Euler diagram showing A is a subset of B, A ⊆ B, and conversely B is a superset of A, B ⊇ A.
In mathematics,
setA 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.
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 on the set ${\mathcal {P}}(S)$ (the
power set of S—the set of all subsets of S^{
[1]}) defined by $A\leq B\iff A\subseteq B$. We may also partially order ${\mathcal {P}}(S)$ by reverse set inclusion by defining $A\leq B{\text{ if and only if }}B\subseteq A.$
When quantified, $A\subseteq B$ is represented as $\forall x\left(x\in A\implies x\in B\right).$^{
[2]}
We can prove the statement $A\subseteq B$ by applying a proof technique known as the element argument^{
[3]}:
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: 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 ${\mathcal {P}}(A)$. The set of all $k$-subsets of $A$ is denoted by ${\tbinom {A}{k}}$, in analogue with the notation for
binomial coefficients, which count the number of $k$-subsets of an $n$-element set. In
set theory, the notation $[A]^{k}$ is also common, especially when $k$ is a
transfinitecardinal number.
Properties
A set A is a subset of Bif and only if their intersection is equal to A.
Formally:
$A\subseteq B{\text{ if and only if }}A\cap B=A.$
A set A is a subset of B if and only if their union is equal to B.
Formally:
$A\subseteq B{\text{ if and only if }}A\cup B=B.$
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:
$A\subseteq B{\text{ if and only if }}|A\cap B|=|A|.$
⊂ and ⊃ symbols
Some authors use the symbols $\subset$ and $\supset$ to indicate subset and superset respectively; that is, with the same meaning as and instead of the symbols $\subseteq$ and $\supseteq .$^{
[4]} 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 proper (also called strict) subset and proper superset respectively; that is, with the same meaning as and instead of the symbols $\subsetneq$ and $\supsetneq .$^{
[5]} 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.
The set A = {1, 2} is a proper subset of B = {1, 2, 3}, thus both expressions $A\subseteq B$ and $A\subsetneq B$ are true.
The set D = {1, 2, 3} is a subset (but not a proper subset) of E = {1, 2, 3}, 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 {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 power) than the former set.
$A\subseteq B$ and $B\subseteq C$ implies $A\subseteq C.$
Inclusion is the canonical
partial order, 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 $[n]$ of all ordinals less than or equal to n, then $a\leq b$ if and only if $[a]\subseteq [b].$
For the
power set$\operatorname {\mathcal {P}} (S)$ 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 $\{0,1\}$ for which $0<1.$ This can be illustrated by enumerating $S=\left\{s_{1},s_{2},\ldots ,s_{k}\right\},$, and associating with each subset $T\subseteq S$ (i.e., each element of $2^{S}$) the k-tuple from $\{0,1\}^{k},$ of which the ith coordinate is 1 if and only if $s_{i}$ is a
member of T.