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 A231920 #8 Nov 16 2013 14:14:53
%S A231920 0,11,1,10,100,12,2,20,101,22,110,202,220,1000,102,21,111,122,212,221,
%T A231920 1001,112,121,200,211,222,1002,120,201,210,1011,1022,1101,1110,1202,
%U A231920 1220,2012,2021,2102,2120,2201,2210,10000,1010,1100,1111,1122,1212,1221,2002,2020
%N A231920 Working in base 3: 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 A231920 This is a permutation of the nonnegative integers in base 3 - see the Comments in A228407 for the proof.
%t A231920 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, 60, 0]; FromDigits@# & /@ IntegerDigits[s, 3]
%Y A231920 Cf. A230891, A231922, A231924, A231926, A231928, A231930, A231932, A228407, A231921.
%K A231920 nonn,base,easy
%O A231920 0,2
%A A231920 _N. J. A. Sloane_ and _Robert G. Wilson v_, Nov 15 2013