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

Terms

a(0) =2a(1) =8a(2) =32a(3) =128a(4) =512a(5) =2048a(6) =8192a(7) =32768a(8) =131072a(9) =524288a(10) =2097152a(11) =8388608a(12) =33554432a(13) =134217728a(14) =536870912a(15) =2147483648a(16) =8589934592a(17) =34359738368a(18) =137438953472a(19) =549755813888