Introduction
摘要
Like the theory of groups or rings or fields or ordered fields, etc. set theory is also a mathematical theory and an interesting one. Each conventional theory starts with some undefined concepts. For example, the group theory starts with the group operation and the group identity, and the theory of ordered fields starts with undefined addition, multiplication, additive identity, multiplicative identity, and an order relation. Similarly, set theory starts with the “belongs to” relation, denoted by \(\in \) .