# Set Theory

Jul 17, 2023

## # Basic Sets

N := Natural Number
Z := Integer
Q := Rational Number
R := Real Number
C := Complex Number

## # Term

### # Disjoint

When two sets have no elements in common, they are said to be disjoint

### # Complement

$A ^ { \prime } = { x : x \in U \text{ and } x \notin A }$

### # Difference

$A \backslash B = A \cap B ^ { \prime } = { x : x \in A \text{ and } x \notin b}$

### # Relations

Subsets of $A × B$ are called relations

A relation is well-defined if each element in the domain is assigned to a unique element in the range.

### # Surjective or onto

满射
each element in B has an A

### # Injective or One-to-one

单射
$a_1 \neq a_2 \Leftrightarrow f(a_1) \neq f(a_2)$

• Countable: can be put in 1-1 correspondence with positive integers
• Integers
• Rational Numbers
• Uncountable
• Interval [0, 1]

### # Bijective

双射
A map that is both one-to-one and onto is called bijective.

### # Identity Mapping

If S is any set, we will use ids or id to denote the identity mapping from S to itself.
Define this map by id(s)= s for all s $\in$ S.

### # Invertible

A mapping is invertible if and only if it is both one-to-one and onto.

Equivalence relation