# CHAPTER 4

# RELATIONS AND FUNCTIONS

## 4.1 RELATIONS

A relation is an association between objects. A book on a table is an example of the relation of one object being *on* another. It is especially common to speak of relations among people. For example, one person could be the niece of another. In mathematics, there are many relations such as equals and less-than that describe associations between numbers. To formalize this idea, we make the next definition.

**DEFINITION 4.1.1**

A set *R* is an (** n-ary**)

**relation**if there exist sets

*A*

_{0},

*A*

_{1}, ... ,

*A*

_{n−1}such that

In particular, *R* is a **unary** relation if *n* = 1 and a **binary** relation if *n* = 2. If *R* ⊆ *A* × *A* for some set *A*, then *R* is a **relation on** *A* and we write (*A*, *R*).

The relation *on* can be represented as a subset of the Cartesian product of the set of all books and the set of all tables. We could then write (*dictionary, desk*) to mean that the

dictionary is on the desk. Similarly, the set {(2, 4), (7, 3), (0, 0)} is a relation because it is a subset of **Z** × **Z**. The ordered pair (2, 4) means that 2 is related to 4. Likewise, **R** × **Q** is a relation where every real number is related to every rational number, and according to Definition 4.1.1, the empty set is also a relation because ∅ = ∅ × ∅.

**EXAMPLE 4.1.2**

For any set *A*, define

Call this set the **identity ...**

Get *A First Course in Mathematical Logic and Set Theory* now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.