Xenodromes: all digits in base 4 are different.

Terms

a(0) =0a(1) =1a(2) =2a(3) =3a(4) =4a(5) =6a(6) =7a(7) =8a(8) =9a(9) =11a(10) =12a(11) =13a(12) =14a(13) =18a(14) =19a(15) =24a(16) =27a(17) =28a(18) =30a(19) =33a(20) =35a(21) =36a(22) =39a(23) =44a(24) =45a(25) =49a(26) =50a(27) =52a(28) =54a(29) =56