a(n) = binomial coefficient C(2n, n-1).

Terms

a(0) =0a(1) =1a(2) =4a(3) =15a(4) =56a(5) =210a(6) =792a(7) =3003a(8) =11440a(9) =43758a(10) =167960a(11) =646646a(12) =2496144a(13) =9657700a(14) =37442160a(15) =145422675a(16) =565722720a(17) =2203961430a(18) =8597496600a(19) =33578000610a(20) =131282408400a(21) =513791607420