a(n) = 6*9^n+1.

Terms

a(0) =7a(1) =55a(2) =487a(3) =4375a(4) =39367a(5) =354295a(6) =3188647a(7) =28697815a(8) =258280327a(9) =2324522935a(10) =20920706407a(11) =188286357655