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 A033001 #26 Mar 13 2021 17:04:12
%S A033001 4,8,36,44,72,76,328,332,396,400,652,656,684,692,2952,2960,2988,2992,
%T A033001 3568,3572,3600,3608,5868,5876,5904,5908,6160,6164,6228,6232,26572,
%U A033001 26576,26640,26644,26896,26900,26928,26936,32112,32120
%N A033001 Every run of digits of n in base 3 has length 2.
%C A033001 See A043291 for the base 2 version (which has a very simple formula), A033002 - A033014 for bases 4 through 16, A033015 - A033029 for the variants with runs of length >= 2. - _M. F. Hasler_, Feb 01 2014
%H A033001 Vincenzo Librandi, <a href="/A033001/b033001.txt">Table of n, a(n) for n = 1..250</a>
%F A033001 a(n)=4*A043307(n). - _M. F. Hasler_, Feb 01 2014
%t A033001 Select[Range[10000], Union[Length/@Split[IntegerDigits[#, 3]]]=={2}&] (* _Vincenzo Librandi_, Feb 05 2014 *)
%o A033001 From _M. F. Hasler_, Feb 01 2014 (Start)
%o A033001 (PARI) is_A033001(n)=!until(!n\=9,bittest(4588304,n%27)||return)
%o A033001 (PARI) for(n=1,9999,is_A033001(n)&&print1(n",")) \\ (End)
%o A033001 (PARI) a(n) = my(v=binary(n+1)); v[1]=0; for(i=2,#v, v[i]+=(v[i]>=v[i-1])); 4*fromdigits(v,9); \\ _Kevin Ryde_, Mar 13 2021
%K A033001 nonn,base
%O A033001 1,1
%A A033001 _Clark Kimberling_