OFFSET
1,6
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
Michael Somos, Introduction to Ramanujan theta functions
Eric Weisstein's World of Mathematics, Ramanujan Theta Functions
FORMULA
Expansion of q * f(-q) * f(-q^2) * f(-q^16)^3 / f(-q^2, -q^14) in powers of q where f() is a Ramanujan theta function.
Expansion of q * psi(-q) * psi(-q^2) * psi(-q^4) * f(-q^6, -q^10) in powers of q where psi(), f() are Ramanujan theta functions.
Euler transform of period 16 sequence [ -1, -1, -1, -2, -1, -2, -1, -2, -1, -2, -1, -2, -1, -1, -1, -4, ...].
EXAMPLE
G.f. = q - q^2 - q^3 - q^5 + 2*q^6 + 2*q^8 + q^11 - 2*q^12 - q^13 - 2*q^14 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ q QPochhammer[ q^6, q^16] QPochhammer[ q^10, q^16] QPochhammer[ q^16]^2 QPochhammer[ q] QPochhammer[ q^4], {q, 0, n}];
PROG
(PARI) {a(n) = local(A, m); if( n<1, 0, n--; A = x * O(x^n); polcoeff( eta(x + A) * eta(x^4 + A) * eta(x^16 + A) * sum( k=0, n\2, if( issquare( 16*k + 1, &m), (-1)^((m + 1) \ 8) * x^(2*k), 0), A), n))};
(Sage) A = CuspForms( Gamma1(16), 2, prec=80).basis(); A[0] - A[1];
(Magma) A := Basis( CuspForms( Gamma1(16), 2), 80); A[1] - A[2]; /* Michael Somos, Jan 08 2015 */
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Jul 17 2013
STATUS
approved