a(n) = T(n,n+2), T given by A027052.

Terms

a(0) =1a(1) =4a(2) =9a(3) =18a(4) =35a(5) =66a(6) =123a(7) =228a(8) =421a(9) =776a(10) =1429a(11) =2630a(12) =4839a(13) =8902a(14) =16375a(15) =30120a(16) =55401a(17) =101900a(18) =187425a(19) =344730a(20) =634059a(21) =1166218a(22) =2145011a(23) =3945292a(24) =7256525a(25) =13346832a(26) =24548653a(27) =45152014a(28) =83047503a(29) =152748174