%I A005560 M2987
%S A005560 1,3,15,45,189,588,2352,7560,29700,98010,382239,1288287,5010005,17177160,
%T A005560 66745536,232092432,901995588,3173688180,12342120700,43861998180
%N A005560 Number of walks on square lattice.
%D A005560 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%H A005560 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 A005560 a(n) = C(n+3, ceil(n/2))*C(n+2, floor(n/2)) - C(n+3, ceil((n-1)/2))*C(n+2, 
               floor((n-1)/2)). - Paul D. Hanna (pauldhanna(AT)juno.com), Apr 16 
               2004
%o A005560 (PARI) {a(n)=binomial(n+3,ceil(n/2))*binomial(n+2,floor(n/2)) - binomial(n+3,
               ceil((n-1)/2))*binomial(n+2,floor((n-1)/2))}
%Y A005560 Cf. A005558-A005562, A093768.
%Y A005560 Sequence in context: A048099 A030505 A074355 this_sequence A100747 A100737 
               A110464
%Y A005560 Adjacent sequences: A005557 A005558 A005559 this_sequence A005561 A005562 
               A005563
%K A005560 nonn
%O A005560 2,2
%A A005560 N. J. A. Sloane (njas(AT)research.att.com).