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Week 8 - Wednesday

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What did we talk about last time? Cardinality Countability Relations

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There is a Grand Hotel with an infinite number of rooms Their slogan is: "We're always full, but we always have room for you!" Although completely full, when a single guest arrives, he or she can be accommodated by moving into Room 1 The guest in Room 1 will move into Room 2, the guest in Room 2 will move into Room 3, and so on, with each guest in Room n moving into Room n + 1 Unfortunately, tragedy has struck! The hotel across the street, Inn Finite, which also has a (countably) infinite number of rooms, has just burned down Describe how all of Inn Finite's guests can be accommodated at the Grand Hotel

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To solve sequence a k = Aa k-1 + Ba k-2 Find its characteristic equation t 2 – At – B = 0 If the equation has two distinct roots r and s Substitute a 0 and a 1 into a n = Cr n + Ds n to find C and D If the equation has a single root r Substitute a 0 and a 1 into a n = Cr n + Dnr n to find C and D

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Student Lecture

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We will discuss many useful properties that hold for any binary relation R on a set A (that is from elements of A to other elements of A) Relation R is reflexive iff for all x A, (x, x) R Informally, R is reflexive if every element is related to itself R is not reflexive if there is an x A, such that (x, x) R

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Relation R is symmetric iff for all x, y A, if (x, y) R then (y, x) R Informally, R is symmetric if for every element related to another element, the second element is also related to the first R is not symmetric if there is an x, y A, such that (x, y) R but (y, x) R

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Relation R is transitive iff for all x, y, z A, if (x, y) R and (y, z) R then (x, z) R Informally, R is transitive when an element is related to a second element and a second element is related to a third, then it must be the case that the first element is also related to the third R is not transitive if there is an x, y, z A, such that (x, y) R and (y, z) R but (x, z) R

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Let A = {0, 1, 2, 3} Let R = {(0,0), (0,1), (0,3), (1,0), (1,1), (2,2), (3,0), (3,3)} Is R reflexive? Is R symmetric? Is R transitive? Let S = {(0,0), (0,2), (0,3), (2,3)} Is S reflexive? Is S symmetric? Is S transitive? Let T = {(0,1), (2,3)} Is T reflexive? Is T symmetric? Is T transitive?

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If you have a set A and a binary relation R on A, the transitive closure of R called R t satisfies the following properties: R t is transitive R RtR Rt If S is any other transitive relation that contains R, then R t S Basically, the transitive closure just means adding in the least amount of stuff to R to make it transitive If you can get there in R, you can get there directly in R t

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Let A = {0, 1, 2, 3} Let R = {(0,1), (1,2), (2,3)} Is R transitive? Then, find the transitive closure of R

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Let R be a relation on real numbers R such that x R y x = y Is R reflexive? Is R symmetric? Is R transitive? Let S be a relation on real numbers R such that x S y x < y Is S reflexive? Is S symmetric? Is S transitive? Let T be a relation on positive integers N such that m T n 3 | (m – n) Is T reflexive? Is T symmetric? Is T transitive?

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A partition of a set A (as we discussed earlier) is a collection of nonempty, mutually disjoint sets, whose union is A A relation can be induced by a partition For example, let A = {0, 1, 2, 3, 4} Let A be partitioned into {0, 3, 4}, {1}, {2} The binary relation induced by the partition is: x R y x and y are in the same subset of the partition List the ordered pairs in R

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Given set A with a partition Let R be the relation induced by the partition Then, R is reflexive, symmetric, and transitive As it turns out, any relation R is that is reflexive, symmetric, and transitive induces a partition We call a relation with these three properties an equivalence relation

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We say that m is congruent to n modulo d if and only if d | (m – n) We write this: m n (mod d) Congruence mod d defines an equivalence relation Reflexive, because m m (mod d) Symmetric because m n (mod d) means that n m (mod d) Transitive because m n (mod d) and n k (mod d) mean that m k (mod d) Which of the following are true? 12 7 (mod 5) 6 -8 (mod 4) 3 3 (mod 7)

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Let A be a set and R be an equivalence relation on A For each element a in A, the equivalence class of a, written [a], is the set of all elements x in A such that a R x Example Let A be { 0, 1, 2, 3, 4, 5, 6, 7, 8} Let R be congruence mod 3 What's the equivalence class of 1? For A with R as an equivalence relation on A If b [a], then [a] = [b] If b [a], then [a] [b] =

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Modular arithmetic has many applications For those of you in Security, you know how many of them apply to cryptography To help us, the following statements for integers a, b, and n, with n > 1, are all equivalent 1. n | (a – b) 2. a b (mod n) 3. a = b + kn for some integer k 4. a and b have the same remainder when divided by n 5. a mod n = b mod n

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Let a, b, c, d and n be integers with n > 1 Let a c (mod n) and b d (mod n), then: 1. (a + b) (c + d) (mod n) 2. (a – b) (c – d) (mod n) 3. ab cd (mod n) 4. a m c m (mod n), for all positive integers m If a and n are relatively prime (share no common factors), then there is a multiplicative inverse a -1 such that a -1 a 1 (mod n) I'd love to have us learn how to find this, but there isn't time

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Let R be a relation on a set A R is antisymmetric iff for all a and b in A, if a R b and b R a, then a = b That is, if two different elements are related to each other, then the relation is not antisymmetric Let R be the "divides" relation on the set of all positive integers Is R antisymmetric? Let S be the "divides" relation on the set of all integers Is S antisymmetric?

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A relation that is reflexive, antisymmetric, and transitive is called a partial order The subset relation is a partial order Show it's reflexive Show it's antisymmetric Show it's transitive The less than or equal to relation is a partial order Show it's reflexive Show it's antisymmetric Show it's transitive

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Review for Exam 2

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Work on Homework 6 Due Friday before midnight Study for Exam 2 Next Monday in class Review on Friday

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