a(n) = 8^n + 1.

Terms

a(0) =2a(1) =9a(2) =65a(3) =513a(4) =4097a(5) =32769a(6) =262145a(7) =2097153a(8) =16777217a(9) =134217729a(10) =1073741825a(11) =8589934593a(12) =68719476737a(13) =549755813889