a(n) = binomial(2n-3,n-1) + binomial(2n-2,n-2).

Terms

a(0) =1a(1) =2a(2) =7a(3) =25a(4) =91a(5) =336a(6) =1254a(7) =4719a(8) =17875a(9) =68068a(10) =260338a(11) =999362a(12) =3848222a(13) =14858000a(14) =57500460a(15) =222981435a(16) =866262915a(17) =3370764540a(18) =13135064250a(19) =51250632510a(20) =200205672810a(21) =782920544640