a(n) = 9*4^n.

Terms

a(0) =9a(1) =36a(2) =144a(3) =576a(4) =2304a(5) =9216a(6) =36864a(7) =147456a(8) =589824a(9) =2359296a(10) =9437184a(11) =37748736a(12) =150994944a(13) =603979776a(14) =2415919104a(15) =9663676416a(16) =38654705664a(17) =154618822656a(18) =618475290624