OFFSET
1,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..2500
FORMULA
a(n) is multiplicative with a(2^(2*k)) = (-8)^k, a(2^(2*k+1)) = 4 * (-8)^k, a(3^e) = 0^e, a(p^(2*k)) = (-p)^(3^k) and a(p^(2*k+1)) = 0 if p == 5 (mod 6), a(p^e) = a(p) * a(p^(e-1)) - p^3 * a(p^(e-2)) if p == 1 (mod 6).
Convolution square of A261277.
EXAMPLE
G.f. = x + 4*x^2 - 8*x^4 + 20*x^7 - 32*x^8 - 70*x^13 + 80*x^14 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ x QPochhammer[ x^3]^8 + 4 x^2 QPochhammer[ x^6]^8, {x, 0, n}];
PROG
(PARI) {a(n) = my(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( eta(x^3 + A)^8 + 4 * x * eta(x^6 + A)^8, n))};
(Magma) A := Basis( CuspForms( Gamma0(18), 4), 72); A[1] + 4*A[2] - 8*A[4];
(Sage) A = CuspForms( Gamma0(18), 4, prec=20).basis(); A[0] + 4*A[1] - 8*A[3];
CROSSREFS
KEYWORD
sign,mult
AUTHOR
Michael Somos, Aug 14 2015
STATUS
approved