Mathematical structure

In mathematics, a structure on a set is some additional mathematical objects that, loosely speaking, attach to the set, making it easier to visualize or work with.

A partial list of possible structures are measures, algebraic structures (groups, fields, etc.), topologies, metric structures (geometries), orders, and equivalence relations.

Sometimes, a set is endowed with more than one structure simultaneously; this enables mathematicians to study it more richly. For example, an order induces a topology. As another example, if a set both has a topology and is a group, and the two structures are related in a certain way, the set becomes a topological group.

Example: the real numbers

The set of real numbers has several standard structures. It has an order: each number is either less or more than every other number. It has algebraic structure: there are operations of multiplication and addition. It has a measure: intervals along the real line have a certain length. It has a geometry: it is equipped with a metric and is flat. And it has a topology: numbers are close to or far from one another. Its order and, independently, its metric structure induce its topology.