A small collection of problems, books, and other things I have come across and found interesting enough to keep a note of. Some problems have solutions, others may not, and their original sources are not always known. There is no particular theme or order.
1. Three Circles and Seven Tokens. Seven white tokens occupy the seven regions of three overlapping circles. In one move, we may either flip all four tokens inside a chosen circle (white to black and black to white), or make all tokens in that circle white. Can we reach a configuration in which only the central token, common to all three circles, is black? Problem and solution (PDF)
A list of books I have read, with occasional notes.