OFFSET
0,1
COMMENTS
a(n) is the number of walks of length 2n+2 in an infinite square lattice that begin at the origin and end at (1,1) using steps (1,0), (-1,0), (0,1), (0,-1).
LINKS
S. Hollos and R. Hollos, Lattice Paths and Walks.
FORMULA
G.f.: 2*2F1(3/2,3/2; 3; 16*x). - Mark van Hoeij, Apr 06 2013
D-finite with recurrence n*(n+2)*a(n) -4*(2*n+1)^2*a(n-1)=0. - R. J. Mathar, Jul 14 2013
E.g.f.: Sum_{n>0} a(n-1) * x^(2*n)/(2*n)! = BesselI(1, 2*x)^2. - Michael Somos, Oct 17 2019
EXAMPLE
G.f. = 2 + 24*x + 300*x^2 + 3920*x^3 + 731808*x^4 + 10306296*x^5 + ... - Michael Somos, Oct 17 2019
MAPLE
series( 2*hypergeom([3/2, 3/2], [3], 16*x), x=0, 20); # Mark van Hoeij, Apr 06 2013
MATHEMATICA
Table[Binomial[2n + 2, n] Binomial[2n + 2, n + 1], {n, 0, 19}] (* Alonso del Arte, Apr 06 2013 *)
CROSSREFS
KEYWORD
easy,nonn,walk
AUTHOR
Stefan Hollos (stefan(AT)exstrom.com), Dec 11 2007
STATUS
approved