# Basics

## Induction, Counterexample, Contraposition

- contraposition
- conversion of a proposition from all A is B to all not-B is not-A.

## Conjunction, Disjunction

Ref: wikipedia-disjunction.

- Logical disjunction
- In logic and mathematics,
*or*is the truth-functional operator of (inclusive) disjunction, also known as alternation; the*or*of a set of operands is true if and only if one or more of its operands is true.