a(n) = (1/18)*(8^n - (-1)^n - 9).

Terms

a(0) =0a(1) =3a(2) =28a(3) =227a(4) =1820a(5) =14563a(6) =116508a(7) =932067a(8) =7456540a(9) =59652323a(10) =477218588a(11) =3817748707a(12) =30541989660a(13) =244335917283