OFFSET
0,3
LINKS
Alois P. Heinz, Table of n, a(n) for n = 0..1000 (terms n=1..197 from R. H. Hardin)
FORMULA
a(n) = (2n)!/2^n = A000680(n) for n<=2.
MAPLE
a:= proc(n) option remember; `if`(n<4, [1$2, 6, 12][n+1],
((8*n-31)*a(n-1) -(4*n-19)*a(n-2) -(3*n-10)*a(n-3)
+(2*n-10)*a(n-4)) / (3*n-11))
end:
seq(a(n), n=0..40); # Alois P. Heinz, Jan 14 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
R. H. Hardin, May 06 2010
STATUS
approved