OFFSET
0,40
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..10000
Wikipedia, Euler function
FORMULA
From Michael Somos, Jun 07 2012: (Start)
G.f.: phi(-q^3)*phi(q)/phi(q^2).
|a(n)|<2 for n<39. |a(n)|<3 for n<56.
Euler transform of period 12 sequence [-1, 0, 0, 0, -1, -2, -1, 0, 0, 0, -1, -1, ...]. (End)
EXAMPLE
G.f. = 1 - x - x^5 - x^6 + x^7 + x^8 + x^11 - x^14 + x^16 - x^18 + x^19 - x^21 + ...
MATHEMATICA
a[ n_] := SeriesCoefficient[ QPochhammer[ -x^3] QPochhammer[ x] / QPochhammer[ x^2], {x, 0, n}]; (* Michael Somos, May 03 2015 *)
PROG
(PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff(eta(x + A) * eta(x^6 + A)^3 / (eta(x^2 + A) * eta(x^3 + A) * eta(x^12 + A)), n))}; /* Michael Somos, Jun 07 2012 */
CROSSREFS
KEYWORD
sign
AUTHOR
Gary Hinger (ghinger(AT)gmail.com), Aug 16 2009
STATUS
approved