a(n) = [ C(2n,n)/n ].

Terms

a(0) =2a(1) =3a(2) =6a(3) =17a(4) =50a(5) =154a(6) =490a(7) =1608a(8) =5402a(9) =18475a(10) =64130a(11) =225346a(12) =800046a(13) =2865471a(14) =10341168a(15) =37567524a(16) =137270954a(17) =504174183a(18) =1860277042a(19) =6892326441a(20) =25631327354a(21) =95640861987a(22) =357975249026