I have an exam in combinatorics on Friday and the pigeonhole principle is a part of the material. Can someone give me a reference to a book with the hardest(!) questions on this material? Thank you ...
How small parallelograms are we guaranteed to get, when we select the two sides from different plane lattices?
This a shortened version (motivation from telecommunications stripped away) of a question I asked in MO in late May (no answers). I am mostly checking, if somebody has seen this or a related question ...