a(n) = A027052(n, 2n-6).

Terms

a(0) =1a(1) =1a(2) =3a(3) =11a(4) =37a(5) =123a(6) =401a(7) =1281a(8) =4023a(9) =12461a(10) =38175a(11) =115939a(12) =349701a(13) =1049063a(14) =3133493a(15) =9327357a(16) =27687947a(17) =82009215a(18) =242473197a(19) =715889685a(20) =2111215763a(21) =6220468653a(22) =18314669783a(23) =53892395679a(24) =158512474561a(25) =466071547105