Xenodromes: all digits in base 7 are different.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =4a(5) =5a(6) =6a(7) =7a(8) =9a(9) =10a(10) =11a(11) =12a(12) =13a(13) =14a(14) =15a(15) =17a(16) =18a(17) =19a(18) =20a(19) =21a(20) =22a(21) =23a(22) =25a(23) =26a(24) =27a(25) =28a(26) =29a(27) =30a(28) =31a(29) =33