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.

## Responses

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.

• $(\sqrt 9\cdot\sqrt 9)/9$ 😉

3. You are a professional indeed 🙂

4. $9^{9-9}$

• $=(\frac 99)^9$ 😉

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 zazzle.com (they do have a lot more math clocks, but no 4 4’s).

6. First google hit for four fours closk is hilarious: http://www.taliscope.com/clock_4444_Base.html

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)