OFFSET
0,3
COMMENTS
Differs from A247007 first at n=27. - Alois P. Heinz, Sep 09 2014
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..450
FORMULA
E.g.f.: (1-x^3)^(1/3)/(1-x).
a(n) ~ n! * 3^(1/3) / (GAMMA(2/3) * n^(1/3)). - Vaclav Kotesovec, Mar 15 2014
EXAMPLE
G.f. = 1 + x + 2*x^2 + 4*x^3 + 16*x^4 + 80*x^5 + 400*x^6 + 2800*x^7 + ...
MAPLE
a:= proc(n) option remember; `if`(n=0, 1, add(`if`(
irem(j, 3)=0, 0, a(n-j)*(j-1)!*binomial(n-1, j-1)), j=1..n))
end:
seq(a(n), n=0..27); # Alois P. Heinz, Jul 31 2017
MATHEMATICA
nn=21; a=Sum[x^n/n, {n, 3, nn, 3}]; Range[0, nn]!CoefficientList[Series[Exp[Log[1/(1-x)]-a], {x, 0, nn}], x] (* Geoffrey Critzer, Nov 11 2012 *)
a[ n_] := If[ n < 0, 0, n! With[{m = Quotient[n, 3]}, (-1)^m Binomial[-2/3, m]]]; (* Michael Somos, Aug 05 2016 *)
PROG
(PARI) {a(n) = my(m); if( n<0, 0, m = n\3; n! * (-1)^m * binomial(-2/3, m))}; /* Michael Somos, Aug 05 2016 */
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Vladeta Jovovic, Feb 08 2005
STATUS
approved