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 A228722 #5 Aug 31 2013 17:13:19 %S A228722 0,1,2,3,4,5,6,7,8,9,10,10,12,12,14,14,16,16,18,18,18,21,21,23,23,25, %T A228722 25,27,27,29,30,30,32,32,34,34,36,36,38,38,38,41,41,43,43,45,45,47,47, %U A228722 49,50,50,52,52,54,54,56,56,58,58,58,61,61,63,63,65,65 %N A228722 Largest number <= n with alternating parities of digits in decimal representation. %C A228722 a(n) <= n <= A228723(n) and A228710(a(n)) = 1; %C A228722 a(A030141(n)) = A030141(n); a(A228709(n)) < A228709(n). %H A228722 Reinhard Zumkeller, <a href="/A228722/b228722.txt">Table of n, a(n) for n = 0..10000</a> %e A228722 a(100) = 98; a(1000) = 989; a(10000) = 9898; a(100000) = 98989; %e A228722 a(101) = 101; a(1001) = 989. %o A228722 (Haskell) %o A228722 a228722 n = head [x | x <- [n, n-1 ..], a228710 x == 1] %K A228722 nonn,base %O A228722 0,3 %A A228722 _Reinhard Zumkeller_, Aug 31 2013