Number of n-step walks on a line starting from the origin but not returning to it.

Terms

a(0) =1a(1) =2a(2) =2a(3) =4a(4) =6a(5) =12a(6) =20a(7) =40a(8) =70a(9) =140a(10) =252a(11) =504a(12) =924a(13) =1848a(14) =3432a(15) =6864a(16) =12870a(17) =25740a(18) =48620a(19) =97240a(20) =184756a(21) =369512a(22) =705432a(23) =1410864a(24) =2704156a(25) =5408312a(26) =10400600a(27) =20801200a(28) =40116600a(29) =80233200