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 A231922 #7 Nov 16 2013 14:15:12
%S A231922 0,11,1,10,100,12,2,20,101,22,3,13,31,103,30,110,33,111,21,102,120,
%T A231922 201,210,1000,122,133,212,221,313,331,1001,112,23,32,113,131,223,232,
%U A231922 300,311,322,333,1003,123,132,213,231,312,321,1002,121,200,211,222,233,323,332
%N A231922 Working in base 4: a(0)=0, thereafter a(n+1) is the smallest number not already in the sequence such that the bits of a(n) and a(n+1) together can be rearranged to form a palindrome.
%C A231922 This is a permutation of the nonnegative integers in base 4 - see the Comments in A228407 for the proof.
%t A231922 a[0] = 0; a[n_] := a[n] = Block[{k = 1, idm = IntegerDigits[ a[n - 1], 4], t = a@# & /@ Range[n - 1]}, Label[ start]; While[ MemberQ[t, k], k++]; While[ Select[ Permutations[ Join[ idm, IntegerDigits[k, 4]]], #[[1]] != 0 && # == Reverse@# &] == {}, k++; Goto[ start]]; k]; s = Array[a, 60, 0]; FromDigits@# & /@ IntegerDigits[s, 4]
%Y A231922 Cf. A230891, A231920, A231924, A231926, A231928, A231930, A231932, A228407, A231923.
%K A231922 nonn,base,easy
%O A231922 0,2
%A A231922 _N. J. A. Sloane_ and _Robert G. Wilson v_, Nov 15 2013