Numbers whose base-10 representation has the same nonzero number of 2's and 7's.

Terms

a(0) =27a(1) =72a(2) =127a(3) =172a(4) =207a(5) =217a(6) =237a(7) =247a(8) =257a(9) =267a(10) =270a(11) =271a(12) =273a(13) =274a(14) =275a(15) =276a(16) =278a(17) =279a(18) =287a(19) =297a(20) =327a(21) =372a(22) =427a(23) =472a(24) =527a(25) =572a(26) =627a(27) =672a(28) =702a(29) =712