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The definition of set can be stated in several different ways, all of them complicated. The most widely used definition is based on the Zermelo-Fraenkel axioms.

It is not necessary to understand or even know the Zermelo-Fraenkel axioms to understand sets as they are used in undergraduate math or large parts of graduate math. Nearly everything having to do with sets in ordinary mathematical practice derives from the Method of Comprehension, so there is usually no need for the axiomatic definition. In that sense, the following specification contains everything you need to know about sets for most mathematical purposes.

The concept of specification of a math object is discussed in the chapter on Definitions.

Specification for sets

A set is a single math object distinct from but completely determined by what its elements are.

This specification tells you the operative properties of a set rather than giving a definition in terms of previously known objects.

An embarrassing difficulty

The specification just given is not a mathematical definition. In particular, in some situations that usually do not occur in most branches of math, a bunch of elements may not correspond to a set. One such example is this:

There is no "set of all sets".

In other words, you can't have a set that is completely determined by the fact that its elements are all the sets that exist. This follows from Cantor's Theorem. In most cases, if you think up a bunch of elements, they do form a set.

The important thing to understand is what the specification means in practice. That is the subject of the next section:

Consequences of the specification for sets

Single math object

Take the definition of a set seriously

If someone defines $S$ as the set of all integers bigger than $3$, then the spec means you know all these things:

Order does not matter

In list notation, the order in which you list the elements of a set is irrelevant for the purposes of determining what the set is.  This follows directly from the specification.


Repetition does not matter

The list notation $\{3, 3, 4\}$ defines a set with two elements $3$ and $4$.  The first occurrence of ‘$3$’ in the list says that $3$ is in the set.  The second occurrence says the same thing.  Saying a true thing twice has no effect (except to irritate the reader). So repetition in list notation does not matter.

Warning: Mathematica uses curly brackets to denote lists, not sets. So in Mathematica, $\{1, 2, 4, 5\}$ and $\{1,5,4,2\}$ are two different lists and so are $\{3,4\}$ and $\{3,3,4\}$.

Set equality

If $A$ and $B$ are sets, then $A=B$ if and only if $A$ and $B$ have the same elements. In other words:

$A = B$ if and only if every element of $A$ is an element of $B$
and every element of $B$ is an element of $A$.


Sets as elements of sets

A set, being a math object, can be an element of another set. Furthermore, if it is, its elements are not necessarily elements of that other set because the specification says that a set is a math object that is distinct from its elements.


Let $A= \{ \{1, 2\}, \{3\}, 1, 6\}$.  


Let \[B=\left\{ \left\{ 1 \right\},\,\left\{ 2 \right\},\,\left\{ 1,\,2 \right\},\,\varnothing  \right\}.\]Then $B$ is the set of all subsets of the set $\{1, 2\}$.  In particular, $\varnothing \in B$ (the empty set is an element of $B$.)  Note that the empty set is not an element of the set $A$ of the preceding example.

It is a myth that the empty set is an element of every set.

On the other hand:

The empty set is a subset of every set.

Sets as elements of sets in practice

Most of the time in practice either none of the elements of a set are sets or all of them are. In fact, sets such as $A$ and $B$ in the preceding examples, which have both sets and numbers as elements, rarely occur in mathematical writing except as examples in texts such as the one you are reading which are intended to bring out the difference between "element of" and "included in". See also contain.


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