a(n) = 27*8^n.

Terms

a(0) =27a(1) =216a(2) =1728a(3) =13824a(4) =110592a(5) =884736a(6) =7077888a(7) =56623104a(8) =452984832a(9) =3623878656a(10) =28991029248a(11) =231928233984