Symmetric closure

In mathematics, the symmetric closure of a binary relation

on a set

is the smallest symmetric relation on

is a set of airports and

means "there is a direct flight from airport

", then the symmetric closure of

is the relation "there is a direct flight either from

is the set of humans and

is the relation 'parent of', then the symmetric closure of

is a parent or a child of

The symmetric closure

In other words, the symmetric closure of

with its converse relation,