OFFSET
1,3
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = binomial(3n, n) - 3*binomial(2n, n) + 3.
From G. C. Greubel, Feb 08 2016: (Start)
E.g.f.: 3*exp(x) - 3*exp(2*x)*BesselI_{0}(2*x) + Hypergeometric2F2[1/3,2/3; 1/2,1; 27*x/4].
G.f.: (1/((x-1)sqrt(a*b)))*[3*sqrt(a)*(1-x) - 3*sqrt(a*b) - 2*(1-x)*sqrt(b)*cos(c/3)], where a = 4-27*x, b = 1-4*x, c = arcsin(3*sqrt(3*x)/2). (End)
MATHEMATICA
Table[Binomial[3 n, n] - 3*Binomial[2 n, n] + 3, {n, 1, 100}] (* G. C. Greubel, Feb 08 2016 *)
PROG
(PARI) a(n) = binomial(3*n, n) - 3*binomial(2*n, n) + 3 \\ Michel Marcus, Jul 15 2013
(Magma) [Binomial(3*n, n)-3*Binomial(2*n, n)+3: n in [1..40]]; // Vincenzo Librandi, Feb 09 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Hieronymus Fischer, Jan 21 2006
STATUS
approved