a(n) = T(2*n, n-2), where T is given by A026584.

Terms

a(0) =1a(1) =3a(2) =14a(3) =65a(4) =251a(5) =1288a(6) =4830a(7) =25518a(8) =95388a(9) =510532a(10) =1910821a(11) =10309234a(12) =38656462a(13) =209766714a(14) =787912030a(15) =4294635438a(16) =16155375825a(17) =88371236851a(18) =332859949946