# Resource: Types of proofs

Methods/types of proofs exist for various purposes and scientific fields. Philosophical, psychological, medical, biological etc. Here we focus on mathematical and physical proofs. Mathematical proofs are in their nature accurate, and we are sure that their results are true, whereas physical proofs rather are methods for testing, checking, measuring and evaluating until we are sure beyond reasonable doubt. An accurate (and often mathematical) proof is called **analytical**, whereas a proof that is based on many experiments, measurements or much data is called **numerical**.

Some relevant terms regarding proofs are the following:^{[1,2]}

**Theorem**: Already-proven expression involved in a proof for something new.**Lemma**: “Side-expression” that is proven separately during a proof with the sole purpose of finishing that proof.**Axioms**: Assumptions that are not proven but*defined*or*assumed**true*.

Types of proofs^{[4]} |
||
---|---|---|

Proof | Description | Example |

Direct proof | Combine something already known (axioms and theorems) to reach the result | If $a$ and $b$ are even integers, then they are the double of two other numbers: $a+b=\underbrace{2c}_a+\underbrace{2d}_b=2(c+d)$. Their sum is also the double of another number, so it is also even |

Proof by contradiction | Show that if something is true, an absurd contradiction appears. So, it must be false. | |

Proof by induction | Prove a base-case. Then prove an induction rule that says that the base-case counts for all other cases. (For an infinite row/series) | |

Proof by contraposition | In order to know if a claim is true, prove its contrapositive (“inverted”) case true. | If we want to prove that if $n$ is even, then $n^2$ is even, then we can instead say that if $n$ is odd then $n^2$ is odd. When proven true, then the contrapositive must also be true. |

Proof by construction | Find one example that either proves (for an existence claim) or disproves (for a truth claim) something. | Example: ‘I have an even number here, so even numbers do exist’. Counterexample: ‘I have an odd number here, so not all numbers are even’. |

Proof by exhaustion / proof by cases | Prove each occurrence individually (for a non-infinite set) | Claim: $n^2+n$ is always even, when $n$ is an integer. Prove this for even and then for odd $n$’s separately, covering all. |

Geometric proof | Make the result clear from accurate drawing | Draw the limiting case of an obtuse triangle to see that it always will have two angles below $90^\circ$ |

Probabilistic proof | Show that something must be true with 100 % probability | The cat must be inside the house, since it can’t get out |

Plausibility proof | Show that something most likely is true with very high probability | The cat most likely is inside the house, since it could only get out through the chimney |

Statistical proof | Collect much data that shows a clear pattern | It is a human feature to have 5 fingers, because 99.6 % of all babies (2996 out of 3000) are born with 5 fingers.^{[3]} |

Empirical proof | Perform many measurements that all show the same result | Things will always fall down when dropped, because every time it has ever been tried, it happened. |

Several of the proofs involve elements of falsification, such as the empirical proof and the proof by construction. We can call a proof **combinatorial**, when it includes more than one type (when more than one proof gives the same result, and we thus believe it to be true).

References:

- ‘
**What is the difference between lemma, axiom, definition, corollary, etc?**’ (web page, answer to forum post),*57Jimmy*, Mathematics Stack Exchange, 2018, math.stackexchange.com/a/2716230/13230 (accessed Oct. 9th, 2019) - ‘
**Mathematical Proof/Methods of Proof/Constructive Proof**’ (encyclopedia/dictionary), Wikibooks, 2017, en.wikibooks.org/wiki/Mathematical_Proof/Methods_of_Proof/Constructive_Proof (accessed Oct. 9th, 2019)

## Responses