Lesson 2: Relations and Functions

Entry conditions

Use relations and functions only when you can:

• List or decide all relevant pairs of elements.
• Decide whether a specific ordered pair is included.
• For functions, assign exactly one output to each input.

Definitions

• A relation $R$ on a set $S$ is a subset of $S\times S$.
• A function $f:S\to T$ assigns exactly one output in $T$ to each input in $S$.

Vocabulary (plain language)

• Ordered pair: a two-item list where order matters, written $(x,y)$.
• Relation: a list of ordered pairs saying which items are related.
• Function: a rule that gives one output for each input.
• Domain: the set of allowed inputs.
• Codomain: the set of possible outputs.

Symbols used

• $S\times S$: all ordered pairs of elements of $S$.
• $(x,y)\in R$: the pair $(x,y)$ is in the relation.
• $f:S\to T$: a function from $S$ to $T$.

Intuition

A relation is a declared list of which items go with which. A function is a stricter relation: each input must have one, and only one, output.

Worked examples

Example 1: A relation

Let $S=\{1,2,3\}$. Define

$$R=\{(1,1),(1,2),(2,3)\}.$$

Then $1$ is related to $2$, but $2$ is not related to $1$ unless $(2,1)$ is also listed.

Example 2: A function

Let $S=\{a,b,c\}$ and $T=\{0,1\}$. Define

$$f(a)=0,f(b)=1,f(c)=1.$$

Every input in $S$ has exactly one output in $T$, so this is a function.

Example 3: Not a function

Let $g$ map $a$ to both $0$ and $1$. Then $g$ is not a function because one input has two outputs.

How to recognize the structure

• Relation: you can answer “is $(x,y)$ in $R$?” for any pair.
• Function: you can answer “what is $f(x)$?” for any $x$ and the answer is unique.

Common mistakes

• Treating “similarity” as a function when one item can be similar to many.
• Treating “can influence” as a function when it can target many outputs.
• Failing to define the domain or codomain.

Minimal data

• A set $S$.
• A list of ordered pairs for $R$.
• For a function, a total mapping from $S$ to $T$.