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 A231921 #7 Nov 16 2013 14:15:03
%S A231921 0,4,1,3,9,5,2,6,10,8,12,20,24,27,11,7,13,17,23,25,28,14,16,18,22,26,
%T A231921 29,15,19,21,31,35,37,39,47,51,59,61,65,69,73,75,81,30,36,40,44,50,52,
%U A231921 56,60,68,70,72,76,80,82,32,34,38,42,46,48,54,58,62,64,66,74,78
%N A231921 Terms of A231920 written in base 10: the binary expansions of a(n) and a(n+1) taken together can be rearranged to form a palindrome.
%C A231921 See A231920 for precise definition.
%C A231921 This is a permutation of the nonnegative integers - see the Comments in A228407 for the proof.
%t A231921 a[0] = 0; a[n_] := a[n] = Block[{k = 1, idm = IntegerDigits[ a[n - 1], 3], t = a@# & /@ Range[n - 1]}, Label[ start]; While[ MemberQ[t, k], k++]; While[ Select[ Permutations[ Join[ idm, IntegerDigits[k, 3]]], #[[1]] != 0 && # == Reverse@# &] == {}, k++; Goto[ start]]; k]; s = Array[a, 100, 0]
%Y A231921 Cf. A230892, A231923, A231925, A231927, A231929, A231931, A231933, A228407, A231920.
%K A231921 nonn,base,easy
%O A231921 0,2
%A A231921 _N. J. A. Sloane_ and _Robert G. Wilson v_, Nov 15 2013