Education Development Center
Making Mathematics: Proof
This is a site which links to specific techniques of proving including conditional statements, proof by contradiction, mathematical induction, parity arguments, and the pigeonhole principle. The site contains a link to various example...
National Council of Teachers of Mathematics
Nctm: Figure This: Pigeonhole Principle
What is the possibility that another student in your school will have your exact same initials? Try this math challenge using logical reasoning and the "Pigeonhole principle," to discover how this could actually happen. An engaging...