a(n) = (1/2)*(binomial(2n, n) - binomial(2n-2, n-1)).

Terms

a(0) =2a(1) =7a(2) =25a(3) =91a(4) =336a(5) =1254a(6) =4719a(7) =17875a(8) =68068a(9) =260338a(10) =999362a(11) =3848222a(12) =14858000a(13) =57500460a(14) =222981435a(15) =866262915a(16) =3370764540a(17) =13135064250a(18) =51250632510a(19) =200205672810a(20) =782920544640