OFFSET
0,3
COMMENTS
FORMULA
a(n) = sum(binomial(n,k)*C(k)*C(n-k),k=floor(n/2)+1..n), n>=0, with C(n)=A000108(n), the Catalan numbers.
E.g.f.: (C(x)^2 - C2(x^2))/2 with the e.g.f. C(x) of A000108, and the o.g.f. C2(x) of the sequence {(C(n)/n!)^2}. Compare this with the e.g.f. of A203577.
C(x) = hypergeom([1/2],[2],4*x) (see the e.g.f. of A000108 for the version involving BesselI functions), and
C2(x) = hypergeom([1/2,1/2],[1,2,2],16*x).
EXAMPLE
With A000108 = {1, 1, 2, 5, 14, 42,...}
a(4) = 4*5*1 + 1*14*1 = 34.
a(5) = 10*5*2 + 5*14*1 + 1*42*1 = 212.
MAPLE
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Jan 16 2012
STATUS
approved