a(n) = (7^n - 2*n - 1)/4.

Terms

a(0) =0a(1) =1a(2) =11a(3) =84a(4) =598a(5) =4199a(6) =29409a(7) =205882a(8) =1441196a(9) =10088397a(10) =70618807a(11) =494331680a(12) =3460321794a(13) =24222252595a(14) =169555768205