Inspired by this question...
A friend and I have been meeting informally with a retired professor to do math on a weekly basis for several semesters now. He is a dear mentor to both of us. We would like to get him a Christmas present, but he is a very private person and rarely discusses his home life, so we know almost nothing about him outside of his mathematical interests. We could get him a book, but chances are good that he would already have it. (His library is enormous, and he's always bringing in obscure texts.)
- A copy of The Emperor's Club? This is a movie about a teacher and his influence on his students. I'm worried that might be a bit cheesy or inappropriate.
- Something mathematical, but more "popular" that he is less likely to have, like a film or a mathematics-strewn crime novel. (No textbooks!)
- A pie with a $\pi$ carved in the crust.
We're really stumped.