Semiprimes whose decimal representation has only digits in {4,5,7}.

Terms

a(0) =4a(1) =55a(2) =57a(3) =74a(4) =77a(5) =445a(6) =447a(7) =454a(8) =545a(9) =554a(10) =745a(11) =755a(12) =4474a(13) =4555a(14) =4574a(15) =4577a(16) =4747a(17) =4754a(18) =4757a(19) =4777a(20) =5447a(21) =5455a(22) =5545a(23) =5554a(24) =5747a(25) =5755a(26) =5774a(27) =5777a(28) =7445a(29) =7447