# Equivalence relation

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When you are talking about of an **relation**, you gotta to think that this is
inter two given sets. So a relation *R* inter set *A* and a set *B* is a subset
of their cartesian product:

An **equivalence relation** in a set *A* is a reation
i.e. a *endo-relation* in a set, which obeys the conditions:

- reflexivity
- symmetry
- transitivity

An example is used when we use sum of fractional numbers. Here a rational number can be represented of several different fractions, so by taking the fractions having a common denominator we can simplify the result.