OFFSET
1,2
COMMENTS
a(n) is odd for n = 1,2,3 and if n is of the form 2^m or 2^m+1 (with m>= 2) (i.e. a(n) is odd if n=1,3,4,5,8,9,16,17,32,33,64,65,128,129,256,257...)
PROG
(PARI) a(n)=if(n<0, 0, (n!/2)*sum(k=1, n, (n+k)!/(k!)^3))
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Benoit Cloitre, Aug 03 2002
STATUS
approved