a(n) = 6^n-5.

Terms

a(0) =1a(1) =31a(2) =211a(3) =1291a(4) =7771a(5) =46651a(6) =279931a(7) =1679611a(8) =10077691a(9) =60466171a(10) =362797051a(11) =2176782331a(12) =13060694011a(13) =78364164091a(14) =470184984571