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 A023804 #17 Mar 25 2021 12:39:46
%S A023804 0,1,2,3,4,5,6,7,8,9,11,12,13,14,15,16,17,18,19,21,22,23,24,25,26,27,
%T A023804 28,29,31,32,33,34,35,36,37,38,39,41,42,43,44,45,46,47,48,49,51,52,53,
%U A023804 54,55,56,57,58,59,61,62,63,64,65,66,67,68,69,71,72,73,74,75
%N A023804 Xenodromes: all digits in base 9 are different.
%C A023804 Last term is a(876809) = 381367044. - _Charles R Greathouse IV_, Jun 16 2012
%H A023804 Michael De Vlieger, <a href="/A023804/b023804.txt">Table of n, a(n) for n = 1..10000</a>
%H A023804 Michael De Vlieger, <a href="/A023804/a023804.txt">Extended b-file style table of all terms</a>
%e A023804 From _Michael De Vlieger_, Mar 24 2021: (Start)
%e A023804 Numbers 0 through 8 are in the sequence because these are single digits in base 9 (nonary).
%e A023804 9 is in the sequence because 9 = "10" in base 9, and both nonary digits are distinct.
%e A023804 11 is in the sequence because, though in decimal the number repeats the digit 1, in base 9, 11 is written "13", with 2 distinct digits.
%e A023804 (End)
%t A023804 Select[Range[0,80],Max[DigitCount[#,9]]==1&] (* _Harvey P. Dale_, Apr 26 2011 *)
%t A023804 (* Second program: generate all terms (less than a second): *)
%t A023804 Union@ Flatten@ Map[FromDigits[#, 9] & /@ Permutations[-1 + Position[Reverse@ #, 1][[All, 1]] ] &, IntegerDigits[Range[2, 2^9] - 1, 2] ] (* _Michael De Vlieger_, Mar 24 2021 *)
%o A023804 (Python)
%o A023804 from itertools import permutations
%o A023804 A023804_list = sorted(set(int(''.join(d),9) for k in range(1,10) for d in permutations('012345678',k))) # _Chai Wah Wu_, Mar 25 2021
%K A023804 nonn,base,fini,easy
%O A023804 1,3
%A A023804 _Olivier Gérard_