%I A005561 M3596
%S A005561 1,4,24,84,392,1344,5760,19800,81675,283140,1145144,4008004,16032016,
%T A005561 56632576,225059328,801773856,3173688180,11392726800,44986664800
%N A005561 Number of walks on square lattice.
%D A005561 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%H A005561 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 A005561 a(n) = C(n+4, ceil(n/2))*C(n+3, floor(n/2)) - C(n+4, ceil((n-1)/2))*C(n+3, 
               floor((n-1)/2)). - Paul D. Hanna (pauldhanna(AT)juno.com), Apr 16 
               2004
%o A005561 (PARI) {a(n)=binomial(n+4,ceil(n/2))*binomial(n+3,floor(n/2)) - binomial(n+4,
               ceil((n-1)/2))*binomial(n+3,floor((n-1)/2))}
%Y A005561 Cf. A005558-A005562, A093768.
%Y A005561 Sequence in context: A112742 A158494 A069145 this_sequence A061612 A097875 
               A026694
%Y A005561 Adjacent sequences: A005558 A005559 A005560 this_sequence A005562 A005563 
               A005564
%K A005561 nonn,walk
%O A005561 3,2
%A A005561 N. J. A. Sloane (njas(AT)research.att.com).