Number of walks on square lattice. Column y=3 of A052174.

Terms

a(0) =1a(1) =4a(2) =24a(3) =84a(4) =392a(5) =1344a(6) =5760a(7) =19800a(8) =81675a(9) =283140a(10) =1145144a(11) =4008004a(12) =16032016a(13) =56632576a(14) =225059328a(15) =801773856a(16) =3173688180a(17) =11392726800a(18) =44986664800a(19) =162594659920a(20) =641087516256