In mathematics, and more specifically set theory, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero. Some axiomatic set theories ensure that the empty set exists by including an axiom of empty set; in other theories, its existence can be deduced. Many possible properties of sets are vacuously true for the empty set.
Null set was once a common synonym for "empty set", but is now a technical term in measure theory and describes a set that is not necessarily empty. The empty set may also be called the void set.
Notation
Common notations for the empty set include "{}", "
$\backslash emptyset$", and "∅". The latter two symbols were introduced by the
Bourbaki group (specifically André Weil) in 1939, inspired by the letter Ø in the Norwegian and Danish alphabets (and not related in any way to the Greek letter Φ).
[ Earliest Uses of Symbols of Set Theory and Logic.] In the past, "0" was occasionally used as a symbol for the empty set, but this is now considered to be an improper use of notation.
The symbol ∅ is available at Unicode point U+2205.[ Unicode Standard 5.2] It can be coded in HTML as ∅ and in LaTeX as \varnothing. The symbol $\backslash emptyset$ is coded in LaTeX as \emptyset; it is not available in HTML/Unicode.
Properties
In standard axiomatic set theory, by the principle of extensionality, two sets are equal if they have the same elements; therefore there can be only one set with no elements. Hence there is but one empty set, and we speak of "the empty set" rather than "an empty set".
The mathematical symbols employed below are explained here.
For any set A:

The empty set is a subset of A:

:$\backslash forall\; A:\; \backslash varnothing\; \backslash subseteq\; A$

The union of A with the empty set is A:

:$\backslash forall\; A:\; A\; \backslash cup\; \backslash varnothing\; =\; A$

The intersection of A with the empty set is the empty set:

:$\backslash forall\; A:\; A\; \backslash cap\; \backslash varnothing\; =\; \backslash varnothing$

The Cartesian product of A and the empty set is the empty set:

:$\backslash forall\; A:\; A\; \backslash times\; \backslash varnothing\; =\; \backslash varnothing$
The empty set has the following properties:

Its only subset is the empty set itself:

:$\backslash forall\; A:\; A\; \backslash subseteq\; \backslash varnothing\; \backslash Rightarrow\; A\; =\; \backslash varnothing$

The power set of the empty set is the set containing only the empty set:

:$2^\{\backslash varnothing\; \}\; =\; \backslash \{\backslash varnothing\backslash \}$

Its number of elements (that is, its cardinality) is zero:

:$\backslash mathrm\{card\}(\backslash varnothing)\; =\; 0$
The connection between the empty set and zero goes further, however: in the standard settheoretic definition of natural numbers, sets are used to model theory the natural numbers. In this context, zero is modelled by the empty set.
For any property:

For every element of $\backslash varnothing$ the property holds (vacuous truth);

There is no element of $\backslash varnothing$ for which the property holds.
Conversely, if for some property and some set V, the following two statements hold:

For every element of V the property holds;

There is no element of V for which the property holds,
 then $V\; =\; \backslash varnothing$.
By the definition of subset, the empty set is a subset of any set A. That is, every element x of $\backslash varnothing$ belongs to A. Indeed, if it were not true that every element of $\backslash varnothing$ is in A then there would be at least one element of $\backslash varnothing$ that is not present in A. Since there are no elements of $\backslash varnothing$ at all, there is no element of $\backslash varnothing$ that is not in A. Any statement that begins "for every element of $\backslash varnothing$" is not making any substantive claim; it is a vacuous truth. This is often paraphrased as "everything is true of the elements of the empty set."
Operations on the empty set
When speaking of the
summation of the elements of a finite set, one is inevitably led to the convention that the sum of the elements of the empty set is zero. The reason for this is that zero is the
identity element for addition. Similarly, the
multiplication of the elements of the empty set should be considered to be one (see
empty product), since one is the identity element for multiplication.
A derangement is a permutation of a set without fixed points. The empty set can be considered a derangement of itself, because it has only one permutation ($0!=1$), and it is vacuously true that no element (of the empty set) can be found that retains its original position.
In other areas of mathematics
Extended real numbers
Since the empty set has no members, when it is considered as a subset of any
ordered set, then every member of that set will be an upper bound and lower bound for the empty set. For example, when considered as a subset of the real numbers, with its usual ordering, represented by the real number line, every real number is both an upper and lower bound for the empty set.
[Bruckner, A.N., Bruckner, J.B., and Thomson, B.S., 2008. Elementary Real Analysis, 2nd ed. Prentice Hall. P. 9.] When considered as a subset of the
extended reals formed by adding two "numbers" or "points" to the real numbers, namely negative infinity, denoted
$\backslash infty\backslash !\backslash ,,$ which is defined to be less than every other extended real number, and positive infinity, denoted
$+\backslash infty\backslash !\backslash ,,$ which is defined to be greater than every other extended real number, then:
 $\backslash sup\backslash varnothing=\backslash min(\backslash \{\backslash infty,\; +\backslash infty\; \backslash \}\; \backslash cup\; \backslash mathbb\{R\})=\backslash infty,$
and
 $\backslash inf\backslash varnothing=\backslash max(\backslash \{\backslash infty,\; +\backslash infty\; \backslash \}\; \backslash cup\; \backslash mathbb\{R\})=+\backslash infty.$
That is, the least upper bound (sup or supremum) of the empty set is negative infinity, while the greatest lower bound (inf or infimum) is positive infinity. By analogy with the above, in the domain of the extended reals, negative infinity is the identity element for the maximum and supremum operators, while positive infinity is the identity element for minimum and infimum.
Topology
In any topological space
X, the empty set is
open set by definition, as is
X. Since the complement of an open set is
closed set and the empty set and
X are complements of each other, the empty set is also closed, making it a
clopen set. Moreover, the empty set is
compact set by the fact that every
finite set is compact.
The closure of the empty set is empty. This is known as "preservation of nullary unions."
Category theory
If
A is a set, then there exists precisely one function
f from {} to
A, the
empty function. As a result, the empty set is the unique
initial object of the
category theory of sets and functions.
The empty set can be turned into a topological space, called the empty space, in just one way: by defining the empty set to be open set. This empty topological space is the unique initial object in the category of topological spaces with continuous maps. In fact, it is a strict initial object: only the empty set has a function to the empty set.
Set theory
In the von Neumann construction of the ordinals, 0 is defined as the empty set, and the successor of an ordinal is defined as
$S(\backslash alpha)=\backslash alpha\backslash cup\backslash \{\backslash alpha\backslash \}$. Thus, we have
$0=\backslash varnothing$,
$1\; =\; 0\backslash cup\backslash \{0\backslash \}=\backslash \{\backslash varnothing\backslash \}$,
$2=1\backslash cup\backslash \{1\backslash \}=\backslash \{\backslash \{\backslash varnothing\backslash \},\backslash varnothing\backslash \}$, and so on. The von Neumann construction, along with the axiom of infinity, which guarantees the existence of at least one infinite set, can be used to construct the set of natural numbers,
$\backslash mathbb\{N\}\_0$, such that the
Peano axioms of arithmetic are satisfied.
Questioned existence
Axiomatic set theory
In Zermelo set theory, the existence of the empty set is assured by the axiom of empty set, and its uniqueness follows from the axiom of extensionality. However, the axiom of empty set can be shown redundant in either of two ways:

There is already an axiom implying the existence of at least one set. Given such an axiom together with the axiom of separation, the existence of the empty set is easily proved.

In the presence of , it is easy to prove that at least one set exists, viz. the set of all urelements (assuming there is not a proper class of them). Again, given the axiom of separation, the empty set is easily proved.
Philosophical issues
While the empty set is a standard and widely accepted mathematical concept, it remains an
ontological curiosity, whose meaning and usefulness are debated by philosophers and logicians.
The empty set is not the same thing as nothing; rather, it is a set with nothing inside it and a set is always something. This issue can be overcome by viewing a set as a bag—an empty bag undoubtedly still exists. Darling (2004) explains that the empty set is not nothing, but rather "the set of all triangles with four sides, the set of all numbers that are bigger than nine but smaller than eight, and the set of all chess opening in chess that involve a king."[
]
The popular syllogism
 Nothing is better than eternal happiness; a ham sandwich is better than nothing; therefore, a ham sandwich is better than eternal happiness
is often used to demonstrate the philosophical relation between the concept of nothing and the empty set. Darling writes that the contrast can be seen by rewriting the statements "Nothing is better than eternal happiness" and "A ham sandwich is better than nothing" in a mathematical tone. According to Darling, the former is equivalent to "The set of all things that are better than eternal happiness is
$\backslash varnothing$" and the latter to "The set {ham sandwich} is better than the set
$\backslash varnothing$". It is noted that the first compares elements of sets, while the second compares the sets themselves.
Jonathan Lowe argues that while the empty set:
 "...was undoubtedly an important landmark in the history of mathematics, … we should not assume that its utility in calculation is dependent upon its actually denoting some object."
it is also the case that:
 "All that we are ever informed about the empty set is that it (1) is a set, (2) has no members, and (3) is unique amongst sets in having no members. However, there are very many things that 'have no members', in the settheoretical sense—namely, all nonsets. It is perfectly clear why these things have no members, for they are not sets. What is unclear is how there can be, uniquely amongst sets, a set which has no members. We cannot conjure such an entity into existence by mere stipulation."
George Boolos argued that much of what has been heretofore obtained by set theory can just as easily be obtained by plural quantification over individuals, without sets as singular entities having other entities as members.[*George Boolos, 1984, "To be is to be the value of a variable," The Journal of Philosophy 91: 430–49. Reprinted in his 1998 Logic, Logic and Logic (Richard Jeffrey, and Burgess, J., eds.) Harvard Univ. Press: 54–72.]
See also
Further reading

Paul Halmos, Naive Set Theory. Princeton, NJ: D. Van Nostrand Company, 1960. Reprinted by SpringerVerlag, New York, 1974. (SpringerVerlag edition). Reprinted by Martino Fine Books, 2011. (Paperback edition).
External links