a(n) = 3*2^(n-1)*(2^n-1).

Terms

a(0) =3a(1) =18a(2) =84a(3) =360a(4) =1488a(5) =6048a(6) =24384a(7) =97920a(8) =392448a(9) =1571328a(10) =6288384a(11) =25159680a(12) =100651008a(13) =402628608a(14) =1610563584a(15) =6442352640a(16) =25769607168a(17) =103078821888a(18) =412316073984