a(n) = 6^n - n.

Terms

a(0) =1a(1) =5a(2) =34a(3) =213a(4) =1292a(5) =7771a(6) =46650a(7) =279929a(8) =1679608a(9) =10077687a(10) =60466166a(11) =362797045a(12) =2176782324a(13) =13060694003a(14) =78364164082a(15) =470184984561