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# Set

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A set is an idea from mathematics. A set can hold zero or more things. A set cannot hold a particular item more than once: either that item is in the set or it is not. Multisets or bags are like sets in quite a few ways but can hold a certain type of item more than once.

## Notation

Most mathematicians use uppercase italic (usually Roman) letters to write about sets. The things that are seen as elements of sets are usually written with lowercase Roman letters.

For example, X={1,2,3,...} is a set of numbers, and the set is called X. Three dots means that the numbers of the set go on for infinity from the number 3.

## What to do with sets

### How to tell others about the set

Usually, when things are put into a bag, all the things that are put in have something in common. If someone else needs to get the same set, there are different options on how to tell them:

• All elements could simply be stated (like a shopping list).
• Some common thing could be stated (e.g. green vegetables).

### Element of

Various things can be put into a bag. Later on, a valid question would be if a certain thing is in the bag. Mathematicians call this element of. Something is an element of a set, if that thing can be found in the respective bag. The symbol used for this is $\in$

$a \in \mathbf{A}$

means that $a$ is in the bag $\mathbf{A}$

### Empty set

Like a bag, a set can also be empty. The empty set is like an empty bag: it has no things in it.

### Comparing sets

Two sets can be compared. This is like looking at two different bags. If they contain the same things, they are equal.

### Cardinality of a set

When mathematicians talk about a set, they sometimes want to know how big a set is. They do this by counting how many elements are in the set (how many items are in the bag). The cardinality can be a simple number. The empty set has a cardinality of 0. The set $\{ apple, orange \}$ has a cardinality of 2.

Two sets have the same cardinality if we can pair up their elements—if we can join two elements, one from each set. The set $\{ apple, orange \}$ and the set $\{ sun, moon \}$ have the same cardinality. We can pair apple with sun, and orange with moon. The order does not matter. It is possible to pair the elements, and none is left out. But the set $\{ dog, cat, bird \}$ and the set $\{ 5, 6 \}$ have different cardinality. If we try to pair them up, we always leave out one animal.

#### Infinite cardinality

At times cardinality is not a number. Sometimes a set has infinite cardinality. The set of integers is a set with infinite cardinality. Some sets with infinite cardinality are bigger (have a bigger cardinality) than others. There are more real numbers than there are natural numbers, for example. That means we cannot pair up the set of integers and the set of real numbers, even if we worked forever. If a set has the same cardinality as the set of integers, it is called a countable set. But if a set has the same cardinality as the real numbers, it is called an uncountable set.

### Subsets

If you look at the set {a,b} and the set {a,b,c,d}, you can see that all elements in the first set are also in the second set.
We say: {a,b} is a subset of {a,b,c,d}
As a formula it looks like this:
$\{a,b\} \subseteq \{a,b,c,d\}$

When all elements of A are also elements of B, we call A a subset of B:
$A \subseteq B$
It is usually read "A is contained in B"

Example:
Every Chevrolet is an American car. So the set of all Chevrolets is contained in the set of all American cars.

### Set operations

There are different ways to combine sets.

#### Intersections

The intersection $A \cap B$ of two sets A and B is a set that contains all the elements,
that are both in set A and in set B.
When A is the set of all cheap cars, and B is the set of all American cars,
then $A \cap B$ is the set of all cheap American cars.

#### Unions

The union $A \cup B$ of two sets A and B is a set that contains all the elements,
that are in set A or in set B.

This "or" is the inclusive disjunction, so the union contains also the elements, that are in set A and in set B.
By the way: This means, that the intersection is a subset of the union:
$(A \cap B) \subseteq (A \cup B)$

When A is the set of all cheap cars, and B is the set of all American cars,
then $A \cup B$ is the set of all cars, without all expensive cars that are not from America.

#### Complements

Complement can mean two different things:

• The complement of A is the universe U without all the elements of A:

$A^{\rm C} = U \setminus A$
The universe U is the set of all things you speak about.
When U is the set of all cars, and A is the set of all cheap cars,
then AC is the set of all expensive cars.

• The relative complement of A in B is the set B without all the elements of A:

$B \setminus A$
It is often called the set difference.
When A is the set of all cheap cars, and B is the set of all American cars,
then $B \setminus A$ is the set of all expensive American cars.

If you exchange the sets in the set difference, the result is different:
In the example with the cars, the difference $A \setminus B$ is the set of all cheap cars, that are not made in America.

## Special sets

Some sets are very important to mathematics. They are used very often. One of these is the empty set. Many of these sets are written using blackboard bold typeface, as shown below. Special sets include:

• $\mathbb{P}$, denoting the set of all primes.
• $\mathbb{N}$, denoting the set of all natural numbers. That is to say, $\mathbb{N}$ = {1, 2, 3, ...}, or sometimes $\mathbb{N}$ = {0, 1, 2, 3, ...}.
• $\mathbb{Z}$, denoting the set of all integers (whether positive, negative or zero). So $\mathbb{Z}$ = {..., -2, -1, 0, 1, 2, ...}.
• $\mathbb{Q}$, denoting the set of all rational numbers (that is, the set of all proper and improper fractions). So, $\mathbb{Q} = \left\{ \begin{matrix}\frac{a}{b} \end{matrix}: a,b \in \mathbb{Z}, b \neq 0\right\}$, meaning all fractions $\begin{matrix} \frac{a}{b} \end{matrix}$ where a and b are in the set of all integers and b is not equal to 0. For example, $\begin{matrix} \frac{1}{4} \end{matrix} \in \mathbb{Q}$ and $\begin{matrix}\frac{11}{6} \end{matrix} \in \mathbb{Q}$. All integers are in this set since every integer a can be expressed as the fraction $\begin{matrix} \frac{a}{1} \end{matrix}$.
• $\mathbb{R}$, denoting the set of all real numbers. This set includes all rational numbers, together with all irrational numbers (that is, numbers which cannot be rewritten as fractions, such as $\pi,$ $e,$ and √2).
• $\mathbb{C}$, denoting the set of all complex numbers.

Each of these sets of numbers has an infinite number of elements, and $\mathbb{P} \subset \mathbb{N} \subset \mathbb{Z} \subset \mathbb{Q} \subset \mathbb{R} \subset \mathbb{C}$. The primes are used less frequently than the others outside of number theory and related fields.

A mathematician called Bertrand Russell found that there are problems with this theory of sets. He stated this in a paradox called Russell's paradox. An easier to understand version, closer to real life, is called the Barber paradox:

There is a small town somewhere. In that town, there is a barber. All the men in the town do not like beards, so they either shave themselves, or they go to the barber shop to be shaved by the barber.

We can therefore make a statement about the barber himself: The barber shaves all men that do not shave themselves. He only shaves those men (since the others shave themselves and do not need a barber to give them a shave).

This of course raises the question: What does the barber do each morning to look clean-shaven? This is the paradox.

• If the barber does not shave himself, he will follow the rule and shave himself (go to the barber shop to have a shave)
• If the barber does indeed shave himself, he will not shave himself, according to the rule given above.