, 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
, while in other theories, its existence can be deduced. Many possible properties of sets are vacuously true
for the empty set.
In some textbooks and popularizations, the empty set is referred to as the "null set".
However, ''null set
'' is a distinct notion within the context of measure theory
, in which it describes a set of measure zero (which is not necessarily empty). The empty set may also be called the ''void set''.
thumb|upright=0.4|A symbol for the empty set
Common notations for the empty set include "", "
", and "∅".
The latter two symbols were introduced by the Bourbaki group
(specifically André Weil
) in 1939, inspired by the letter Ø
in the Danish
alphabets. 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. It can be coded in HTML
and as ∅
. It can be coded in LaTeX
. The symbol
is coded in LaTeX as \emptyset
When writing in languages such as Danish and Norwegian, where the empty set character may be confused with the alphabetic letter Ø (as when using the symbol in linguistics), the Unicode character U+29B0 REVERSED EMPTY SET ⦰ may be used instead.
In standard axiomatic set theory
, by the principle of extensionality
, two sets are equal if they have the same elements. As a result, there can be only one set with no elements, hence the usage of "the empty set" rather than "an empty set".
The following lists document of some of the most notable properties related to the empty set. For more on the mathematical symbols used therein, see ''List of mathematical symbols
* The empty set is a subset
* The union
of ''A'' with the empty set is ''A'':
* The intersection
of ''A'' with the empty set is the empty set:
* The Cartesian product
of ''A'' and the empty set is the empty set:
The empty set has the following properties:
* Its only subset is the empty set itself:
* The power set
of the empty set is the set containing only the empty set:
* The number of elements of the empty set (i.e., its cardinality
) is zero:
The connection between the empty set and zero goes further, however: in the standard set-theoretic definition of natural numbers
, sets are used to model
the natural numbers. In this context, zero is modelled by the empty set.
For any property
* For every element of
, the property ''P'' holds (vacuous truth
* There is no element of
for which the property ''P'' holds.
Conversely, if for some property ''P'' and some set ''V'', the following two statements hold:
* For every element of ''V'' the property ''P'' holds
* There is no element of ''V'' for which the property ''P'' holds
then ''V'' = ∅.
By the definition of subset
, the empty set is a subset of any set ''A''. That is, ''every'' element ''x'' of
belongs to ''A''. Indeed, if it were not true that every element of
is in ''A'', then there would be at least one element of
that is not present in ''A''. Since there are ''no'' elements of
at all, there is no element of
that is not in ''A''. Any statement that begins "for every element of
" 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 sum
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 product
of the elements of the empty set should be considered to be one
(see empty product
), since one is the identity element for multiplication.
is a permutation
of a set without fixed point
s. The empty set can be considered a derangement of itself, because it has only one permutation (
), 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 member when it is considered as a subset of any ordered set
, 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. When considered as a subset of the extended reals
formed by adding two "numbers" or "points" to the real numbers (namely negative infinity
which is defined to be less than every other extended real number, and positive infinity
which is defined to be greater than every other extended real number), we have that:
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 the minimum and infimum operators.
In any topological space
''X'', the empty set is open
by definition, as is ''X''. Since the complement
of an open set is closed
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
by the fact that every finite set
of the empty set is empty. This is known as "preservation of nullary unions
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
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
. 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.
In the von Neumann construction of the ordinals
, 0 is defined as the empty set, and the successor of an ordinal is defined as
. Thus, we have
, 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,
, such that the Peano axioms
of arithmetic are satisfied.
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 at least two ways:
*Standard first-order logic
implies, merely from the logical axiom
s, that ''something'' exists, and in the language of set theory, that thing must be a set. Now the existence of the empty set follows easily from the axiom of separation
*Even using free logic
(which does not logically imply that something exists), there is already an axiom implying the existence of at least one set, namely the axiom of infinity
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 opening moves
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 "
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
" and the latter to "The set is better than the set
". The first compares elements of sets, while the second compares the sets themselves.
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 set-theoretical sense—namely, all non-sets. 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."
argued that much of what has been heretofore obtained by set theory can just as easily be obtained by plural quantification
over individuals, without reifying
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 1998, ''Logic, Logic and Logic'' (Richard Jeffrey, and Burgess, J., eds.) Harvard University Press, 54–72.]
, ''Naive Set Theory
''. Princeton, NJ: D. Van Nostrand Company, 1960. Reprinted by Springer-Verlag, New York, 1974. (Springer-Verlag edition). Reprinted by Martino Fine Books, 2011. (paperback edition).
Category:Basic concepts in set theory