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1 Math/CSE 1019C: Discrete Mathematics for Computer Science Fall 2011 Suprakash Datta datta@cse.yorku.ca Office: CSEB 3043 Phone: 416-736-2100 ext 77875 Course page: http://www.cse.yorku.ca/course/1019

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2 Last class: quick recap Sections 1.1 -- 1.3 Why logic? Why Propositional logic? What is a proposition? Tautology, contradiction Manipulating propositions –not, and, or, implication, biconditional Truth tables Propositional equivalences Table 6 (page 27)

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3 Last class: quick recap – contd. Sections 1.4 -- 1.5: Predicate logic Why Predicate logic? What is a predicate? Translating English sentences to predicates Negation of quantifiers

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4 Translation into predicates “All students in this class are from FSE” “There are digital media majors in this class” “Every student has a smartphone or has a friend that owns a smartphone” “There are no students in this class that does not send text messages daily”

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5 Scope of Quantifiers have higher precedence than operators from Propositional Logic; so x P(x) Q(x) x (P(x) Q(x)) Use brackets E.g. x (P(x) Q(x)) x R(x) Say P(x): x is odd, Q(x): x is divisible by 3, R(x): (x=0) (2x >x) /

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6 Negation of Quantifiers Consider the statement Q: x P(x) where P(x) is a given predicate over a given domain. What does “Q is false” mean? Similarly what does “ x P(x) is false” mean?

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7 Negation of Quantifiers - contd x P(x) x P(x) x P(x) x P(x) E.g.: – “There is no student who can …” –“Not all professors are bad” “There is no Toronto Raptor that can dunk like Vince …” Caution: The negation of “Every Canadian loves Hockey” is NOT “No Canadian loves Hockey”! Many, many students make this mistake!

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8 Nested Quantifiers Allows simultaneous quantification of many variables. E.g. – domain positive integers, x y z x 2 + y 2 = z 2 n>2 x y z x n + y n = z n (Fermat’s Last Theorem) Domain real numbers: x y z (x < z < y) (y < z < x) x y z (x=y) (x < z < y) (y < z < x) x y z (x y) (x < z < y) (y < z < x) Is it true?

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9 Nested Quantifiers - 2 x y (x + y = 0) is true over the integers Proof: Assume an arbitrary integer x. To show that there exists a y that satisfies the requirement of the predicate, choose y = -x. Clearly y is an integer, and thus is in the domain. So x + y = x + (-x) = x – x = 0. Since we assumed nothing about x (other than it is an integer), the argument holds for any integer x. Therefore, the predicate is TRUE.

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10 Nested Quantifiers - 3 Analogy: quantifiers are like loops: An inner quantified variable can depend on the outer quantified variable. E.g. in x y (x + y = 0) we chose y=-x, so for different x we need different y to satisfy the statement. p j Accept (p,j) does NOT say that there is a j that will accept all p. p,j have different domains

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11 Nested Quantifiers - 4 Caution: In general, order matters! Consider the following propositions over the integer domain: x y (x < y) and y x (x < y) x y (x < y) : “there is no maximum integer” y x (x < y) : “there is a maximum integer” Not the same meaning at all!!!

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12 Negation of Nested Quantifiers Use the same rule as before carefully. Ex 1: x y (x + y = 0) –This is equivalent to x y (x + y = 0) –This is equivalent to x y (x + y = 0) –This is equivalent to x y (x + y 0) Ex 2: x y (x < y) –This is equivalent to x y (x < y) –This is equivalent to x y (x < y) –This is equivalent to x y (x y)

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13 Logical Equivalence of statements Page 45: P Q if and only if they have same truth value no matter which domain is used and no matter which predicates are assigned to predicate variables.

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14 How to prove Truth tables may not help Domain may be infinite (e.g. integers) Prove each direction separately Use intuitive ideas (example 19 on page 45).

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15 Proof and counterexamples 1.To prove a statement of the form x P(x) it is not enough to show that P(a) is true for one or some a’s. 2.To show that a statement of the form x P(x) is FALSE, it is enough to show that P(a) is false for one a (“counterexample”). 3.To prove a statement of the form x P(x) it is enough to show that P(a) is true for one a.

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16 Exercises Check that: x y (x + y = 0) is not true over the positive integers. x y (x + y = 0) is not true over the integers. x 0 y (y = 1/x) is true over the real numbers.

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17 Readings and problems Read 1.4-1.5. Practice: Q2,8,16,30 (pg 65-67) Next: Rules of inference (1.6).

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18 Inference rules Recall: the reason for studying logic was to formalize derivations and proofs. How can we infer facts using logic? Let’s start with Propositional logic.

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19 Inference rules - 2 Simple inference rule (Modus Ponens) : From (a) p q and (b) p is TRUE, we can infer that q is TRUE. Example: (a) if these lecture slides (ppt) are online then you can print them out (b) these lecture slides are online Similarly, From p q, q r and p is TRUE, we can infer that r is TRUE.

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20 Inference rules - 3 ((p q) p ) q is a TAUTOLOGY. Modus Tollens, Hypothetical syllogism and disjunctive syllogism can be seen as alternative forms of Modus Ponens Other rules like “From p is true we can infer p q ” are very intuitive

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21 Inference rules - 4 Resolution: From (a) p q and (b) p r, we can infer that q r Exercise: check that ((p q) ( p r)) (q r) is a TAUTOLOGY. Very useful in computer generated proofs.

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22 Inference rules - 5 Read rules on page 72. Understanding the rules is crucial, memorizing is not. You should be able to see that the rules make sense and correspond to our intuition about formal reasoning.

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23 Inference rules for quantified statements Very intuitive, e.g. Universal instantiation – If x P(x) is true, we infer that P(a) is true for any given a E.g.: Universal Modus Ponens: x P(x) Q(x) and P(a) imply Q(a) If x is odd then x 2 is odd, a is odd. So a 2 is odd.

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24 Inference rules for quantified statements-2 Read rules on page 76 Again, understanding is required, memorization is not.

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25 Aside: Inference and Planning The steps in an inference are useful for planning an action. Example: your professor has assigned reading from an out-of-print book. How do you do it? Example 2: you are participating in the television show “Amazing race”. How do you play?

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26 Aside 2: Inference and Automatic Theorem-Proving The steps in an inference are useful for proving assertions from axioms and facts. Why is it important for computers to prove theorems? –Proving program-correctness –Hardware design –Data mining –…..

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27 Aside 2: Inference and Automatic Theorem-Proving – contd. Sometimes the steps of an inference (proof) are useful. E.g. on Amazon book recommendations are made. You can ask why they recommended a certain book to you (reasoning).

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28 Next Introduction to Proofs (Sec 1.7) What is a (valid) proof? Why are proofs necessary?

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29 Introduction to Proof techniques Why are proofs necessary? What is a (valid) proof? What details do you include/skip? “Obviously”, “clearly”…

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30 Assertions Axioms Proposition, Lemma, Theorem Corollary Conjecture

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31 Logic-based proof Every step should follow from axioms or previous step(s) using an inference rule. Problems: –Axiomatization is hard and often long (see Appendix 1) –Proofs are often very long and tedious Intuitive proofs :

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32 Types of Proofs Direct proofs (including Proof by cases) Proof by contraposition Proof by contradiction Proof by construction Proof by Induction Other techniques

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33 Direct Proofs The average of any two primes greater than 2 is an integer. Every prime number greater than 2 can be written as the difference of two squares, i.e. a 2 – b 2.

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34 Proof by cases If n is an integer, then n(n+1)/2 is an integer Case 1: n is even. or n = 2a, for some integer a So n(n+1)/2 = 2a*(n+1)/2 = a*(n+1), which is an integer. Case 2: n is odd. n+1 is even, or n+1 = 2a, for an integer a So n(n+1)/2 = n*2a/2 = n*a, which is an integer.

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35 Proof by contraposition If (pq) (p+q)/2, then p q Direct proof left as exercise Contrapositive: If p = q, then (pq) = (p+q)/2 Easy: (pq) = (pp) = (p 2 ) = p = (p+p)/2 = (p+q)/2.

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36 Proof by contraposition- 2 Prove: If x 2 is even, x is even Proof: if x is not even, x is odd. Therefore x 2 is odd. This is the contrapositive of the original assertion. Note that the problem is to prove an implication. Universal generalization

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37 Proof by Contradiction 2 is irrational Suppose 2 is rational. Then 2 = p/q, such that p, q have no common factors. Squaring and transposing, p 2 = 2q 2 (even number) So, p is even (previous slide) Or p = 2x for some integer x So 4x 2 = 2q 2 or q 2 = 2x 2 So, q is even (previous slide) So, p,q are both even – they have a common factor of 2. CONTRADICTION. So 2 is NOT rational. Q.E.D.

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38 Proof by Contradiction - 2 In general, start with an assumption that statement A is true. Then, using standard inference procedures infer that A is false. This is the contradiction. Recall: for any proposition p, p p must be false

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39 Existence Proofs There exists integers x,y,z satisfying x 2 +y 2 = z 2 Proof: x = 3, y = 4, z = 5. This is a constructive proof (produce an example).

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40 Existence Proofs - 2 There exists irrational b,c, such that b c is rational (page 97) Nonconstructive proof: Consider 2 2. Two cases are possible: Case 1: 2 2 is rational – DONE (b = c = 2). Case 2: 2 2 is irrational – Let b = 2 2, c = 2. Then b c = ( 2 2 ) 2 = ( 2) 2* 2 = ( 2) 2 = 2

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41 Uniqueness proofs E.g. the equation ax+b=0, a,b real, a 0 has a unique solution.

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42 The Use of Counterexamples All prime numbers are odd Every prime number can be written as the difference of two squares, i.e. a 2 – b 2.

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43 Examples Show that if n is an odd integer, there is a unique integer k such that n is the sum of k-2 and k+3. Prove that there are no solutions in positive integers x and y to the equation 2x 2 + 5y 2 = 14. If x 3 is irrational then x is irrational Prove or disprove – if x, y are irrational, x + y is irrational.

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44 Alternative problem statements “show A is true if and only if B is true” “show that the statements A,B,C are equivalent”

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45 Exercises Q8, 10, 26, 28 on page 91

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46 What can we prove? The statement must be true We must construct a valid proof

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47 The role of conjectures 3x+1 conjecture Game: Start from a given integer n. If n is even, replace n by n/2. If n is odd, replace n with 3n+1. Keep doing this until you hit 1. e.g. n=5 16 8 4 2 1 Q: Does this game terminate for all n?

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48 Elegance in proofs Q: Prove that the only pair of positive integers satisfying a+b=ab is (2,2). Many different proofs exist. What is the simplest one you can think of?

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49 Next Ch. 2: Introduction to Set Theory Set operations Functions Cardinality

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