A set that has one and only one element is called ___.
an empty set
an infinite set
a singleton set
a universal set
A set that has one and only one element is called a singleton set. The cardinal number of this set is 1.
If U = {x:x=n2,n∈N,n≤5}, then the subset A = {even numbers} is
why an empty set is called a set,when a set is a collection of elements?