Number of walks on square lattice. Column y=2 of A052174.

Terms

a(0) =1a(1) =3a(2) =15a(3) =45a(4) =189a(5) =588a(6) =2352a(7) =7560a(8) =29700a(9) =98010a(10) =382239a(11) =1288287a(12) =5010005a(13) =17177160a(14) =66745536a(15) =232092432a(16) =901995588a(17) =3173688180a(18) =12342120700a(19) =43861998180a(20) =170724392916a(21) =611947174608