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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

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.

CROSSREFS

Cf. A098550, A251542-A251544, A253056.

Sequence in context: A276319 A038805 A192162 * A228642 A230834 A120535

Adjacent sequences:  A251542 A251543 A251544 * A251546 A251547 A251548

KEYWORD

nonn

AUTHOR

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

STATUS

approved

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Last modified May 27 22:57 EDT 2017. Contains 287210 sequences.