OFFSET
1,3
COMMENTS
G.f. is a newform level 15 weight 3 and nontrivial character.
The terms of A115155 differ only in sign from this sequence. - Michael Somos, Jun 14 2023
FORMULA
a(n) is multiplicative with a(3^e) = 3^e, a(5^e) = (-5)^e, a(p^e) = p^e * (1 + (-1)^e) / 2 if p == 7, 11, 13, 14 (mod 15), a(p^e) = a(p) * a(p^(e-1)) - p^2 * a(p^(e-2)) if p == 1, 2, 4, 8 (mod 15).
G.f. is a period 1 Fourier series which satisfies f(-1 / (15 t)) = 15^(3/2) (t/i)^3 f(t) where q = exp(2 Pi i t). - Michael Somos, Aug 29 2013
EXAMPLE
G.f. = q - q^2 + 3*q^3 - 3*q^4 - 5*q^5 - 3*q^6 + 7*q^8 + 9*q^9 + 5*q^10 + ...
MATHEMATICA
QP = QPochhammer; s = (QP[q^3]*QP[q^5])^3-q*(QP[q]*QP[q^15])^3 + O[q]^100; CoefficientList[s, q] (* Jean-François Alcover, Nov 25 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( (eta(x^3 + A) * eta(x^5 + A))^3 - x * (eta(x + A) * eta(x^15 + A))^3, n))};
(Magma) A := Basis( CuspForms( Gamma1(15), 3), 80); A[1] - A[2] + 3*A[3] - 3*A[4] - 5*A[5] - 3*A[6] + 7*A[8]; /* Michael Somos, Oct 13 2015 */
CROSSREFS
KEYWORD
sign,mult
AUTHOR
Michael Somos, Jan 05 2008
STATUS
approved