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CSE115/ENGR160 Discrete Mathematics 05/03/11 Ming-Hsuan Yang UC Merced 1

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8.4 Equivalence relation In traditional C, only the first 8 characters of a variable are checked by the complier Let R be relation on the set of strings of characters s.t. sRt where s and t are two strings, if s and t are at least 8 characters long and the first 8 characters of s and t agree, or s=t Easy to see R is reflexive, symmetric, and transitive 2

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Example The integers a and b are related by the “congruence modulo 4” relation when 4 divides a-b We will see this relation is reflexive, symmetric and transitive The above-mentioned two relations are examples of equivalence relations, namely, relations that are reflexive, symmetric, and transitive 3

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Equivalence relation A relation on a set A is called an equivalence relation if it is reflexive, symmetric, and transitive Important property in mathematics and computer science Two elements a and b are related by an equivalence relation are called equivalent, denoted by a ∼ b, a and b are equivalent elements w.r.t. a particular equivalence relation 4

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Example Let R be the relation on the set of integers s.t. aRb iff a=b or a=-b We previously showed that R is reflexive, symmetric, and transitive – R is reflexive, aRa iff a=a or a=-a – R is symmetric, aRb if a=b or a=-b, then b=a or b=-a and so bRa (also for the only if part) – R is transitive, if aRb and bRc, then (a=b or a=-b) and (b=c or b=-c). So a=c or a=-c. Thus aRc (also for the only if part) If follows that R is an equivalence relation 5

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Example Let R be the relation on the set of real numbers s.t. aRb iff a-b is an integer. Is R an equivalence relation? Since a-a=0 is an integer for all real number a, aRa for all a. Hence R is reflexive Now suppose aRb, then a-b is an integer, so b-a is an integer. Hence bRa. Thus R is symmetric If aRb and bRc, then a-b and b-c are integers. So, a- c=(a-b)+(b-c) is an integer. Hence aRc. Thus R is transitive Consequently, R is an equivalence relation 6

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Example Let m be a positive integer with m>1. Show that the relation R={(a,b)|a ≡ b (mod m)} is an equivalence relation on the set of integers Recall a ≡ b (mod m) iff m divides a-b. Note that a-a=0 is divided by m. Hence a ≡ a (mod m). So congruence modulo m is reflexive Suppose a ≡ b(mod m), then a-b is divisible by m, so a-b=km, where k is an integer. It follows b-a=(-k)m, so b ≡ a(mod m). Hence, congruence modulo m is symmetric 7

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Example Suppose a ≡ b(mod m) and b ≡ c(mod m). Then m divides both a-b and b-c. Thus, there are integers k and l with a-b=km and b-c=lm Put them together a-c=(a-b)+(b-c) = km+lm = (k+l)m. Thus a ≡ c(mod m). So, congruence modulo m is transitive It follows that congruence modulo m is an equivalence relation 8

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Example Let R be the relation on the set of real numbers s.t. xRy iff x and y are real numbers that differ by less than 1, that is |x-y|<1. Show that R is not an equivalence relation R is reflexive as |x-x|=0<1 where x ∊ R R is symmetric for if xRy, then |x-y|<1, which tells us |y-x|<1. So yRx R is not transitive. Take x=2.8, y=1.9, z=1.1, so that |x-y|=0.9 1 So R is not an equivalence relation 9

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8.6 Partial orderings Often use relations to order some or all of the elements of sets Example: order words, schedule projects A relation R on a set S is called partial ordering or partial order if it is reflexive, antisymmetric, and transitive A set S together with a partial ordering R is called partially ordered set, or poset, and is denoted by (S,R) Members of S are called elements of the poset 10

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Example Show that the “greater than or equal” relation ( ≥ ) is a partial ordering on the set of integers ≥ is reflexive as a ≥ a ≥ is antisymmetric as if a ≥ b and b ≥ a then a=b ≥ is transitive as if if a ≥ b and b ≥ c then a ≥ c (Z, ≥ ) is a poset 11

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Example The divisbility relation | is a partial ordering on the set of positive integers, as it is reflexive, antisymmetric, and transitive We see that (Z +, |) is a poset 12

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Example Show that inclusion ⊆ is a partial ordering (the relation of one set being a subset of another is called inclusion) on the power set of a set S Example: power set of {0,1,2} is P({0,1,2})= { ∅, {0}, {1}, {2}, {0,1}, {0,2}, {1,2}, {0,1,2}} A ⊆ A whenever A is a subset of S, so ⊆ is reflexive It is antiysmmetric as A ⊆ B and B ⊆ A imply that A=B It is transitive as A ⊆ B and B ⊆ C imply that A ⊆ C. Hence ⊆ is a partial ordering on P(S), and (P(S), ⊆ ) is a poset 13

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Example Let R be the relation on the set of people s.t. xRy if x and y are people and x is older than y. Show that R is not a partial ordering R is antisymmetric if a person x is order than a person y, then y is not order than x R is transitive R is not reflexive as no person is older than himself/herself 14

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Relation in any poset In different posets different symbols such as ≤, ⊆ and | are used for a partial ordering Need a symbol we can use when we discuss the ordering in an arbitrary poset The notation a ≼ b is used to denote (a,b) ∊ R in an arbitrary poset (S,R) ≼ is used as the “less than or equal to” relation on the set of real numbers is the most familiar example of a partial ordering, and similar to ≤ symbol ≼ is used for any poset, not just “less than or equals” relation, i.e., (S, ≼ ) 15

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Comparable The elements a and b of a poset (S, ≼ ) are called comparable if either a ≼ b or b ≼ a. When a and b are elements of S s.t. neither a ≼ b nor b ≼ a, a and b are incomparable In the poset (Z+, |), are the integers 3 and 9 comparable? Are 5 and 7 comparable? The integers 3 and 9 are comparable as 3|9. the integers 5 and 7 are incomparable as 5 does not divide 7 and 7 does not divide 5 16

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Total ordering Pairs of elements may be incomparable and thus we have “partial” ordering When every 2 elements in the set are comparable, the relation is called total ordering If (S, ≼ ) is a poset and every two elements of S are comparable, S is called a totally ordered or linearly ordered set, and ≼ is called a total order or a linear order A totally ordered set is also called a chain 17

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Example The poset (Z, ≤ ) is totally ordered as a ≤ b or b ≤ a whenever a and b are integers The poset (Z +, |) is not tally ordered as it contains elements that are incomparable, such as 5 and 7 18

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Well-ordered set (S, ≼ ) is a well-ordered set if it is a poset s.t. ≼ is a total ordering and every nonempty subset of S has a least element The set of ordered pairs of positive integers, Z + × Z + with (a 1, a 2 ) (b 1, b 2 ) if a 1 < a 2 or if a 1 =b 1 and a 2 ≤ b 2 (the lexicographic ordering), is a well-ordered set The set Z, with the usual ordering, is not well- ordered as the set of negative integers has no least element 19

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