Wrong, But Useful: Episode 19


In this month's WBU spectacular:

  • Big MathsJam is coming up - book quickly!
  • The Relatively Prime Series 2 Kickstarter closes today. Donate quickly!
  • Dave's got a chapter in a book - order quickly!
  • Colin sails into the wind (virtually)
  • Dave's moving house over half term: what maths are the movers doing?
  • When will the first two-hour marathon be run? (Dave says the world record is currently 2:02:55, although it's actually 2:02:57).
  • @chrishazell72 got the last puzzle:

    WBUPuzzle solution: Alphabetically Begin Counting - Disregard Eight Forwards, Gale

  • This month's puzzle: Show that $n^4 + 4$ is not prime for integer $n \ge 2$
  • What Dave Would Like To Know More About: shuffling


Colin is a Weymouth maths tutor, author of several Maths For Dummies books and A-level maths guides. He started Flying Colours Maths in 2008. He lives with an espresso pot and nothing to prove.


4 comments on “Wrong, But Useful: Episode 19

  • Stephen Morris

    On shuffling. With 2N cards lable the cards from 0 to 2N-1 from bottom to the top of the pack. Suppose a perfect shuffle leaves 0 and 2N in place, moves 1 to 2, 2 to 4, … N to 1, N+1 to 3, …, 2N-1 to 2N-2. Ignoring the top card (which doesn’t move) each card, c, moves to 2c mod 2N-1. The number of shuffles to bring it back to the start is s where 2^s = 1 mod 2N-1.
    Thre is an alternative riffle shuffle does move the top and bottom cards. Then we have 2^s = 1 mod 2N+1.

    • Dave Gale

      Hi Stephen,
      Is it as simple as that? I was under the impression that we didn’t know a lot about it yet. (Sadly, all my maths books are packed up at present so I can’t find where I got that from.)
      I think I’d go with the ‘IN’ shuffle as an ‘Out’ shuffle can appear suspicious. Thanks for your comment.

  • Pingback: A nice little thought puzzle | cavmaths

  • Johan Strandberg

    On the first MathsJam, apparently everybody gave a talk. What percentage of the participants gave a talk at the zeroth MathsJam?

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