Numbers with digits 4 and 7 only.

Terms

a(0) =4a(1) =7a(2) =44a(3) =47a(4) =74a(5) =77a(6) =444a(7) =447a(8) =474a(9) =477a(10) =744a(11) =747a(12) =774a(13) =777a(14) =4444a(15) =4447a(16) =4474a(17) =4477a(18) =4744a(19) =4747a(20) =4774a(21) =4777a(22) =7444a(23) =7447a(24) =7474a(25) =7477a(26) =7744a(27) =7747a(28) =7774a(29) =7777