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 A043090 #8 Nov 24 2018 15:19:31
%S A043090 0,1,2,3,4,6,7,8,9,11,12,13,14,17,18,19,24,25,27,28,29,30,33,34,35,36,
%T A043090 38,39,44,45,46,49,50,51,52,54,55,56,57,59,68,70,71,72,73,75,76,77,78,
%U A043090 97,98,99,100,102,103,108,109,110,113,114,115
%N A043090 Every string of 2 consecutive base 4 digits contains exactly 2 distinct numbers.
%C A043090 No two consecutive base 4 digits are the same. - _Harvey P. Dale_, Nov 24 2018
%F A043090 A031942 UNION {0}. [From _R. J. Mathar_, Oct 20 2008]
%t A043090 Select[Range[0,120],SequenceCount[IntegerDigits[#,4],{x_,x_}]==0&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Nov 24 2018 *)
%K A043090 nonn,base
%O A043090 0,3
%A A043090 _Clark Kimberling_