Primes that contain digits 4 and 7 only.

Terms

a(0) =7a(1) =47a(2) =4447a(3) =7477a(4) =44777a(5) =47777a(6) =74747a(7) =77447a(8) =77477a(9) =77747a(10) =4444747a(11) =4447747a(12) =4747747a(13) =4774477a(14) =4774747a(15) =7444477a(16) =7447777a(17) =7474477a(18) =7477777a(19) =7747477a(20) =7774777a(21) =7777447a(22) =44447747a(23) =44747447a(24) =44747777a(25) =44774777a(26) =47447747a(27) =47774477a(28) =47774747