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 A033004 #14 Feb 05 2014 07:07:26
%S A033004 7,14,21,28,35,252,266,273,280,287,504,511,525,532,539,756,763,770,
%T A033004 784,791,1008,1015,1022,1029,1043,1260,1267,1274,1281,1288,9079,9086,
%U A033004 9093,9100,9107,9576,9583,9597,9604,9611,9828,9835
%N A033004 Every run of digits of n in base 6 has length 2.
%C A033004 See A043291 and A033001 through A033014 for the analog in other bases, A033015 - A033029 for the variants with run lengths >= 2. - _M. F. Hasler_, Feb 02 2014
%H A033004 Vincenzo Librandi, <a href="/A033004/b033004.txt">Table of n, a(n) for n = 1..700</a>
%F A033004 a(n) = 7*A043310(n) (= 7*n for n<6). - _M. F. Hasler_, Feb 02 2014
%t A033004 Select[Range[10000], Union[Length/@Split[IntegerDigits[#, 6]]]=={2}&] (* _Vincenzo Librandi_, Feb 05 2014 *)
%Y A033004 Cf. A033007, A033010.
%K A033004 nonn,base
%O A033004 1,1
%A A033004 _Clark Kimberling_