a(n) = 6*4^n.

Terms

a(0) =6a(1) =24a(2) =96a(3) =384a(4) =1536a(5) =6144a(6) =24576a(7) =98304a(8) =393216a(9) =1572864a(10) =6291456a(11) =25165824a(12) =100663296a(13) =402653184a(14) =1610612736a(15) =6442450944a(16) =25769803776a(17) =103079215104a(18) =412316860416