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

Terms

a(0) =4a(1) =24a(2) =112a(3) =480a(4) =1984a(5) =8064a(6) =32512a(7) =130560a(8) =523264a(9) =2095104a(10) =8384512a(11) =33546240a(12) =134201344a(13) =536838144a(14) =2147418112a(15) =8589803520a(16) =34359476224a(17) =137438429184a(18) =549754765312