OFFSET
0,3
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 q * f(q) * phi(q^3) * chi(q^3)^2 * psi(q^9) / (chi(q)^2 * phi(q^9)) in powers of q where phi(), psi(), chi(), f() are Ramanujan theta functions.
Expansion of eta(q)^3 * eta(q^4)^2 * eta(q^6)^9 * eta(q^9) * eta(q^36)^2 / (eta(q^2)^4 * eta(q^3)^4 * eta(q^12)^4 * eta(q^18)^3) in powers of q.
Euler transform of period 36 sequence [ -3, 1, 1, -1, -3, -4, -3, -1, 0, 1, -3, -2, -3, 1, 1, -1, -3, -2, -3, -1, 1, 1, -3, -2, -3, 1, 0, -1, -3, -4, -3, -1, 1, 1, -3, -2, ...].
Moebius transform is period 36 sequence [ 1, -4, 3, 0, -1, 0, 1, 0, 0, 4, -1, 0, 1, -4, -3, 0, -1, 0, 1, 0, 3, 4, -1, 0, 1, -4, 0, 0, -1, 0, 1, 0, -3, 4, -1, 0, ...].
EXAMPLE
G.f. = x - 3*x^2 + 4*x^3 - 3*x^4 + 2*x^7 - 3*x^8 + 4*x^9 + 2*x^13 + ...
MATHEMATICA
a[ n_] := If[ n < 1, 0, {1, -3, 4, -3, 1, 0}[[Mod[ n, 6, 1]]] Sum[ {1, 0, 0, 0, -1, 0}[[Mod[ d, 6, 1]]], {d, Divisors @ n}]];
a[ n_] := If[ n < 1 || Mod[n, 6] == 0, 0, Times @@ (Which[ # == 1, 1, # == 2, -2 - Mod[#2, 2], # == 3, 4, Mod[#, 6] == 5, 1 - Mod[#2, 2], True, #2 + 1] & @@@ FactorInteger @ n)];
a[ n_] := SeriesCoefficient[ QPochhammer[ x] EllipticTheta[ 3, 0, x^3] QPochhammer[ -x^3, x^6]^2 EllipticTheta[ 2, 0, x^(9/2)] / (2 x^(1/8) QPochhammer[-x, x^2]^2 EllipticTheta[ 3, 0, x^9]), {x, 0, n}];
PROG
(PARI) {a(n) = if( n<1, 0, [0, 1, -3, 4, -3, 1][n%6+1] * sumdiv(n, d, [0, 1, 0, 0, 0, -1][n/d%6+1]))};
(PARI) {a(n) = my(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( eta(x + A)^3 * eta(x^4 + A)^2 * eta(x^6 + A)^9 * eta(x^9 + A) * eta(x^36 + A)^2 / (eta(x^2 + A)^4 * eta(x^3 + A)^4 * eta(x^12 + A)^4 * eta(x^18 + A)^3), n))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 05 2015
STATUS
approved