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Relations

# Relations - CSE 1400 Applied Discrete Mathematics Relations...

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CSE 1400 Applied Discrete Mathematics Relations Department of Computer Sciences College of Engineering Florida Tech Fall 2011 Relations and Their Graphs 1 A Relation’s Domain, Co-domain, and Range 2 A Sampling of Relations 3 Equality 3 Less than 3 Divides 3 Congruence Modulo n 4 Perpendicular on Lines 5 The Incestuous and Empty Relations 5 A Relation is a Set of Ordered Pairs 6 The Inverse Relation 6 Counting Relations 7 Relational Properties 7 Reflexive Property 7 Symmetric Property 8 Antisymmetric Property 10 Transitive Property 11 Orders and Equivalences 11 Orders 12 Well-Ordered Sets 13 Equivalences 13 Equivalence Relations Partition a Set 14 Stirling Numbers of the Second Kind 16

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cse 1400 applied discrete mathematics relations 2 Problems on Relations 18 Abstract A relation describes how things are connected. That a thing a is related to a thing b can be represented by 1 . An ordered pair ( a , b ) . 2 . An directed edge a b . 3 . Or more commonly, simply using relational notation a b . A relation is 1 . A set G of ordered pairs . 2 . A directed graph G of nodes and edges. 3 . A matrix of True and False values. Higher-dimensional relations among a , b , c or more parameters can be defined. Higher-dimensional relations occur as tables in relational databases and as data in multi-variable problems. Relations and Their Graphs A relation is a set of ordered pairs . A relation can be pictured of as a graph G . a b c d e f h g This graph represents the relation G = { ( a , b ) , ( b , c ) , ( c , d ) , ( d , h ) , ( h , f ) , ( f , e ) , ( e , a ) } G = { ( x , y ) : x y } = { ( x , y ) : x is related to y . } There are many examples of relations. You are, no doubt, familiar with relations among people: Mother-Daughter, Father-Son, Parent- Child, Aunt-Nephew. Familial relations often become murky. We study relations that can be precisely defined. A few common rela- tions are equality, congruence mod n , less than, divides, subset, and perpendicular. In this course, relationships will be between two things a and b . Relationships among 3 or more things are common and useful, but The course studies binary relations. these ideas are not within the scope of this course. A Relation’s Domain, Co-domain, and Range T he things involved in a relation need names : C all them the things x and y . Write x y to express the phrase x is related to y .”
cse 1400 applied discrete mathematics relations 3 The value x belongs to a set X called the domain of . The value y belongs to a set Y called the co - domain of . The domain X is the set of elements that appear on the left-hand side of . For this course, you can assume that every element in X appears on the left-hand side of , that is, every relation we en- counter is total , defined on all members of X . A relation is said to be total when every x X occurs on on the left-hand side of for some y Y .

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