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

Terms

a(0) =4a(1) =19a(2) =109a(3) =649a(4) =3889a(5) =23329a(6) =139969a(7) =839809a(8) =5038849a(9) =30233089a(10) =181398529a(11) =1088391169a(12) =6530347009a(13) =39182082049a(14) =235092492289