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 A023706 #12 Oct 04 2018 10:43:16
%S A023706 0,4,8,12,17,18,19,20,24,28,33,34,35,36,40,44,49,50,51,52,56,60,69,70,
%T A023706 71,73,74,75,77,78,79,81,82,83,84,88,92,97,98,99,100,104,108,113,114,
%U A023706 115,116,120,124,133,134,135,137,138,139,141
%N A023706 Numbers with a single 0 in their base 4 expansion.
%C A023706 Each member of the sequence is either 4*a+b for a in the sequence and b in {1,2,3}, or 4*a for a in A023705. - _Robert Israel_, Oct 04 2018
%H A023706 Robert Israel, <a href="/A023706/b023706.txt">Table of n, a(n) for n = 1..10000</a>
%p A023706 R23705:= {1,2,3}:
%p A023706 R:= {}: A:= 0;
%p A023706 for i from 1 to 4 do
%p A023706 R:= map(t ->4*t, R23705) union map(t -> (4*t+1,4*t+2,4*t+3), R);
%p A023706 R23705:= map(t -> (4*t+1,4*t+2,4*t+3), R23705);
%p A023706 A:= A, op(sort(convert(R,list)));
%p A023706 od:
%p A023706 A; # _Robert Israel_, Oct 04 2018
%t A023706 Select[ Range[ 0, 160 ], (Count[ IntegerDigits[ #, 4 ], 0 ]==1)& ]
%o A023706 (PARI) isok(n) = (n==0) || (#select(x->(x==0), digits(n, 4)) == 1); \\ _Michel Marcus_, Oct 04 2018
%Y A023706 Cf. A023705.
%K A023706 nonn,base,easy
%O A023706 1,2
%A A023706 _Olivier Gérard_