# Universal Relation

Universal relation is a relation on set A when A X A $\subseteq$ A X A. In other words, universal-relation is the relation if each element of set A is related to every element of A.
For example : Relation on the set A = {1,2,3,4,5,6} by
R = {(a,b) $\in$ R : |a -b|$\geq$0}
We observe that |a -b|$\geq$0 for all a, b $\in$ A
$\Rightarrow$(a,b)$\in$ R for all (a,b) $\in$ A X A
$\Rightarrow$ each element of set A is related to every element of set A.
$\Rightarrow$ R = A X A
$\Rightarrow$ R is a universal relation on set A.
Note : It is to note here that the void relation and the universal relation on a set A are respectively the smallest and the largest relations on set A.
Both the void and universal relation are sometimes called trivial relations.

## Examples on Universal Relation

Example : 1 Let A be the set of all students of a boys school. Show that the relation R on A given by R = {(a,b) : difference between the heights of a and b is less than 5 meters} is the universal-relation.
Solution : It is obvious that the difference between the heights of any two students of the school has to be less than 5 meters. Therefore (a,b) $\in$ R for all a, b $\in$ A.
$\Rightarrow$ R = A X A
$\Rightarrow$ R is the universal-relation on set A.