Numbers with digits 7 and 9 only.

Terms

a(0) =7a(1) =9a(2) =77a(3) =79a(4) =97a(5) =99a(6) =777a(7) =779a(8) =797a(9) =799a(10) =977a(11) =979a(12) =997a(13) =999a(14) =7777a(15) =7779a(16) =7797a(17) =7799a(18) =7977a(19) =7979a(20) =7997a(21) =7999a(22) =9777a(23) =9779a(24) =9797a(25) =9799a(26) =9977a(27) =9979a(28) =9997a(29) =9999