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 A251237 #14 Oct 01 2018 11:52:28
%S A251237 2,4,6,8,10,12,14,16,18,20,25,27,29,31,33,35,37,39,41,44,46,48,50,52,
%T A251237 54,56,58,60,63,65,67,69,71,73,75,77,80,82,84,86,89,91,93,95,97,99,
%U A251237 102,104,106,108,110,112,115,117,119,121,123,125,128,130,133,135
%N A251237 Indices of even numbers in A098550.
%C A251237 A098550(a(n)) mod 2 = 0;
%C A251237 a(n+1) > a(n) + 1;
%C A251237 there are infinitely many even terms in A098550, for proof: see comment #4 in A098550.
%H A251237 Reinhard Zumkeller, <a href="/A251237/b251237.txt">Table of n, a(n) for n = 1..10000</a>
%H A251237 David L. Applegate, Hans Havermann, Bob Selcoe, Vladimir Shevelev, N. J. A. Sloane, and Reinhard Zumkeller, <a href="http://arxiv.org/abs/1501.01669">The Yellowstone Permutation</a>, arXiv preprint arXiv:1501.01669, 2015 and <a href="https://cs.uwaterloo.ca/journals/JIS/VOL18/Sloane/sloane9.html">J. Int. Seq. 18 (2015) 15.6.7</a>.
%t A251237 f[lst_] := Block[{k = 4}, While[GCD[lst[[-2]], k] == 1 || GCD[lst[[-1]], k] > 1 || MemberQ[lst, k], k++]; Append[lst, k]];
%t A251237 Position[Nest[f, {1, 2, 3}, 140], _?EvenQ] // Flatten (* _Jean-François Alcover_, Oct 01 2018, after _Robert G. Wilson v_ in A098550 *)
%o A251237 (Haskell)
%o A251237 a251237 n = a251237_list !! (n-1)
%o A251237 a251237_list = filter (even . a098550) [1..]
%Y A251237 Cf. A098550, A005843, A251238, A251393.
%Y A251237 First row of array A251716.
%K A251237 nonn
%O A251237 1,1
%A A251237 _Reinhard Zumkeller_, Dec 02 2014