a(n) = 6^n + 1.

Terms

a(0) =2a(1) =7a(2) =37a(3) =217a(4) =1297a(5) =7777a(6) =46657a(7) =279937a(8) =1679617a(9) =10077697a(10) =60466177a(11) =362797057a(12) =2176782337a(13) =13060694017a(14) =78364164097a(15) =470184984577