OFFSET
0,2
FORMULA
The generating function equals 0 when 2 * E(k) = K(k) at q = 0.1076539192... (A072558) the "One-Ninth" constant.
Expansion of (P(q) - 2 * P(q^2) + 4 * P(q^4)) / 3 in powers of q where P() is a Ramanujan Lambert series.
G.f.: 1 - 8 * Sum_{k>0} k * x^k / (1 - (-x)^k) = 1 + 8 * Sum_{k>0} (-x)^k / (1 + (-x)^k)^2.
EXAMPLE
G.f. = 1 - 8*q - 8*q^2 - 32*q^3 - 40*q^4 - 48*q^5 - 32*q^6 - 64*q^7 - 104*q^8 + ...
MATHEMATICA
a[ n_] := If[ n < 1, Boole[n == 0], -(-1)^n 8 Sum[(-1)^d d, {d, Divisors @ n}]]; (* Michael Somos, Apr 07 2015 *)
a[ n_] := SeriesCoefficient[ With[{m = InverseEllipticNomeQ[ q]}, EllipticK[ m] (2 EllipticE[ m] - EllipticK[ m]) (2/Pi)^2], {q, 0, n}]; (* Michael Somos, Apr 07 2015 *)
PROG
(PARI) {a(n) = if( n<1, n==0, -(-1)^n * 8 * sumdiv(n, d, (-1)^d * d))};
CROSSREFS
KEYWORD
sign
AUTHOR
Michael Somos, Aug 09 2008
STATUS
approved