Number of paths on square lattice.

Terms

a(0) =1a(1) =2a(2) =5a(3) =16a(4) =54a(5) =180a(6) =595a(7) =1964a(8) =6485a(9) =21418a(10) =70740a(11) =233640a(12) =771661a(13) =2548622a(14) =8417525a(15) =27801196a(16) =91821114a(17) =303264540a(18) =1001614735a(19) =3308108744a(20) =10925940965a(21) =36085931638a(22) =119183735880a(23) =393637139280