a(n) = T(n,n-3), where T is the array in A026148.

Terms

a(0) =1a(1) =3a(2) =11a(3) =35a(4) =111a(5) =343a(6) =1049a(7) =3177a(8) =9559a(9) =28611a(10) =85293a(11) =253461a(12) =751296a(13) =2222442a(14) =6563598a(15) =19359022a(16) =57038247a(17) =167911721a(18) =493972165a(19) =1452419661a(20) =4268753126a(21) =12542145548a(22) =36841741320a(23) =108202146520a(24) =317748977715a(25) =933052724721