Discussion 3: Solutions ECS 20 (Fall 2014) Patrice Koehl [email protected] October 21, 2014 Exercise 1 Let p and q be two propositions. The proposition p N OR q is true when both p and q are false, and it is false otherwise. It is denoted p ↓ q. a) Write down the truth table for p ↓ q p T T F F p↓q F F F T q T F T F b) Show that p ↓ q is equivalent to ¬(p ∨ q) p T T F F q T F T F p↓q F F F T p∨q T T T F (¬(p ∨ q) F F F T Therefore p ↓ q is equivalent to ¬(p ∨ q) c) Find a compound proposition logically equivalent to p → q using only the logical operator ↓. 1 p T T F F q T F T F p↓p F F T T (p ↓ p) ↓ q F T F F ((p ↓ p) ↓ q) ↓ ((p ↓ p) ↓ q) T F T T p→q T F T T Exercise 2 Let P (x) be the statement “x = x2 . . If the domain consists of the integers, what are the truth values of the following statements: a) P (0) P (0): 0 = 02 : true b) P (1) P (1): 1 = 12 : true c) P (2) P (2): 2 = 22 : false d) P (−1) P (−1): −1 = (−1)2 : false e) ∃x P (x) The statement is true: P (1) is true: proof by example f) ∀x P (x) The statement is false: P (2) is false: proof by counter-example Exercise 3 Express each of these statements using quantifiers. Then form the negation of the statement so that no negation is to the left of a quantifier. Next, express the negation in simple English. (Do not simply use the phrase “It is not the case that.”) a) All dogs have fleas. ∀d ∈ Dogs, d has fleas. Negation: There exists a dog that does not have flea. b) There exists a horse that can add. ∃h ∈ Horses, h can count. Negation: All horses cannot add. 2 c) Every koala can climb. ∀k ∈ Koalas, k can climb. Negation: There exists koala that cannot climb. d) No monkey can speak French. ∀m ∈ M onkeys, k m cannot speak French. Negation: There is a monkey that can speak French e) There exists a pig that can swim and catch fish. ∃p ∈ P igs, p can swim and catch fish. Negation: Every pig either cannot swim, or cannot catch fish. Exercise 4 √ √ a) Let a and b be two integers. Prove that if n = ab, then a ≤ n or b ≤ n √ √ We use a proof by contradiction. Let us suppose that a > n and b > n. Then ab > n, i.e. n > n. We have reached a contradiction. Therefore the property is true. b) Prove or disprove that there exists x rational and y irrational such that xy is irrational. √ √ Let x = 2 and y = 2. Then xy = 2 2 . There are two cases: √ – 2 2 is irrational. We are done – 2 2 is rational. Let us define then x = 2 √ √ 2 √ and y = xy = (2 = 2 √ 2 ) 2 4 . Then √ 2 4 √ √ 2 2 4 1 = 22 √ = 2 i.e. xy is irrational. We have shown that there exists x rational and y irrational such that xy is irrational but we do not know the values of x and y: non-constructive proof. 3

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