a(n) = 27*2^n.

Terms

a(0) =27a(1) =54a(2) =108a(3) =216a(4) =432a(5) =864a(6) =1728a(7) =3456a(8) =6912a(9) =13824a(10) =27648a(11) =55296a(12) =110592a(13) =221184a(14) =442368a(15) =884736a(16) =1769472a(17) =3538944a(18) =7077888a(19) =14155776a(20) =28311552a(21) =56623104a(22) =113246208a(23) =226492416a(24) =452984832a(25) =905969664a(26) =1811939328a(27) =3623878656a(28) =7247757312a(29) =14495514624