A251545 A251544, sorted.

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

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

Table of n, a(n) for n=1..56.

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

Cf. A098550, A251542-A251544, A253056.

Sequence in context: A276319 A038805 A192162 * A228642 A334261 A290434

Adjacent sequences: A251542 A251543 A251544 * A251546 A251547 A251548

Keyword

nonn

Author

David Applegate and N. J. A. Sloane, Dec 16 2014

Status

approved