The estimation method is featured in a new book by Rob Eastaway.
Here is the main video on Numberphile:
And here are some extras:
In addition, my resident graph expert Dave Wiley (see previous blog) drew up two graphs to analyse the strength or weaknesses of zequals.
This first one shows what happens when you square a number.
The blue line is the true answer, the red line is a zequal plot.
See a hi-res version of this graph on Flickr.
The second graph is fascinating.
It shows what happens when you multiply two numbers (x and y axis) using the zequals technique.
White areas show where you answer is dead right.
Red areas show where your zequals answer is higher than the true answer... blue areas show where is it is too low!?
Again, a hi-res version is on Flickr.