L is the set of all rational numbers less than
and R is the set of all rational numbers greater than .
In fact, you can use any irrational number in place of .
Find two sets of rational numbers L and R (both does not equal the empty set) such that L union R =Q for each x belong L and y belong R, x<y and set L does not contain a largest number and R does not contain a smallest number.
And does not contain a largest number, since for every , you can always find which is greater than , simply by making the mean of and ; i.e. .
Similarly, of course, does not have a smallest number.
Finally, , since every rational number is in one or other set.