1 and the prime powers p^m where m >= 2, thus excluding the primes.

Terms

a(0) =1a(1) =4a(2) =8a(3) =9a(4) =16a(5) =25a(6) =27a(7) =32a(8) =49a(9) =64a(10) =81a(11) =121a(12) =125a(13) =128a(14) =169a(15) =243a(16) =256a(17) =289a(18) =343a(19) =361a(20) =512a(21) =529a(22) =625a(23) =729a(24) =841a(25) =961a(26) =1024a(27) =1331a(28) =1369a(29) =1681