a(n) = 2^((prime(n) - 1)/2).

Terms

a(0) =2a(1) =4a(2) =8a(3) =32a(4) =64a(5) =256a(6) =512a(7) =2048a(8) =16384a(9) =32768a(10) =262144a(11) =1048576a(12) =2097152a(13) =8388608a(14) =67108864a(15) =536870912a(16) =1073741824a(17) =8589934592a(18) =34359738368a(19) =68719476736a(20) =549755813888