• Number Sequences

    From David Entwistle@qnivq.ragjvfgyr@ogvagrearg.pbz to rec.puzzles on Sun Sep 27 11:59:49 2026
    From Newsgroup: rec.puzzles

    The first puzzle appears as the first question in 'One Hundred Problems in Elementary Mathematics' by Hugo Steinhaus.

    ISBN 0-486-23875-X

    1. Exercise on the multiplication table

    We construct a sequence of numbers as follows: The first number is 2, the
    next is 3,

    2.3 = 6

    the third number in the sequence is 6,

    3.6 = 18

    the fourth number is 1,and the fifth is 8,

    6.1 = 6, 1.8 = 8

    the sixth number is 6, then follows 8, etc.

    This is the sequence we obtain:

    2,3,6,1,8,6,8,4,8,4,8,3,2,3,2,3,2,3,2,2,4...

    Prove that the numbers 5, 7 and 9 never appear in this sequence.

    This second puzzle was chosen as puzzle of the week at MathPuzzle.com.
    This puzzle is copyright Erich Friedman.

    https://erich-friedman.github.io/puzzle/mathpuzzle/

    1. Form a sequence of digits by starting with 1 and 2, and letting each
    the next digit(s) be the product of the two previous digits:

    1,2,2,4,8,3,2,2,4,6,4,8,2,4,2,4,3,2,1,6,8...

    Show that the numbers 0, 5, 7, and 9 never appear, but arbitrarily long sequences of 8's appear.

    I think I'm okay with those questions, but I'm not sure about this - can
    you prove that neither sequence is periodic? I think I can, as I've seen
    it demonstrated for another related sequence.
    --
    David Entwistle
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  • From James Dow Allen@user4353@newsgrouper.org.invalid to rec.puzzles on Tue Sep 29 21:08:07 2026
    From Newsgroup: rec.puzzles



    As spoiler warning space, I show some of the Witches' lines
    from the play MacBeth.

    FIRST WITCH. When shall we three meet again?
    In thunder, lightning, or in rain?
    SECOND WITCH. When the hurlyburly's done,
    When the battle's lost and won.
    THIRD WITCH. That will be ere the set of sun.
    FIRST WITCH. Where the place?
    SECOND WITCH. Upon the heath.
    THIRD WITCH. There to meet with Macbeth.
    FIRST WITCH. I come, Graymalkin.
    ALL. Paddock calls. Anon!
    Fair is foul, and foul is fair.
    Hover through the fog and filthy air. Exeunt.

    Enter the three Witches.

    FIRST WITCH. Where hast thou been, sister?
    SECOND WITCH. Killing swine.
    THIRD WITCH. Sister, where thou?
    FIRST WITCH. A sailor's wife had chestnuts in her lap,
    And mounch'd, and mounch'd, and mounch'd. "Give me," quoth I.
    "Aroint thee, witch!" the rump-fed ronyon cries.
    Her husband's to Aleppo gone, master the Tiger;
    But in a sieve I'll thither sail,
    And, like a rat without a tail,
    I'll do, I'll do, and I'll do.
    SECOND WITCH. I'll give thee a wind.
    FIRST WITCH. Thou'rt kind.
    THIRD WITCH. And I another.
    FIRST WITCH. I myself have all the other,
    And the very ports they blow,
    All the quarters that they know
    I' the shipman's card.
    I will drain him dry as hay:
    Sleep shall neither night nor day
    Hang upon his penthouse lid;
    He shall live a man forbid.
    Weary se'nnights nine times nine
    Shall he dwindle, peak, and pine;
    Though his bark cannot be lost,
    Yet it shall be tempest-toss'd.
    Look what I have.
    SECOND WITCH. Show me, show me.
    FIRST WITCH. Here I have a pilot's thumb,
    Wreck'd as homeward he did come. Drum within.
    THIRD WITCH. A drum, a drum!
    Macbeth doth come.
    ALL. The weird sisters, hand in hand,
    Posters of the sea and land,
    Thus do go about, about,
    Thrice to thine, and thrice to mine,
    And thrice again, to make up nine.
    Peace! The charm's wound up.

    ---------------------------------------------------


    David Entwistle <qnivq.ragjvfgyr@ogvagrearg.pbz> posted:
    1,2,2,4,8,3,2,2,4,6,4,8,2,4,2,4,3,2,1,6,8...

    Gur irel arkg gjb qvtvgf va guvf frdhrapr ner rnpu rvtug,
    guhf cebqhpvat guerr pbafrphgvir rvtug'f -- frr orybj.



    Show that the numbers 0, 5, 7, and 9 never appear, but arbitrarily long sequences of 8's appear.


    Jr svefg cebir gung gur "onq" qvtvgf -- 0, 5, 7, 9 -- pnaabg bpphe. Vg vf fhssvpvrag gb cebir gung gur svefg onq qvtvg qbrfa'g bpphe. Sbe rknzcyr
    (9,8) --> (7,2) ohg gung onq 7 vf bs ab jbeel fvapr vg jnf cebqhprq ol
    na rneyvre onq 9. Vafcrpgvba fubjf gung (3,3) --> (9) vf gur bayl jbeel.
    Ab tbbq qvtvgf cebqhpr (k3) fb (3)(32) naq (3)(3) ner gur bayl jnlf gb
    trg gur haqrfverq (3,3).

    (3) vf cebqhprq bayl ivn 1&3 naq arvgure (1,k) abe (3,k) cebqhpr (32).
    (3,3) pna or cebqhprq ol (1,3,1) be (3,1,3), rvgure bs juvpu erdhverf na rneyvre (3,1).

    Fb ab onq qvtvgf pna nevfr.

    X pbafrphgvir vafgnaprf bs (8) cebqhpr X-1 vafgnaprf bs (64) juvpu gura
    lvryq 2X-3 pbafrphgvir vafgnaprf bs (24) juvpu cebqhpr 4X-7 pbafrphgvir
    (8)f. Vs X=3, 4X-7 vf 5, fb 3 8'f lvryq 5 8'f juvpu lvryq 13 8'f
    lvryqvat 45 8'f naq fb ba. Naq jr QB unir 888 nf jnf zragvbarq nobir.


    I think I'm okay with those questions, but I'm not sure about this - can
    you prove that neither sequence is periodic? I think I can, as I've seen
    it demonstrated for another related sequence.


    Tvira gung OBGU gur frdhraprf lbh qrsvar raq hc jvgu rire-tebjvat
    frdhraprf bs 8'f, V guvax guvf vf gur rnfvrfg dhrfgvba! Nal crevbq
    zhfg or n svavgr crevbq ohg gur frdhraprf bs pbafrphgvir 8'f tebj
    jvgubhg yvzvg.

    Znlor gurer'f n cebbs gung qbrfa'g eryl ba fcrpvsvp tebjvat frdhraprf,
    ohg V'z pbagrag ...


    Purref,
    wnzrfqbjnyyra @ Tznvy

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