Proof by...
Counterexample
- show an example which does not fit with the theorem
- QED (the theorem is disproven)
Contradiction
- assume the opposite of the theorem
- derive a contradiction
- QED (the theorem is proven)
Induction
- prove for a base case (e.g., n = 1)
- assume for an anonymous value (n)
- prove for the next value (n + 1)
- QED