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.
Mathematics under the Microscope 
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?
By: Sergei Yakovenko on June 4, 2013
at 7:42 am
A good point: of course, 1 can be expressed via three 9’s.
By: Anonymous on June 4, 2013
at 8:26 am
By: Sergei Yakovenko on June 4, 2013
at 8:30 am
You are a professional indeed 🙂
By: Anonymous on June 4, 2013
at 8:36 am
$9^{9-9}$
By: temur on June 4, 2013
at 1:20 pm
By: Sergei Yakovenko on June 4, 2013
at 2:29 pm
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.
By: suevanhattum on June 4, 2013
at 4:03 pm
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).
By: Ton Jeursen on December 1, 2016
at 1:43 pm
First google hit for four fours closk is hilarious: http://www.taliscope.com/clock_4444_Base.html
By: suevanhattum on June 4, 2013
at 4:04 pm
My big problem with this clock, is it assumes no negative hours can exist but uses roots!
By: Alice on June 14, 2013
at 1:10 am
5 is supposed to be (sqrt 9)!-(9/9)
By: Ken on January 12, 2017
at 8:00 am