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A251545 A251544, sorted. 3
4, 9, 21, 25, 26, 33, 58, 82, 85, 93, 95, 111, 115, 129, 177, 213, 237, 265, 267, 309, 321, 329, 335, 365, 381, 411, 427, 505, 519, 545, 565, 579, 581, 597, 633, 655, 679, 687, 699, 723, 753, 755, 785, 789, 831, 835, 879, 895, 921, 951, 973, 985, 1043, 1047, 1115, 1135 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
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.)
It would be nice to have a characterization of these numbers that is independent of A098550.
LINKS
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 J. Int. Seq. 18 (2015) 15.6.7.
MATHEMATICA
terms = 56;
max = 15 terms;
f[lst_] := Block[{k = 4}, While[GCD[lst[[-2]], k] == 1 || GCD[lst[[-1]], k] > 1 || MemberQ[lst, k], k++]; Append[lst, k]];
A098550 = Nest[f, {1, 2, 3}, max - 3];
sel = Select[Transpose[{Range[max], A098550}], PrimeQ[#[[2]]]&][[All, 1]]+2;
Sort[A098550[[sel]]][[1 ;; terms]] (* Jean-François Alcover, Sep 05 2018, after Robert G. Wilson v in A098550 *)
CROSSREFS
Sequence in context: A276319 A038805 A192162 * A228642 A334261 A290434
KEYWORD
nonn
AUTHOR
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)