a(n) = nextprime(3^n) - 2^n.

Terms

a(0) =1a(1) =3a(2) =7a(3) =21a(4) =67a(5) =219a(6) =669a(7) =2075a(8) =6307a(9) =19175a(10) =58027a(11) =175119a(12) =527361a(13) =1586139a(14) =4766587a(15) =14316141a(16) =42981211a(17) =129009125a(18) =387158355a(19) =1161737235a(20) =3485735833a(21) =10458256107a(22) =31376865309a(23) =94134790251a(24) =282412759267a(25) =847255055025