a(n) = (3*4^n - 0^n)/2.

Terms

a(0) =1a(1) =6a(2) =24a(3) =96a(4) =384a(5) =1536a(6) =6144a(7) =24576a(8) =98304a(9) =393216a(10) =1572864a(11) =6291456a(12) =25165824a(13) =100663296a(14) =402653184a(15) =1610612736a(16) =6442450944a(17) =25769803776a(18) =103079215104a(19) =412316860416