a(n) = (4*6^n - 3*5^n - 3^n)/6.

Terms

a(0) =0a(1) =1a(2) =10a(3) =77a(4) =538a(5) =3581a(6) =23170a(7) =147197a(8) =923338a(9) =5738621a(10) =35418130a(11) =217421117a(12) =1329029338a(13) =8096512061a(14) =49190221090a(15) =298195475837