OFFSET
0,3
LINKS
Robert Israel, Table of n, a(n) for n = 0..2993
Ran Pan, Exercise C, Project P.
FORMULA
a(n) = 4*binomial(n,floor(n/2)) for n > 4; a(n) = n! for n <= 4.
From Robert Israel, Apr 30 2015: (Start)
G.f.: -3*(1+x)*(1+2*x^2) - 2/x + 2*(2+1/x)/sqrt(1-4*x^2).
a(n+2) = (4*(1+n)*a(n) + 2*a(n+1))/(n+3) for n >= 4. (End)
MAPLE
f:= gfun:-rectoproc({a(n+2)= 4*(1+n)*a(n)/(n+3) + 2*a(n+1)/(n+3), seq(a(n)=[1, 1, 2, 6, 24, 40, 80][n+1], n=0..5)}, a(n), remember):
map(f, [$0..100]); # Robert Israel, May 12 2015
MATHEMATICA
Table[If[n <= 4, n!, 4 Binomial[n, Floor[n/2]]], {n, 31}] (* Michael De Vlieger, Apr 29 2015 *)
PROG
(Magma) [1, 1, 2, 6] cat [4*Binomial(n, Floor(n/2)): n in [4..40]]; // Vincenzo Librandi, Apr 30 2015
CROSSREFS
KEYWORD
easy,nonn,changed
AUTHOR
Ran Pan, Apr 29 2015
EXTENSIONS
More terms from Vincenzo Librandi, Apr 30 2015
STATUS
approved