M.C. Escher: More Mathematics Than Meets the Eye
im-possible.info
im-possible.info
(For the record, I do understand this article explores a different mathematical aspect of Escher than Hofstadter)
One of the most captivating talks I've ever seen. He's a great speaker. I wish I had him as a professor.
EDIT: forgot to add, in the Mathematica blog there is another explanation of the mathematics between the "Droste effect" and the code to obtain it: http://blog.wolfram.com/2009/04/24/droste-effect-with-mathem...
The interpretation of the blank spot is also quite nice and very Escher-esque, with the content literally jumping out of the frame. Probably Escher left the center patch blank because his construction method would have him exceed some technical/physical limitation, so instead of drawing an approximation he preferred a void.