, R = A × A Both the empty relation and the universal relation are some times called trivial
relations Example 1 Let A be the set of all students of a boys school Show that the relation R
in A given by R = {(a, b) : a is sister of b} is the empty relation and R′ = {(a, b) : the
difference between heights of a and b is less than 3 meters} is the universal relation