a(n) = 9*2^n.

Terms

a(0) =9a(1) =18a(2) =36a(3) =72a(4) =144a(5) =288a(6) =576a(7) =1152a(8) =2304a(9) =4608a(10) =9216a(11) =18432a(12) =36864a(13) =73728a(14) =147456a(15) =294912a(16) =589824a(17) =1179648a(18) =2359296a(19) =4718592a(20) =9437184a(21) =18874368a(22) =37748736a(23) =75497472a(24) =150994944a(25) =301989888a(26) =603979776a(27) =1207959552a(28) =2415919104a(29) =4831838208