## When things are similar but arenâ€™t equal

Or when we need to order things, relations are a flexible mathematical notion that allows us to compare terms or elements.

The most familiar relation is equality. But you have encountered many others at this point:

- $ \to $ - implication of propositions
- $ \leq $ - less than (or equality) of numbers
- $ \subseteq $ - subset containment
- $\mid$ - divisibility

Relations unify all these concepts. We will discuss the general concept and then branch off to investigate different common properties of relations.