In mathematics, especially in set theory, a set A is a subset of a set B, or equivalently B is a superset of A, if A is "contained" inside B, that is, all elements of A are also elements of B. A and B may coincide. The relationship of one set being a subset of another is called inclusion or sometimes containment.
The subset relation defines a partial order on sets.
The algebra of subsets forms a Boolean algebra in which the subset relation is called inclusion.
Definitions
If A and B are sets and every element of A is also an element of B, then:
 A is a subset of (or is included in) B, denoted by $A\; \backslash subseteq\; B$,
 or equivalently
 B is a superset of (or includes) A, denoted by $B\; \backslash supseteq\; A.$
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 also a proper (or strict) subset of B; this is written as $A\backslash subsetneq\; B.$
 or equivalently
 B is a proper superset of A; this is written as $B\backslash supsetneq\; A.$
For any set S, the inclusion relation ⊆ is a partial order on the set $\backslash mathcal\{P\}(S)$ of all subsets of S (the power set of S).
The symbols ⊂ and ⊃
Some authors^{[1]}^{:p.6} use the symbols ⊂ and ⊃ to indicate "subset" and "superset" respectively, instead of the symbols ⊆ and ⊇, but with the same meaning. So for example, for these authors, it is true of every set A that A ⊂ A.
Other authors^{[2]} prefer to use the symbols ⊂ and ⊃ to indicate proper subset and superset, respectively, in place of ⊊ and ⊋. This usage makes ⊆ and ⊂ analogous to the inequality symbols ≤ and <. For example, if x ≤ y then x may be equal to y, or maybe not, but if x < y, then x definitely does not equal y, and is strictly less than y. Similarly, using the "⊂ means proper subset" convention, if A ⊆ B, then A may or may not be equal to B, but if A ⊂ B, then A is definitely not equal to B.
Examples
 The set {1, 2} is a proper subset of {1, 2, 3}.
 Any set is a subset of itself, but not a proper subset.
 The empty set { }, denoted by ∅, is also a subset of any given set X. It is also always a proper subset of any set except itself.
 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, and the set of points in a line segment is a proper subset of the set of points in a line. These are examples in which both the part and the whole are infinite, and the part has the same cardinality (number of elements) as the whole; such cases can tax one's intuition.
Other properties of inclusion
Inclusion is the canonical partial order in the sense that every partially ordered set (X, $\backslash preceq$) 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 ≤ b if and only if [a] ⊆ [b].
For the power set $\backslash mathcal\{P\}(S)$ 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 = {s_{1}, s_{2}, …, s_{k}} and associating with each subset T ⊆ S (which is to say with each element of 2^{S}) the ktuple from {0,1}^{k} of which the ith coordinate is 1 if and only if s_{i} is a member of T.
See also
References
External links


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