^(8*k + 1)) = Product_{k>0} (1 - x^(10*k)) * (1 - x^(10*k -2)) * (1 - x^(10*k -8)) / ((1 - x^(10*k -1)) * (1 - x^(10*k -9))).
|a(n)| is the characteristic function of A204221.
The exponents in the q-series q * A(q^15) are the squares of the numbers == +- 1 or +- 4 (mod 15).
EXAMPLE
G.f. = 1 + x - x^8 - x^13 - x^17 - x^24 + x^45 + x^56 + x^64 + x^77 - x^112 + ...
G.f. = q + q^16 - q^121 - q^196 - q^256 - q^361 + q^676 + q^841 + q^961 + q^1156 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ x^2, x^5] QPochhammer[ x^3, x^5] QPochhammer[ x^5] QPochhammer[ -x, x], {x, 0, n}]; (* Michael Somos, Jan 06 2016 *)
a[ n_] := SeriesCoefficient[ Product[ (1 - x^k)^{ -1, 1, 0, 0, 0, 0, 0, 1, -1, 1}[[Mod[k, 10, 1]]], {k, n}], {x, 0, n}]; (* Michael Somos, Jan 06 2016 *)
PROG
(PARI) {a(n) = if( n<0, 0, polcoeff( prod( k=1, n, (1 - x^k) ^ ([1, -1, 1, 0, 0, 0, 0, 0, 1, -1][k%10 + 1]), 1 + x * O(x^n)), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Jan 13 2012
STATUS
approved