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 A088698 #16 Feb 20 2021 23:11:06
%S A088698 0,3,6,15,12,27,30,63,24,51,54,111,60,123,126,255,48,99,102,207,108,
%T A088698 219,222,447,120,243,246,495,252,507,510,1023,96,195,198,399,204,411,
%U A088698 414,831,216,435,438,879,444,891,894,1791,240,483,486,975,492,987,990
%N A088698 Replace 1 with 11 in binary representation of n.
%H A088698 Rémy Sigrist, <a href="/A088698/b088698.txt">Table of n, a(n) for n = 0..8192</a>
%F A088698 a(0)=0, a(2n) = 2a(n), a(2n+1) = 4a(n) + 3.
%e A088698 n=9: 1001 -> 110011 = 51, so a(9) = 51.
%o A088698 (PARI) a(n)=if(n<1,0,if(n%2==0,2*a(n/2),4*a((n-1)/2)+3))
%o A088698 (Python)
%o A088698 def a(n): return int(bin(n)[2:].replace('1', '11'), 2)
%o A088698 print([a(n) for n in range(55)]) # _Michael S. Branicky_, Feb 20 2021
%Y A088698 Ordered terms plus one are in A048297.
%Y A088698 Same sequence sorted into ascending order: A277335, A290258 (without 0).
%Y A088698 Cf. A001196, A084471, A048678, A088697.
%Y A088698 Main diagonal of A341520, right edge of A341521.
%K A088698 nonn,easy,base
%O A088698 0,2
%A A088698 _Ralf Stephan_, Oct 07 2003