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

Terms

a(0) =1a(1) =5a(2) =33a(3) =209a(4) =1281a(5) =7745a(6) =46593a(7) =279809a(8) =1679361a(9) =10077185a(10) =60465153a(11) =362795009a(12) =2176778241a(13) =13060685825a(14) =78364147713a(15) =470184951809