Numbers whose base-10 representation has the same nonzero number of 4's and 7's.

Terms

a(0) =47a(1) =74a(2) =147a(3) =174a(4) =247a(5) =274a(6) =347a(7) =374a(8) =407a(9) =417a(10) =427a(11) =437a(12) =457a(13) =467a(14) =470a(15) =471a(16) =472a(17) =473a(18) =475a(19) =476a(20) =478a(21) =479a(22) =487a(23) =497a(24) =547a(25) =574a(26) =647a(27) =674a(28) =704a(29) =714