a(n) = (3*7^n-3^n)/2.

Terms

a(0) =1a(1) =9a(2) =69a(3) =501a(4) =3561a(5) =25089a(6) =176109a(7) =1234221a(8) =8643921a(9) =60520569a(10) =423683349a(11) =2965901541a(12) =20761665081a(13) =145332718449