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

Terms

a(0) =1a(1) =2a(2) =6a(3) =24a(4) =104a(5) =448a(6) =1888a(7) =7808a(8) =31872a(9) =129024a(10) =519680a(11) =2086912a(12) =8366080a(13) =33505280a(14) =134111232a(15) =536641536a(16) =2146992128a(17) =8588886016a(18) =34357510144a(19) =137434234880a(20) =549745852416