a(n) = 3*6^n-1.

Terms

a(0) =2a(1) =17a(2) =107a(3) =647a(4) =3887a(5) =23327a(6) =139967a(7) =839807a(8) =5038847a(9) =30233087a(10) =181398527a(11) =1088391167a(12) =6530347007a(13) =39182082047a(14) =235092492287