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 A253048 #13 Sep 03 2018 02:53:43
%S A253048 4,6,10,33,18,42,48,58,70,82,90,102,110,120,126,140,144,154,150,182,
%T A253048 180,198,200,220,216,228,272,252,280,270,300,306,312,330,336,340,348,
%U A253048 360,390,392,396,400,442,438,468,440,506,480,484,500,486,518,528,540,574,546,570,572,616
%N A253048 List of composite numbers in A098550 which appear immediately after a prime.
%H A253048 Reinhard Zumkeller, <a href="/A253048/b253048.txt">Table of n, a(n) for n = 1..10000</a>
%H A253048 David L. Applegate, Hans Havermann, Bob Selcoe, Vladimir Shevelev, N. J. A. Sloane, and Reinhard Zumkeller, The Yellowstone Permutation, 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 A253048 f[lst_] := Block[{k = 4}, While[GCD[lst[[-2]], k] == 1 || GCD[lst[[-1]], k] > 1 || MemberQ[lst, k], k++]; Append[lst, k]];
%t A253048 A098550 = Nest[f, {1, 2, 3}, 1000];
%t A253048 sp = SequencePosition[A098550, {_?PrimeQ, _?CompositeQ}] [[All, 2]];
%t A253048 A098550[[sp]] (* _Jean-François Alcover_, Sep 03 2018, after _Robert G. Wilson v_ in A098550 *)
%o A253048 (Haskell)
%o A253048 a253048 n = a253048_list !! (n-1)
%o A253048 a253048_list = filter ((== 0) . a010051') $ map a253049 [1..]
%o A253048 -- _Reinhard Zumkeller_, Dec 30 2014
%Y A253048 Cf. A098550. Subsequence of A253049.
%Y A253048 Cf. A010051, A251239.
%K A253048 nonn
%O A253048 1,1
%A A253048 _N. J. A. Sloane_, Dec 27 2014