a(n) = (6^n - 1)/5.

Terms

a(0) =0a(1) =1a(2) =7a(3) =43a(4) =259a(5) =1555a(6) =9331a(7) =55987a(8) =335923a(9) =2015539a(10) =12093235a(11) =72559411a(12) =435356467a(13) =2612138803a(14) =15672832819a(15) =94036996915a(16) =564221981491