Composite numbers whose prime factors contain no digits other than 4 and 7.

Terms

a(0) =49a(1) =329a(2) =343a(3) =2209a(4) =2303a(5) =2401a(6) =15463a(7) =16121a(8) =16807a(9) =31129a(10) =52339a(11) =103823a(12) =108241a(13) =112847a(14) =117649a(15) =209009a(16) =217903a(17) =313439a(18) =334439a(19) =351419a(20) =366373a(21) =523229a(22) =542129a(23) =542339a(24) =544229a(25) =726761a(26) =757687a(27) =789929a(28) =823543a(29) =1463063