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

Terms

a(0) =4a(1) =32a(2) =256a(3) =2048a(4) =16384a(5) =131072a(6) =1048576a(7) =8388608a(8) =67108864a(9) =536870912a(10) =4294967296a(11) =34359738368a(12) =274877906944