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 A026140 #8 Aug 28 2021 02:53:12
%S A026140 0,1,3,4,7,9,10,12,13,16,19,21,22,25,27,28,30,31,34,36,37,39,40,43,46,
%T A026140 48,49,52,55,57,58,61,63,64,66,67,70,73,75,76,79,81,82,84,85,88,90,91,
%U A026140 93,94,97,100,102,103,106,108,109,111,112,115,117,118
%N A026140 a(n) = (1/2)*(s(n) - 1), where s = A026139.
%H A026140 Michael De Vlieger, <a href="/A026140/b026140.txt">Table of n, a(n) for n = 1..5004</a>
%t A026140 Block[{a, r, s, nn = 119}, a[1] = 1; Do[If[! IntegerQ[a[#1]], Set[a[#1], i], Set[a[#2], i]] & @@ {i - #, i + #} &@ Floor[i/2], {i, 2 nn}]; s = TakeWhile[Array[a[#] &, 2 nn], IntegerQ]; Map[(1/2) (# - 1) &, Union@ FoldList[Max, s]]] (* _Michael De Vlieger_, Aug 27 2021 *)
%Y A026140 Cf. A026136, A026139.
%K A026140 nonn
%O A026140 1,3
%A A026140 _Clark Kimberling_