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 A180412 #13 Apr 16 2023 13:41:16
%S A180412 1,2,3,4,5,6,7,8,9,20,31,40,51,60,71,80,91,200,311,400,511,600,711,
%T A180412 800,911,2000,3111,4000,5111,6000,7111,8000,9111,20000,31111,40000,
%U A180412 51111,60000,71111,80000,91111,200000,311111,400000,511111,600000,711111,800000,911111,2000000
%N A180412 Lexicographically earliest increasing sequence of numbers with all odd digits alternating with numbers with all even digits.
%H A180412 Harvey P. Dale, <a href="/A180412/b180412.txt">Table of n, a(n) for n = 1..1000</a>
%e A180412 11 is all odd digits. The next number in which all of the digits are even is 20. The next number in which all of the digits are odd is 31.
%t A180412 nxt[n_]:=Module[{fidn=First[IntegerDigits[n]],len=IntegerLength[n]},Which[fidn==9,2*10^len, EvenQ[ fidn], FromDigits[PadRight[{fidn+1},len,1]],OddQ[fidn],FromDigits[PadRight[ {fidn+ 1}, len,0]]]]; NestList[nxt,1,50] (* _Harvey P. Dale_, Nov 26 2013 *)
%Y A180412 Cf. A014261, A014263.
%K A180412 easy,nonn,base
%O A180412 1,2
%A A180412 _Dominick Cancilla_, Sep 02 2010
%E A180412 Name edited by _Michel Marcus_, Apr 16 2023