a(n) = 8^n - 5^n.

Terms

a(0) =0a(1) =3a(2) =39a(3) =387a(4) =3471a(5) =29643a(6) =246519a(7) =2019027a(8) =16386591a(9) =132264603a(10) =1063976199a(11) =8541106467a(12) =68475336111a(13) =548535110763