In mathematics, the reflexive closure of a binary relation R on a set X is the smallest reflexive relation on X that contains R.
For example, if X is a set of distinct numbers and x R y means "x is less than y", then the reflexive closure of R is the relation "x is less than or equal to y".
The reflexive closure S of a relation R on a set X is given by
In English, the reflexive closure of R is the union of R with the identity relation on X.
As an example, if
then the relation is already reflexive by itself, so it does not differ from its reflexive closure.
However, if any of the pairs in was absent, it would be inserted for the reflexive closure.
For example, if on the same set
then the reflexive closure is