a(n) = 6^n + 2^n - 1.

Terms

a(0) =1a(1) =7a(2) =39a(3) =223a(4) =1311a(5) =7807a(6) =46719a(7) =280063a(8) =1679871a(9) =10078207a(10) =60467199a(11) =362799103a(12) =2176786431a(13) =13060702207a(14) =78364180479a(15) =470185017343