Pigeonhole Principle

https://docs.google.com/presentation/d/1KVdIgNV7k75T7dqr0JrTKCE-Qusgs2SHRe7hnwG4lHQ/edit?usp=sharing

https://www.mathsisfun.com/sets/injective-surjective-bijective.html

https://staff.informatics.buu.ac.th/~ureerat/886203/Slides/ch6Combinatorics.ppt

https://emagazine.ipst.ac.th/204/index.html#23/z

https://math.uchicago.edu/~tghyde/PHP_sols.pdf