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%I #20 Sep 05 2018 14:45:35
%S 4,9,21,25,26,33,58,82,85,93,95,111,115,129,177,213,237,265,267,309,
%T 321,329,335,365,381,411,427,505,519,545,565,579,581,597,633,655,679,
%U 687,699,723,753,755,785,789,831,835,879,895,921,951,973,985,1043,1047,1115,1135
%N A251544, sorted.
%C 4, 9, and 25 are squares. It appears that all the remaining terms are the products of two distinct primes, usually both odd. (But not all such numbers occur, of course.)
%C It would be nice to have a characterization of these numbers that is independent of A098550.
%H 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 terms = 56;
%t max = 15 terms;
%t f[lst_] := Block[{k = 4}, While[GCD[lst[[-2]], k] == 1 || GCD[lst[[-1]], k] > 1 || MemberQ[lst, k], k++]; Append[lst, k]];
%t A098550 = Nest[f, {1, 2, 3}, max - 3];
%t sel = Select[Transpose[{Range[max], A098550}], PrimeQ[#[[2]]]&][[All,1]]+2;
%t Sort[A098550[[sel]]][[1 ;; terms]] (* _Jean-François Alcover_, Sep 05 2018, after _Robert G. Wilson v_ in A098550 *)
%Y Cf. A098550, A251542-A251544, A253056.
%K nonn
%O 1,1
%A _David Applegate_ and _N. J. A. Sloane_, Dec 16 2014
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