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

Terms

a(0) =8a(1) =36a(2) =176a(3) =876a(4) =4376a(5) =21876a(6) =109376a(7) =546876a(8) =2734376a(9) =13671876a(10) =68359376a(11) =341796876a(12) =1708984376a(13) =8544921876a(14) =42724609376a(15) =213623046876