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Comment: Unions and intersections
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| Deletions are marked like this. | Additions are marked like this. |
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| ''A ⊆ B'' means that ''A'' is a '''subset''' of ''B''. This can be formally expressed as ''∀ a ((a ∈ A) -> (a ∈ B))''. | ''A ⊆ B'' means that ''A'' is a '''subset''' of ''B''. ''∀ a ((a ∈ A) -> (a ∈ B))''. |
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| This gives a test for set equality: if ''A ⊆ B'' and ''B ⊆ A''. | This leads to a test for set equality: if ''A ⊆ B'' and ''B ⊆ A''. |
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=== Complements === The '''complement''' of ''A'' contains all elements that are not members of ''A''. This is usually notated as ''A^c^''. |
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| A pair of sets are '''disjoint''' if there is no intersection, i.e., ''A ⋂ B = ∅''. The '''set difference''' of ''A'' with respect to ''B'' contains all members in ''A'' that are not in ''B''. ''A \ B = {x ∈ A | x ∉ B}''. |
Sets
Sets are a collection of members.
Description
A set is a collection of members. Generally, sets are defined and notated like {a,b,c}.
A set can also be defined by a condition. For example, {x ∈ R | x > 0} should be read as the set of all real values greater than 0.
The empty set is notated Ø.
Commonly Used Sets
The commonly re-used notations for basic sets of numbers are:
Natural numbers (N)
Integers (Z)
Rational numbers (Q)
Real numbers (R)
Logic
Membership
a ∈ A means a is a member of A. Conversely, a ∉ A means a is not a member of A.
Subsets
A ⊆ B means that A is a subset of B. ∀ a ((a ∈ A) -> (a ∈ B)).
This leads to a test for set equality: if A ⊆ B and B ⊆ A.
If A ⊆ B and A != B, then A is a proper subset of B. This is notated as either A ⊂ B or A ⊊ B.
The reverse relation could be notated as B ⊇ A, B ⊃ A, and so on.
Complements
The complement of A contains all elements that are not members of A. This is usually notated as Ac.
Operations
The union of two sets contains all members of both. A ⋃ B = {x | x ∈ A or x ∈ B}.
The union of all subsets Ai can be expressed as:
The intersection of two sets contains all members that are common between the two. A ⋂ B = {x | x ∈ A and x ∈ B}.
The intersection of all subsets Ai can be expressed as:
A pair of sets are disjoint if there is no intersection, i.e., A ⋂ B = ∅.
The set difference of A with respect to B contains all members in A that are not in B. A \ B = {x ∈ A | x ∉ B}.