a(n) = 7*6^n-1.

Terms

a(0) =6a(1) =41a(2) =251a(3) =1511a(4) =9071a(5) =54431a(6) =326591a(7) =1959551a(8) =11757311a(9) =70543871a(10) =423263231a(11) =2539579391a(12) =15237476351a(13) =91424858111a(14) =548549148671