a(n) = binomial(2*n, n+1) + 1.

Terms

a(0) =1a(1) =2a(2) =5a(3) =16a(4) =57a(5) =211a(6) =793a(7) =3004a(8) =11441a(9) =43759a(10) =167961a(11) =646647a(12) =2496145a(13) =9657701a(14) =37442161a(15) =145422676a(16) =565722721a(17) =2203961431a(18) =8597496601a(19) =33578000611a(20) =131282408401a(21) =513791607421