Posted by: Alexandre Borovik | June 4, 2013

A professional skill: parsing







I am proud that, after marking 220 examination scripts in first year linear algebra , I was still able to locate, at a glance, an error in this picture — thanks to skills in parsing of meaningless symbolic input developed over many years of teaching mathematics.



  1. Apart from the error (which indeed strikes the eye, since 9! is far beyond the range 1..12), why 1=9/9 is out of step? can’t one express 1 via three 9’s and not two?

  2. A good point: of course, 1 can be expressed via three 9’s.

  3. You are a professional indeed 🙂

  4. $9^{9-9}$

  5. Ahh, they meant to use those symbols in the opposite order. I wonder where the error crept in. Did someone give handwritten diagrams to someone else, who put it into a computer system wrong? Except for the errors, I love this clock. So much more accessible for students than the other math clocks I’m familiar with. Maybe I’ll search for a site that let’s you make your own, and get a four 4’s clock made.

    • I would like a 4 4’s clock too. But I found a 3 3’s clock on (they do have a lot more math clocks, but no 4 4’s).

  6. First google hit for four fours closk is hilarious:

  7. My big problem with this clock, is it assumes no negative hours can exist but uses roots!

  8. 5 is supposed to be (sqrt 9)!-(9/9)

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