T(2n,n+3), T given by A026780.

Terms

a(0) =1a(1) =9a(2) =57a(3) =316a(4) =1647a(5) =8330a(6) =41558a(7) =206418a(8) =1026392a(9) =5126109a(10) =25763881a(11) =130446758a(12) =665644541a(13) =3423360416a(14) =17740450004a(15) =92599616864a(16) =486607242642