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

Terms

a(0) =2a(1) =5a(2) =37a(3) =215a(4) =1297a(5) =7775a(6) =46657a(7) =279935a(8) =1679617a(9) =10077695a(10) =60466177a(11) =362797055a(12) =2176782337a(13) =13060694015a(14) =78364164097a(15) =470184984575