Subset

{{short description|Set whose elements all belong to another set}} {{redirect|Superset}}

{{redirect|⊃|the logic symbol|Horseshoe (symbol)|other uses|Horseshoe (disambiguation)}} [[File:Venn A subset B.svg|150px|thumb|right|class=skin-invert-image|Euler diagram showing<br /> ''A'' is a subset of ''B'' (denoted <math>A \subseteq B</math>) and, conversely, ''B'' is a superset of ''A'' (denoted <math>B \supseteq A</math>)]]

In mathematics, a set ''A'' is a '''subset''' of a set ''B'' if and only 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.

When quantified, <math>A \subseteq B</math> is represented as <math>\forall x \left(x \in A \Rightarrow x \in B\right).</math><ref>{{cite book|last=Rosen|first=Kenneth H.|title=Discrete Mathematics and Its Applications|url=https://archive.org/details/discretemathemat00rose_408|url-access=limited|date=2012|publisher=McGraw-Hill|location=New York|isbn=978-0-07-338309-5|page=[https://archive.org/details/discretemathemat00rose_408/page/n139 119]|edition=7th}}</ref>

One can prove the statement <math>A \subseteq B</math> by applying a proof technique known as the element argument<ref>{{Cite book|last=Epp|first=Susanna S.|title=Discrete Mathematics with Applications|year=2011|isbn=978-0-495-39132-6|edition=Fourth|pages=337|publisher=Cengage Learning }}</ref>:<blockquote>Let sets ''A'' and ''B'' be given. To prove that <math>A \subseteq B,</math>

# '''suppose''' that ''a'' is a particular but arbitrarily chosen element of A # '''show''' that ''a'' is an element of ''B''. </blockquote>The validity of this technique can be seen as a consequence of universal generalization: the technique shows <math>(c \in A) \Rightarrow (c \in B)</math> for an arbitrarily chosen element ''c''. Universal generalisation then implies <math>\forall x \left(x \in A \Rightarrow x \in B\right),</math> which is equivalent to <math>A \subseteq B,</math> as stated above.

==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 <math>A \subseteq B</math>, or equivalently, :* ''B'' is a '''superset''' of ''A'', denoted by <math>B \supseteq A.</math>

{{anchor|proper subset}} 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 <math>A \subsetneq B</math>, or equivalently, :* ''B'' is a '''proper''' (or '''strict''') '''superset''' of ''A'', denoted by <math>B \supsetneq A.</math>

The empty set, written <math>\{ \}</math> or <math>\varnothing,</math> has no elements, and therefore is vacuously a subset of any set ''X''.

==Basic properties== thumb|alt=Euler diagram: A ⊆ B ⊆ C|<math>A \subseteq B</math> and <math>B \subseteq C</math> implies <math>A \subseteq C.</math> thumb|350px|Topological variety of proper subsets. While <math>A \subsetneq B</math> is a single logical relation, it can be realized as a tangential or non-tangential containment, as shown in these 11 exhaustive cases. * ''Reflexivity'': Given any set <math>A</math>, <math>A \subseteq A</math><ref>{{cite book|first=Robert R.|last=Stoll|date=1 January 1968|title=Set Theory and Logic|publisher=Dover Publications|location=San Francisco, CA|isbn=978-0-486-63829-4}}</ref> * ''Transitivity'': If <math>A \subseteq B</math> and <math>B \subseteq C</math>, then <math>A \subseteq C</math> * ''Antisymmetry'': If <math>A \subseteq B</math> and <math>B \subseteq A</math>, then <math>A = B</math>.

===Proper subset=== * ''Irreflexivity'': Given any set <math>A</math>, <math>A \subsetneq A</math> is False. * ''Transitivity'': If <math>A \subsetneq B</math> and <math>B \subsetneq C</math>, then <math>A \subsetneq C</math> * ''Asymmetry'': If <math>A \subsetneq B</math> then <math>B \subsetneq A</math> is False.

==⊂ and ⊃ symbols== Some authors use the symbols <math>\subset</math> and <math>\supset</math> to indicate {{em|subset}} and {{em|superset}} respectively; that is, with the same meaning as and instead of the symbols <math>\subseteq</math> and <math>\supseteq</math>.<ref>{{Citation|last1=Rudin|first1=Walter|author1-link=Walter Rudin|title=Real and complex analysis|publisher=McGraw-Hill|location=New York|edition=3rd|isbn=978-0-07-054234-1|mr=924157 |year=1987|page=6}}</ref> For example, for these authors, it is true of every set ''A'' that <math>A \subset A.</math> (a reflexive relation).

Other authors prefer to use the symbols <math>\subset</math> and <math>\supset</math> to indicate {{em|proper}} (also called strict) subset and {{em|proper}} superset respectively; that is, with the same meaning as and instead of the symbols <math>\subsetneq</math> and <math>\supsetneq.</math><ref>{{Citation|title=Subsets and Proper Subsets|url=http://it.edgecombe.edu/homepage/killorant/MAT140/Module1/Subsets.pdf|access-date=2012-09-07|archive-url=https://web.archive.org/web/20130123202559/http://it.edgecombe.edu/homepage/killorant/MAT140/Module1/Subsets.pdf|archive-date=2013-01-23|url-status=dead }}</ref> This usage makes <math>\subseteq</math> and <math>\subset</math> analogous to the inequality symbols <math>\leq</math> and <math><.</math> For example, if <math>x \leq y,</math> then ''x'' may or may not equal ''y'', but if <math>x < y,</math> then ''x'' definitely does not equal ''y'', and ''is'' less than ''y'' (an irreflexive relation). Similarly, using the convention that <math>\subset</math> is proper subset, if <math>A \subseteq B,</math> then ''A'' may or may not equal ''B'', but if <math>A \subset B,</math> then ''A'' definitely does not equal ''B''.

== Examples of subsets == [[File:PolygonsSet EN.svg|thumb|The regular polygons form a subset of the polygons.]] * The set A = {1, 2} is a proper subset of B = {1, 2, 3}, thus both expressions <math>A \subseteq B</math> and <math>A \subsetneq B</math> are true. * The set D = {1, 2, 3} is a subset (but {{em|not}} a proper subset) of E = {1, 2, 3}, thus <math>D \subseteq E</math> is true, and <math>D \subsetneq E</math> is not true (false). * The set {''x'': ''x'' is a prime number greater than 10} is a proper subset of {''x'': ''x'' is an odd number greater than 10} * The set of natural numbers 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 numbers. In this example, both sets are infinite, but the latter set has a larger cardinality (or {{em|power}}) than the former set.

Another example in an Euler diagram:

<gallery widths="270"> File:Example of A is a proper subset of B.svg|alt=Euler diagram: A = {1, 9, 11}; B = {1, 4, 8, 9, 11}|A is a proper subset of B. File:Example of C is no proper subset of B.svg|alt=Euler diagram: C = B = {1, 4, 8, 9, 11}|C is a subset but not a proper subset of B. </gallery>

==Power set== The set of all subsets of <math>S</math> is called its power set, and is denoted by <math>\mathcal{P}(S)</math>.<ref>{{Cite web|last=Weisstein|first=Eric W.|title=Subset|url=https://mathworld.wolfram.com/Subset.html|access-date=2020-08-23|website=mathworld.wolfram.com|language=en}}</ref>

The inclusion relation <math>\subseteq</math> is a partial order on the set <math>\mathcal{P}(S)</math> defined by <math>A \leq B \iff A \subseteq B</math>. We may also partially order <math>\mathcal{P}(S)</math> by reverse set inclusion by defining <math>A \leq B \text{ if and only if } B \subseteq A.</math>

For the power set <math>\operatorname{\mathcal{P}}(S)</math> of a set ''S'', the inclusion partial order is—up to an order isomorphism—the Cartesian product of <math>k = |S|</math> (the cardinality of ''S'') copies of the partial order on <math>\{0, 1\}</math> for which <math>0 < 1.</math> This can be illustrated by enumerating <math>S = \left\{ s_1, s_2, \ldots, s_k \right\},</math>, and associating with each subset <math>T \subseteq S</math> (i.e., each element of <math>2^S</math>) the ''k''-tuple from <math>\{0, 1\}^k,</math> of which the ''i''th coordinate is 1 if and only if <math>s_i</math> is a member of ''T''.

The set of all <math>k</math>-subsets of <math>A</math> is denoted by <math>\tbinom{A}{k}</math>, in analogue with the notation for binomial coefficients, which count the number of <math>k</math>-subsets of an <math>n</math>-element set. In set theory, the notation <math>[A]^k</math> is also common, especially when <math>k</math> is a transfinite cardinal number.

== Other properties of inclusion == * A set ''A'' is a '''subset''' of ''B'' if and only if their intersection is equal to A. Formally: :<math> A \subseteq B \text{ if and only if } A \cap B = A. </math> * A set ''A'' is a '''subset''' of ''B'' if and only if their union is equal to B. Formally: :<math> A \subseteq B \text{ if and only if } A \cup B = B. </math> * 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: :<math> A \subseteq B \text{ if and only if } |A \cap B| = |A|.</math> * The subset relation defines a partial order on sets. In fact, the subsets of a given set form a Boolean algebra 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. * Inclusion is the canonical partial order, in the sense that every partially ordered set <math>(X, \preceq)</math> 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>[n]</math> of all ordinals less than or equal to ''n'', then <math>a \leq b</math> if and only if <math>[a] \subseteq [b].</math>

==See also== *{{annotated link|Convex subset}} *{{annotated link|Inclusion order}} *{{annotated link|Mereology}} *{{annotated link|Region (mathematics)|Region}} *{{annotated link|Subset sum problem}} *{{annotated link|Hierarchy#Subsumptive_containment_hierarchy|Subsumptive containment}} *{{annotated link|Subspace (mathematics)|Subspace}} *{{annotated link|Total subset}}

==References== {{Reflist}}

== Bibliography == * {{cite book|author-link=Thomas Jech|author=Jech, Thomas|title=Set Theory|publisher=Springer-Verlag|year=2002|isbn=3-540-44085-2}}

==External links== *{{Commons category-inline|Subsets}} *{{MathWorld |title=Subset |id=Subset }}

Category:Basic concepts in set theory