Composite numbers whose prime factors contain no digits other than 3 and 7.

Terms

a(0) =9a(1) =21a(2) =27a(3) =49a(4) =63a(5) =81a(6) =111a(7) =147a(8) =189a(9) =219a(10) =243a(11) =259a(12) =333a(13) =343a(14) =441a(15) =511a(16) =567a(17) =657a(18) =729a(19) =777a(20) =999a(21) =1011a(22) =1029a(23) =1119a(24) =1323a(25) =1369a(26) =1533a(27) =1701a(28) =1813a(29) =1971