a(n) = 7*5^n-1.

Terms

a(0) =6a(1) =34a(2) =174a(3) =874a(4) =4374a(5) =21874a(6) =109374a(7) =546874a(8) =2734374a(9) =13671874a(10) =68359374a(11) =341796874a(12) =1708984374a(13) =8544921874a(14) =42724609374a(15) =213623046874