R is the complement of a complete set. R is the cornerstone.
Mathematical set is a basic concept in mathematics. The basic concept is a concept that cannot be defined by other concepts, and it is also a concept that cannot be defined by other concepts. The concept of set can be defined in an intuitive and axiomatic way. Set is a basic concept in mathematics. It is the research object of set theory, and the basic theory of set theory was not founded until 19 century.
The simplest statement is in the most primitive set theory. Set is to combine some definite and distinguishable objects in people's intuition or thinking into a whole or monomer. This whole thing is a set.
Setting characteristics
A sure thing
Given a set, any element, whether it belongs to the set or not, must be one of them. No ambiguity? .
Mutual anisotropy
Any two elements in a collection are considered different, that is, each element can only appear once. Sometimes it is necessary to describe the situation where the same element appears many times. You can use multiset, where elements are allowed to appear multiple times.
randomness
In a set, the state of each element is the same and the elements are out of order. You can define an order relation on the set. After defining the order relation, you can sort the elements according to the order relation. But as far as the characteristics of the set itself are concerned, there is no necessary order between elements.