%I A005559 M1832
%S A005559 1,2,8,20,75,210,784,2352,8820,27720,104544,339768,1288287,4294290,
%T A005559 16359200,55621280,212751396,734959368,2821056160,9873696560,38013731756
%N A005559 Number of walks on square lattice.
%D A005559 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%H A005559 R. K. Guy, Catwalks, Sandsteps and Pascal Pyramids, <a href="http://www.cs.uwaterloo.ca/
               journals/JIS/index.html">J. Integer Seqs., Vol. 3 (2000), #00.1.6</
               a>
%F A005559 a(n) = C(n+2, ceil(n/2))*C(n+1, floor(n/2)) - C(n+2, ceil((n-1)/2))*C(n+1, 
               floor((n-1)/2)). - Paul D. Hanna (pauldhanna(AT)juno.com), Apr 16 
               2004
%o A005559 (PARI) {a(n)=binomial(n+2,ceil(n/2))*binomial(n+1,floor(n/2)) - binomial(n+2,
               ceil((n-1)/2))*binomial(n+1,floor((n-1)/2))}
%Y A005559 Cf. A005558-A005560, A093768.
%Y A005559 Sequence in context: A100097 A133467 A091004 this_sequence A001471 A162585 
               A000159
%Y A005559 Adjacent sequences: A005556 A005557 A005558 this_sequence A005560 A005561 
               A005562
%K A005559 nonn
%O A005559 1,2
%A A005559 N. J. A. Sloane (njas(AT)research.att.com).