a(n) = T(n,n+2), array T as in A047130.

Terms

a(0) =1a(1) =3a(2) =9a(3) =28a(4) =104a(5) =367a(6) =1295a(7) =4600a(8) =17229a(9) =64069a(10) =238148a(11) =886823a(12) =3366045a(13) =12793452a(14) =48663267a(15) =185303586a(16) =711046282a(17) =2734845898a(18) =10532016662a(19) =40599379289a(20) =157049408777a(21) =608667818351