%I M3654 #32 Apr 11 2018 02:56:29
%S 4,34,308,3024,31680,349206,4008004,47530912,579058896,7215393640,
%T 91644262864,1183274479040,15497363512800,205519758825150,
%U 2755739674246500,37314861293793600,509727100259638800
%N Number of walks on square lattice.
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H R. K. Guy, <a href="/A005555/a005555.pdf">Letter to N. J. A. Sloane, May 1990</a>
%H R. K. Guy, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL3/GUY/catwalks.html">Catwalks, sandsteps and Pascal pyramids</a>, J. Integer Sequences, Vol. 3 (2000), Article #00.1.6.
%F G.f.: 4_F_3 ( [ 2, 17/5, 5/2, 3/2 ]; [ 4, 5, 12/5 ];16 x ).
%F a(n)(n-1)(n+2)(n+3)(5n+2) = a(n-1)4n(2n-1)(2n+1)(5n+7), n>1. - _Michael Somos_, Mar 04 2003
%o (PARI) a(n)=4*if(n<2,n==1,a(n-1)*n*(2*n-1)*(2*n+1)*(5*n+7)/((n-1)*(n+2)*(n+3)*(5*n+2)))
%K nonn,walk,easy
%O 1,1
%A _N. J. A. Sloane_