I've seen Lenstra's talk about Escher's print gallery some years ago in Rome and then again after a few months in Pisa. One of the most captivating talks I've

I've seen Lenstra's talk about Escher's print gallery some years ago in Rome and then again after a few months in Pisa. 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-mathematica/ http://blog.wolfram.com/2009/04/24/droste-effect-with-mathem...