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

Terms

a(0) =2a(1) =16a(2) =128a(3) =1024a(4) =8192a(5) =65536a(6) =524288a(7) =4194304a(8) =33554432a(9) =268435456a(10) =2147483648a(11) =17179869184a(12) =137438953472