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mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
,
set Set, The Set, SET or SETS may refer to: Science, technology, and mathematics Mathematics *Set (mathematics), a collection of elements *Category of sets, the category whose objects and morphisms are sets and total functions, respectively Electro ...
''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset of ''B''. The relationship of one set being a subset of another is called inclusion (or sometimes containment). ''A'' is a subset of ''B'' may also be expressed as ''B'' includes (or contains) ''A'' or ''A'' is included (or contained) in ''B''. A ''k''-subset is a subset with ''k'' elements. The subset relation defines a
partial order In mathematics, especially order theory, a partially ordered set (also poset) formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary ...
on sets. In fact, the subsets of a given set form a
Boolean algebra In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas i ...
under the subset relation, in which the join and meet are given by intersection and union, and the subset relation itself is the Boolean inclusion relation.


Definition

If ''A'' and ''B'' are sets and every element of ''A'' is also an element of ''B'', then: :*''A'' is a subset of ''B'', denoted by A \subseteq B, or equivalently, :* ''B'' is a superset of ''A'', denoted by B \supseteq A. If ''A'' is a subset of ''B'', but ''A'' is not equal to ''B'' (i.e. there exists at least one element of B which is not an element of ''A''), then: :*''A'' is a proper (or strict) subset of ''B'', denoted by A \subsetneq B, or equivalently, :* ''B'' is a proper (or strict) superset of ''A'', denoted by B \supsetneq A. The empty set, written \ or \varnothing, is a subset of any set ''X'' and a proper subset of any set except itself, the inclusion relation \subseteq is a
partial order In mathematics, especially order theory, a partially ordered set (also poset) formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary ...
on the set \mathcal(S) (the power set of ''S''—the set of all subsets of ''S'') defined by A \leq B \iff A \subseteq B. We may also partially order \mathcal(S) by reverse set inclusion by defining A \leq B \text B \subseteq A. When quantified, A \subseteq B is represented as \forall x \left(x \in A \implies x \in B\right). We can prove the statement A \subseteq B by applying a proof technique known as the element argument:
Let sets ''A'' and ''B'' be given. To prove that A \subseteq B, # suppose that ''a'' is a particular but arbitrarily chosen element of A # show that ''a'' is an element of ''B''.
The validity of this technique can be seen as a consequence of
Universal generalization In predicate logic, generalization (also universal generalization or universal introduction,Moore and Parker GEN) is a valid inference rule. It states that if \vdash \!P(x) has been derived, then \vdash \!\forall x \, P(x) can be derived. Gener ...
: the technique shows c \in A \implies c \in B for an arbitrarily chosen element ''c''. Universal generalisation then implies \forall x \left(x \in A \implies x \in B\right), which is equivalent to A \subseteq B, as stated above. The set of all subsets of A is called its powerset, and is denoted by \mathcal(A). The set of all k-subsets of A is denoted by \tbinom, in analogue with the notation for binomial coefficients, which count the number of k-subsets of an n-element set. In
set theory Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly concern ...
, the notation k is also common, especially when k is a transfinite
cardinal number In mathematics, cardinal numbers, or cardinals for short, are a generalization of the natural numbers used to measure the cardinality (size) of sets. The cardinality of a finite set is a natural number: the number of elements in the set. ...
.


Properties

* A set ''A'' is a subset of ''B''
if and only if In logic and related fields such as mathematics and philosophy, "if and only if" (shortened as "iff") is a biconditional logical connective between statements, where either both statements are true or both are false. The connective is bic ...
their intersection is equal to A. :Formally: : A \subseteq B \text A \cap B = A. * A set ''A'' is a subset of ''B'' if and only if their union is equal to B. :Formally: : A \subseteq B \text A \cup B = B. * A finite set ''A'' is a subset of ''B'', if and only if the cardinality of their intersection is equal to the cardinality of A. :Formally: : A \subseteq B \text , A \cap B, = , A, .


⊂ and ⊃ symbols

Some authors use the symbols \subset and \supset to indicate and respectively; that is, with the same meaning as and instead of the symbols \subseteq and \supseteq. For example, for these authors, it is true of every set ''A'' that A \subset A. Other authors prefer to use the symbols \subset and \supset to indicate (also called strict) subset and superset respectively; that is, with the same meaning as and instead of the symbols \subsetneq and \supsetneq. This usage makes \subseteq and \subset analogous to the inequality symbols \leq and <. For example, if x \leq y, then ''x'' may or may not equal ''y'', but if x < y, then ''x'' definitely does not equal ''y'', and ''is'' less than ''y''. Similarly, using the convention that \subset is proper subset, if A \subseteq B, then ''A'' may or may not equal ''B'', but if A \subset B, then ''A'' definitely does not equal ''B''.


Examples of subsets

* The set A = is a proper subset of B = , thus both expressions A \subseteq B and A \subsetneq B are true. * The set D = is a subset (but a proper subset) of E = , thus D \subseteq E is true, and D \subsetneq E is not true (false). * Any set is a subset of itself, but not a proper subset. (X \subseteq X is true, and X \subsetneq X is false for any set X.) * The set is a proper subset of * The set of
natural number In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country"). Numbers used for counting are called '' cardinal ...
s is a proper subset of the set of rational numbers; likewise, the set of points in a line segment is a proper subset of the set of points in a line. These are two examples in which both the subset and the whole set are infinite, and the subset has the same cardinality (the concept that corresponds to size, that is, the number of elements, of a finite set) as the whole; such cases can run counter to one's initial intuition. * The set of rational numbers is a proper subset of the set of
real number In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s. In this example, both sets are infinite, but the latter set has a larger cardinality (or ) than the former set. Another example in an Euler diagram: File:Example of A is a proper subset of B.svg, A is a proper subset of B File:Example of C is no proper subset of B.svg, C is a subset but not a proper subset of B


Other properties of inclusion

Inclusion is the canonical
partial order In mathematics, especially order theory, a partially ordered set (also poset) formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary ...
, in the sense that every partially ordered set (X, \preceq) is isomorphic to some collection of sets ordered by inclusion. The ordinal numbers are a simple example: if each ordinal ''n'' is identified with the set /math> of all ordinals less than or equal to ''n'', then a \leq b if and only if \subseteq For the power set \operatorname(S) of a set ''S'', the inclusion partial order is—up to an order isomorphism—the Cartesian product of k = , S, (the cardinality of ''S'') copies of the partial order on \ for which 0 < 1. This can be illustrated by enumerating S = \left\,, and associating with each subset T \subseteq S (i.e., each element of 2^S) the ''k''-tuple from \^k, of which the ''i''th coordinate is 1 if and only if s_i is a
member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...
of ''T''.


See also

* Convex subset *
Inclusion order In the mathematical field of order theory, an inclusion order is the partial order that arises as the subset-inclusion relation on some collection of objects. In a simple way, every poset ''P'' = (''X'',≤) is ( isomorphic to) an inclusion or ...
*
Region In geography, regions, otherwise referred to as zones, lands or territories, are areas that are broadly divided by physical characteristics ( physical geography), human impact characteristics ( human geography), and the interaction of humanity an ...
* Subset sum problem * Subsumptive containment *
Total subset In mathematics, more specifically in functional analysis, a subset T of a topological vector space X is said to be a total subset of X if the linear span of T is a dense subset of X. This condition arises frequently in many theorems of functional ...


References


Bibliography

*


External links

* * {{Common logical symbols Basic concepts in set theory