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# lecnotes3.3 - R or by describing the property that relates...

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Lecture Notes, Concepts of Mathematics (21-127) Lecture 1, Recitation A–D, Spring 2008 3 Relations 3.3 Relations, Equivalence Relations, and Partitions Recall the deﬁnition of the Cartesian product X × Y of two sets X and Y . From basic set theory, we have: Theorem 3.1 If A , B , C , and D are sets, then (a) A × ( B C ) = ( A × B ) ( A × C ) (b) A × ( B C ) = ( A × B ) ( A × C ) (c) A × ∅ = (d) ( A × B ) ( C × D ) = ( A C ) × ( B D ) (e) ( A × B ) ( C × D ) ( A C ) × ( B D ) Proof: (Part (a)) The ordered pair ( x,y ) A × ( B C ) iﬀ x A and y B C iﬀ x A and ( y B or y C ) iﬀ ( x A and x B ) or ( x A and y C ) iﬀ ( x,y ) A × B or ( x,y ) A × C iﬀ ( x,y ) ( A × B ) ( A × C ). Therefore A × ( B C ) = ( A × B ) ( A × C ). The other parts of the proof are left as an exercise. ± Deﬁnition 3.1 A relation R from a set X to a set Y is a subset of X × Y : R = { ( x,y ) X × Y | xRy } . Subsets of X × X are called relations on X . Deﬁnition 3.2 The domain of the relation R from X to Y is the set Dom ( R ) = { x X | there exists y Y such that xRy } . The range of the relation R from X to Y is the set Rng ( R ) = { y Y | there exists x X such that xRy } . 1

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A relation R can be deﬁned by listing the elements of
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Unformatted text preview: R or by describing the property that relates the two elements. Example 3.1 Let A = { 1 , 2 , 3 } and B = { a, 5 , { b } ,c } and R be given by R = { (1 ,a ) , (1 ,b ) , (2 ,a ) , (2 ,c ) } . Then • we have 1 Ra , 1 R { b } , 2 Ra , 2 Rc , 3 6 R 5 • Dom ( R ) = { 1 , 2 } ⊆ A • Rng ( R ) = { a, { b } ,c } ⊆ B Example 3.2 Let P be the set of all people. Let L = { ( a,b ) ∈ P × P | a and b have the same last name } . Then L is a relation on P . Example 3.3 Let S be a relation on the set N × N deﬁned by ( m,n ) S ( k,j ) iﬀ m + n = k + j . Then (3 , 17) S (12 , 8) but (5 , 4) 6 S (6 , 15) . Note that we can write S as S = { (( m,n ) , ( k,j )) ∈ N 2 × N 2 | m + n = k + j } . What are the domain and range of S ? 2...
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## This note was uploaded on 01/25/2010 for the course MATH 21127 taught by Professor Howard during the Spring '08 term at Carnegie Mellon.

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lecnotes3.3 - R or by describing the property that relates...

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