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 A065031 #13 Mar 31 2017 22:06:16
%S A065031 2,1,2,1,2,1,2,1,2,1,12,11,12,11,12,11,12,11,12,11,22,21,22,21,22,21,
%T A065031 22,21,22,21,12,11,12,11,12,11,12,11,12,11,22,21,22,21,22,21,22,21,22,
%U A065031 21,12,11,12,11,12,11,12,11,12,11,22,21,22,21,22,21,22,21,22,21
%N A065031 In the decimal expansion of n, replace each odd digit with 1 and each even digit with 2.
%C A065031 A196563(a(n)) = A196563(n); A196564(a(n)) = A196564(n).
%H A065031 Reinhard Zumkeller, <a href="/A065031/b065031.txt">Table of n, a(n) for n = 0..10000</a>
%e A065031 a(123)=121 because 1 and 3 are odd and 2 is even.
%t A065031 Table[FromDigits[If[OddQ[#],1,2]&/@IntegerDigits[n]],{n,0,120}] (* _Harvey P. Dale_, Jun 08 2014 *)
%o A065031 (Haskell)
%o A065031 a065031 n = f n where
%o A065031 f x | x < 10 = 2 - x `mod` 2
%o A065031 | otherwise = 10 * (f x') + 2 - m `mod` 2
%o A065031 where (x',m) = divMod x 10
%o A065031 -- _Reinhard Zumkeller_, Feb 22 2012
%K A065031 base,nonn
%O A065031 0,1
%A A065031 _Santi Spadaro_, Nov 03 2001