a(n) = T(2n, n-2), T given by A026780.

Terms

a(0) =1a(1) =11a(2) =84a(3) =557a(4) =3446a(5) =20514a(6) =119336a(7) =684227a(8) =3886460a(9) =21939528a(10) =123347842a(11) =691644044a(12) =3871738018a(13) =21652138770a(14) =121026492186a(15) =676391629701