This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A308540 #15 Jul 24 2019 14:45:43 %S A308540 0,1,2,3,4,5,6,7,8,9,10,22,33,44,55,66,77,88,99,100,332,554,776,998, %T A308540 1001,3322,5544,7766,9988,33220,33221,55445,77667,99889,332200,332211, %U A308540 776674,776675,3322008,3322009,7766741,7766755,33220088,33220099,77667411 %N A308540 a(n) is the least nonnegative number whose digits do not appear in order (not necessarily consecutively) in the concatenation of all previous terms. %C A308540 This sequence is a variant of A048991. %H A308540 Rémy Sigrist, <a href="/A308540/a308540.txt">C program for A308540</a> %e A308540 The first terms are necessarily the one-digit numbers: a(1) = 0, a(2) = 1, ..., a(10) = 9. %e A308540 The number 10 does not appear in "0123456789", hence a(11) = 10. %e A308540 The digits of every number from 11 to 21 appear in order in "012345678910", but this is not the case for the number 22, hence a(12) = 22. %o A308540 (C) See Links section. %Y A308540 See A309340 for the binary variant. %Y A308540 Cf. A048991. %K A308540 nonn,base %O A308540 1,3 %A A308540 _Rémy Sigrist_, Jul 24 2019