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Slide08 Relations

# Slide08 Relations - D iscrete Mathematics Discrete...

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Discrete Mathematics Discrete Mathematics (2009 Spring) Relations (Chapter 8, 5 hours) Chih-Wei Yi Dept. of Computer Science National Chiao Tung University May 25, 2009

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Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Binary Relations De°nition Let A and B be any two sets. A binary relation R from A to B , written R : A \$ B , is a subset of A ° B . The notation aRb means ( a , b ) 2 R . If aRb , we may say ± a is related to b (by relation R )², or ± a relates to b (under relation R )². Example < : N \$ N : ± f ( n , m ) j n < m g . a < b means ( a , b ) 2 < . A binary relation R corresponds to a predicate function P R : A ° B ! f T , F g de°ned over the 2 sets A and B .
Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Examples of Binary Relations Let A = f 0 , 1 , 2 g and B = f a , b g . Then R = f ( 0 , a ) , ( 0 , b ) , ( 1 , a ) , ( 2 , b ) g is a relation from A to B . For instance, we have 0 Ra , 0 Rb , etc.. Can we have visualized expressions of relations? Let A be the set of all cities, and let B be the set of the 50 states in the USA. De°ne the relation R by specifying that ( a , b ) belongs to R if city a is in state b . For instance, (Boulder,Colorado), (Bangor,Maine), (Ann Arbor,Michigan), (Middletown,New Jersey), (Middletown,New York), (Cupertino,California), and (Red Bank,New Jersey) are in R . ±eats² : ± f ( a , b ) j organism a eats food b g .

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Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Complementary Relations De°nition Let R : A \$ B be any binary relation. Then, R : A \$ B , the complement of R , is the binary relation de°ned by R : ± f ( a , b ) j ( a , b ) / 2 R g = ( A ° B ) ² R . Note this is just R if the universe of discourse is U = A ° B ; thus the name complement. The complement of R is R .
Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Inverse Relations De°nition Any binary relation R : A \$ B has an inverse relation R ² 1 : B \$ A , de°ned by R ² 1 : ± f ( b , a ) j ( a , b ) 2 R g . Examples 1 < ² 1 = f ( b , a ) j a < b g = f ( b , a ) j b > a g = > . 2 If R : People ! Foods is de°ned by " aRb , a eats b ", then bR ² 1 a , b is eaten by a .

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Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Examples Example Let A = f 1 , 2 , 3 , 4 , 5 g and R : A \$ A : ± f ( a , b ) : a j b g . What are R and R ² 1 ? Solution R = ° ( 1 , 1 ) , ( 1 , 2 ) , ( 1 , 3 ) , ( 1 , 4 ) , ( 1 , 5 ) , ( 2 , 2 ) , ( 2 , 4 ) , ( 3 , 3 ) , ( 4 , 4 ) , ( 5 , 5 ) ±
Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Examples Example Let A = f 1 , 2 , 3 , 4 , 5 g and R : A \$ A : ± f ( a , b ) : a j b g . What are R and R ² 1 ? Solution R = ° ( 1 , 1 ) , ( 1 , 2 ) , ( 1 , 3 ) , ( 1 , 4 ) , ( 1 , 5 ) , ( 2 , 2 ) , ( 2 , 4 ) , ( 3 , 3 ) , ( 4 , 4 ) , ( 5 , 5 ) ± R = 8 < : ( 2 , 1 ) , ( 2 , 3 ) , ( 2 , 5 ) , ( 3 , 1 ) , ( 3 , 2 ) , ( 3 , 4 ) , ( 3 , 5 ) , ( 4 , 1 ) , ( 4 , 2 ) , ( 4 , 3 ) , ( 4 , 5 ) , ( 5 , 1 ) , ( 5 , 2 ) , ( 5 , 3 ) , ( 5 , 4 ) 9 = ;

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Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Examples Example Let A = f 1 , 2 , 3 , 4 , 5 g and R : A \$ A : ± f ( a , b ) : a j b g . What are R and R ² 1 ? Solution R = ° ( 1 , 1 ) , ( 1 , 2 ) , ( 1 , 3 ) , ( 1 , 4 ) , ( 1 , 5 ) , ( 2 , 2 ) , ( 2 , 4 ) , ( 3 , 3 ) , ( 4 , 4 ) , ( 5 , 5 ) ± R = 8 < : ( 2 , 1 ) , ( 2 , 3 ) , ( 2 , 5 ) , ( 3 , 1 ) , ( 3 , 2 ) , ( 3 , 4 ) , ( 3 , 5 ) , ( 4 , 1 ) , ( 4 , 2 ) , ( 4 , 3 ) , ( 4 , 5 ) , ( 5 , 1 ) , ( 5 , 2 ) , ( 5 , 3 ) , ( 5 , 4 ) 9 = ; R ² 1 = ° ( 1 , 1 ) , ( 2 , 1 ) , ( 3 , 1 ) , ( 4 , 1 ) , ( 5 , 1 ) , ( 2 , 2 ) , ( 4 , 2 ) , ( 3 , 3 ) , ( 4 , 4 ) , ( 5 , 5 ) ±
Discrete Mathematics Chapter 8 Relations §8.1 Relations and Their Properties Combining Relations Since relations from A to B are subsets of A ° B , two relations from A to B can be combined through set operations.

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