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 A291790 Numbers whose trajectory under iteration of the map k -> (sigma(k)+phi(k))/2 consists only of integers and is unbounded. 9
 270, 290, 308, 326, 327, 328, 352, 369, 393, 394, 395, 396, 410, 440, 458, 459, 465, 496, 504, 510, 525, 559, 560, 570, 606, 616, 620, 685, 686, 702, 712, 725, 734, 735, 737, 738, 745, 746, 747, 783, 791, 792, 805, 806, 813, 814, 815, 816, 828 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It would be nice to have a proof that these trajectories are integral and unbounded, or, of course, that they eventually reach a fractional value (and die), or reach a prime (which is then a fixed point). (Cf. A291787.) If either of the last two things happen, then that value of n will be removed from the sequence. AT PRESENT ALL TERMS ARE CONJECTURAL. When this sequence was submitted, there was a hope that it would be possible to prove that these trajectories were indeed integral and unbounded. This has not yet happened, although see the remarks of Andrew R. Booker in A292108. - N. J. A. Sloane, Sep 25 2017 LINKS Hugo Pfoertner, Table of n, a(n) for n = 1..82 N. J. A. Sloane, Three (No, 8) Lovely Problems from the OEIS, Experimental Mathematics Seminar, Rutgers University, Oct 05 2017, Part I, Part 2, Slides. (Mentions this sequence) CROSSREFS Cf. A000010, A000203, A289997, A290001, A291789 (the trajectory of 270), A291787, A292108. For the "seeds" see A292766. Sequence in context: A053322 A274328 A025393 * A025394 A291789 A292766 Adjacent sequences:  A291787 A291788 A291789 * A291791 A291792 A291793 KEYWORD nonn AUTHOR N. J. A. Sloane, Sep 03 2017 EXTENSIONS More terms from Hugo Pfoertner, Sep 03 2017 STATUS approved

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Last modified January 17 04:29 EST 2021. Contains 340214 sequences. (Running on oeis4.)