a(n) = T(2n,n-1), where T is defined in A026022.

Terms

a(0) =1a(1) =4a(2) =15a(3) =56a(4) =209a(5) =780a(6) =2912a(7) =10880a(8) =40698a(9) =152456a(10) =572033a(11) =2150040a(12) =8095425a(13) =30535260a(14) =115377660a(15) =436698240a(16) =1655607390a(17) =6286707000a(18) =23908446510a(19) =91057063344a(20) =347281885818