Question: what is the basic relationship between set?
Answer: The basic relation in set theory is that of elementhood, or membership. We write a∈A a ∈ A to indicate that the object a is an element, or a member, of the set A . We also say that a belongs to A . Thus, a set A is equal to a set B if and only if for every a , a∈A a ∈ A if and only if a∈B a ∈ B .