Chapter 2: The Logic of Quantified Statements. January 22, 2010

Transcription

1 Chapter 2: The Logic of Quantified Statements January 22, 2010

2 Outline Introduction to Predicates and Quantified Statements I Introduction to Predicates and Quantified Statements II Statements Containing Multiple Quantifiers

3 2.1- Introduction to Predicates and Quantified Statements I

4 Basic Facts and Notation for Sets A set is some collection of objects, which we call elements of the set. For example, and are examples of sets. A = { blue, red, yellow} B = {0, 2, 5, 13, 4} To write the fact that an object x is an element of a set A, we write x A x / A will denote the fact that x is not an element of A.

5 We will use the following notation for familiar sets of numbers: (1) R for the set of all real numbers (2) Z for the set of all integers (3) Q for the set of all rational numbers (4) N for the set of all natural numbers (i.e. positive integers) By putting + or in the superscript, we indicate the set of positive or negative numbers. For example, R + denotes the set of all positive real numbers.

6 Ex. x + 4 > 9 is not a statement because its truth (or falsity) depends on what value we assign to x. Ex. If we do not define who she is, then She is a student at Ryerson is technically not a statement. Such statements are called predicates. They are not statements unless we interpret the variables in them as particular elements of some prescribed set.

7 DEFINITION: A predicate is a sentence that contains a finite number of variables and becomes a statement once these variables are assigned specific values. The domain of a predicate variable is the set of all values that can be substituted for the variable. Suppose P(x) is the predicate x + 4 > 9 the domain can be any set of numbers (R, Z,...,) in which its operations (+,,... ) make sense. A predicate could involve any number of variables, for example Q(x, y) is the predicate x is divisible by y 3.

8 DEFINITION: If P(x) is a predicate whose truth domain is D, the truth set of P(x) is the set of all elements of D which make P(x) true when they are substituted for x. We write the truth set for P(x) as {x D P(x)} which we read as the set of all x in D such that P(x) is true. Ex. Let Q(n) be the predicate n divides 5 If the truth domain is Z, then the truth set is { 5, 1, 1, 5} If the truth domain is N then the truth set is {1, 5}

9 One way to turn a predicate P(x) into a statement is to use the universal quantifier, which is the symbol and means for all ( for any, for each ). DEFINITION: Let Q(x) be a predicate and D be the truth domain for x. A universal statement is a statement of the form x D, Q(x). It is true if and only if Q(x) is true for every x in D. If there is any x in D for which Q(x) is false, the universal statement will be false. The value of x for which Q(x) is false is called a counterexample for the universal statement.

10 Ex. Let D = { 5, 8, 9, 11} and consider the statement x D, x 2 + x > 2 Show that this statement is true: Solution: We check that x 2 + x > 2 is true for all x D. (This is called the method of exhaustion) x D, x 2 + x > 2 is true. ( 5) 2 + ( 5) > > > > 2

11 Ex. Now show that x R, x 2 + x > 2 is false Solution: It suffices to find one value of x R for which it is not the case that x 2 + x > 2. In other words, we are looking for a counterexample in R. One such counterexample is x = 1: ( 1) 2 + ( 1) 2 Therefore x R, x 2 + x > 2 is false.

12 Another way to turn a predicate Q(x) into a statement is to use the existential quantifier,, which means there exists ( there is a, for some, there is at least one ). DEFINITION: Let Q(x) be a predicate and D the truth domain of x. An existential statement is a statement of the form x D such that Q(x). It is true if and only if Q(x) is true for at least one element of x of D. It will be false if Q(x) is false for all x D.

13 Ex. Show that the statement n Z such that n 2 = n is true. Solution: It suffices to find one integer such that n 2 = n. Obviously, one such integer is n = 1. Ex. Given the truth domain A = {2, 7, 3, 5}, show that the existential statement n A such that n 2 = n is false. Solution: Clearly ( 7) Since there is no element n A for which n 2 = n, this existential statement is false.

14 Given a statement involving quantifiers, it is generally possible to translate it into English in a variety of ways: Ex. x R, x 2 0 This can be translated as The square of every real number is non-negative. Every real number has a non-negative square. x has a non-negative square, for any real number x. No real number can have a negative square.

15 One of the most common types of universal statements is the universal conditional statement: x, if P(x) then Q(x). Ex. Translate the following statements: (a) x R, if x 3, then x 2 9 translation: if a real number is at least 3, then its square is at least 9. (b) no bird is a mammal translation: x, if x is a bird, then x is not a mammal.

16 Consider the statement x, if x Z, then x Q. We can rewrite it as: x Z, x Q. [we restricted the truth domain to integers] Consider the statement x D, P(x). We can rewrite it as: x, if x D, then P(x).

17 Consider the statement if a number is an integer, then it is a rational number. Although it is a universal statement, it does not contain the key words all or every or any or each. This is an example of an implicit universal quantification. Consider the statement the number 24 can be written as a sum of two even integers. This is an example of an implicit existential quantification and we could rewrite it as even integers m and n such that 24 = m + n.

18 DEFINITION: If P(x) and Q(x) are predicates with the same domain D, the notation P(x) Q(x) means that every element of x D which is in the truth set for P(x) is also in the truth set of Q(x). Equivalently, x, P(x) Q(x). The notation P(x) Q(x) means that the universal statement x, P(x) Q(x) is true (P(x) and Q(x) have the same truth sets).

19 Ex. Let Q(n) be n is a factor of 8 ; R(n) be n is a factor of 4 ; S(n) be n < 5 and n 3 and suppose the domain of n is N. The truth sets are: Q(n) : {1, 2, 4, 8} R(n) : {1, 2, 4} S(n) : {1, 2, 4} Then R(n) S(n), R(n) Q(n), S(n) Q(n).

20 2.2 - Introduction to Predicates and Quantified Statements II

21 NEGATION OF A UNIVERSAL STATEMENT THEOREM: The negation of a statement of the form x D, Q(x) is logically equivalent to a statement of the form x D such that Q(x). We can write it as ( ) x D, Q(x) x D such that Q(x). Ex. the negation of All engineering students must take calculus is there are some engineering students who do not have to take calculus.

22 Ex. the negation of primes p, p is odd is a prime p such that p is not odd. NEGATION OF AN EXISTENTIAL STATEMENT THEOREM: The negation of a statement of the form x D such that Q(x) is logically equivalent to a statement of the form x D, Q(x). We can write it as ( ) x D such that Q(x) x D, Q(x).

23 Ex. the negation of some birds are mammals is no birds are mammals. Ex. the negation of a triangle T such that the sum of the angles of T equals 200 o is triangles T, the sum of the angles of T does not equal 200 o.

24 Suppose we want to negate a universal conditional statement x, P(x) Q(x). Then ( ) ( ) x, P(x) Q(x) x such that P(x) Q(x). But, So ( ) P(x) Q(x) ( ) x, P(x) Q(x) P(x) Q(x) x such that ( ) P(x) Q(x).

25 Ex. The negation of integers n, if n is even, then n is divisible by 4 is divisible by 4. an integer n such that n is even and n is not Ex. The negation of a person p such that p is blond and p does not have blue eyes is people p, if p is blond then p has blue eyes.

26 Suppose we are looking at quantified statements which refer to a finite domain D = {x 1, x 2,..., x n }. Saying the statement x D, P(x) is true, is equivalent to saying that P(x) is true for every x i D: P(x 1 ) P(x 2 ) P(x n ). Saying the statement x D such that P(x) is equivalent to the saying that P(x) is true for some x i D: P(x 1 ) P(x 2 ) P(x n ).

27 Ex. Suppose D = {1, 1} where P(x) is the predicate x 2 = 1 and Q(x) is the predicate x 2 = x. Then x D, P(x) is true because P(1) P( 1) is true. And x D such that Q(x) is true because Q(1) Q( 1) is true. If we have a universal statement x D, if P(x) then Q(x), then the statement is vacuously true if and only if P(x) is false for every x D.

28 DEFINITION: Consider a universal conditional statement x D, if P(x) then Q(x). (1) Its contrapositive is x D, if Q(x) then P(x). (2) Its converse is x D, if Q(x) then P(x). (3) Its inverse x D, if P(x) then Q(x). Ex. Consider the statement x R, if x > 2 then 3x + 2 > 8. Contrapositive: x R, if 3x then x 2. Converse: x R, if 3x + 2 > 8 then x > 2. Inverse: x R, if x 2 then 3x

29 The usual equivalences (and non-equivalences) are also true for universal conditional statements; namely: conditional statement its contrapositive conditional statement its converse conditional statement its inverse converse inverse.

30 Ex. Negate the following statements: Added: January 22 (1) some people like red" it is not true that some people like red no people like red people p, p does not like red (2) all dogs are loyal it is not true that all dogs are loyal some dogs are disloyal a dog d such that d is disloyal (3) The sum of any two irrational numbers is irrational it is not true that the sum of any two irrational numbers is irrational" there are some irrational numbers whose sum is rational irrational numbers m, n such that m + n is rational

31 Added: January 22 (4) The product of any irrational number and any rational number is irrational It is not true that the product of any irrational number and rational number is irrational there exists an irrational number and a rational number whose product is not irrational

32 Consider rewording the original sentence: Added: January 22 rational numbers q and irrational numbers i, (q i) is irrational q Q and i I, (q i) I (where I is the set of irrational numbers) The negation was: there exists an irrational number and a rational number whose product is not irrational" In other words: q Q and i I such that (q i) / I Generally, ( x D, Q(x)) x D such that Q(x) and ( x D such that Q(x)) x D, Q(x)

33 Added: January 22 (5) For all integers n, if n 2 is even then n is even. It is not true that for all integers n, if n 2 is even then n is even. ambiguous So how to we negate conditional statements? Let Q(n) be if n 2 is even, then n is even. So our statement becomes integers n, Q(n). The negation is: it is not true that integers n, Q(n) Or simply some integer n such that Q(n) ( ) Now we need to negate Q(n): if n 2 is even, then n is even

34 ( ) if n 2 is even, then n is even Added: January 22 We did this back in Section 1.2: ( (p q) ( p q) ( p) ( q) p q ) if n 2 is even, then n is even n 2 is even and n is odd

35 Added: January 22 So back to the question: (5) For all integers n, if n 2 is even then n is even. We gave the negation to be: some integer n such that Q(n) But Q(n) is n 2 is even and n is odd We now have: some integer n such that n 2 is even and n is odd. In ( general, ) x D, if P(x) then Q(x) x D such that P(x) and Q(x)

36 Added: January 22 (6) people p, if p is blond, then p has blue eyes. Again, we can treat this as people p, Q(p). Then the negation is people p such that Q(p). Which is simply people p such that p is blond and p does not have blue eyes.

37 Added: January 22 Ex. Negate the following: (7) If an integer is divisible by 2, then it is even. (8) For every real number x, if x(x + 1) > 0 then x > 0 or x < 1

38 Added: January 22 Ex. Negate the following: (7) If an integer is divisible by 2, then it is even. Equivalently, x Z, if x is divisible by 2, then x is even. x Z, if P(x) then Q(x) Negation: x Z such that P(x) and Q(x) Answer: x Z such that x is divisible by 2 and x is not even.

39 Added: January 22 (8) For every real number x, if x(x + 1) > 0 then x > 0 or x < 1 Equivalently, x R, if P(x) then Q(x) where P(x) = x(x + 1) > 0 and Q(x) = x > 0 or x < 1 Negation: x R such that P(x) and Q(x). DeMorgan s ( Law: ) (x > 0) (x < 1) (x > 0) (x < 1) (x 0) (x 1) Answer: x R such that x(x + 1) > 0 and both x 0 and x 1

40 Modified: January 22 DEFINITION: (1) x, P(x) is a sufficient condition for Q(x) means x, if P(x) then Q(x). Ex. people p, if p is eligible to vote, then p is at least 18 years old. The truth of the condition of p is eligible to vote" is sufficient to ensure the truth of the condition p is at least 18 years old." (2) x, P(x) only if Q(x) means x, if Q(x), then P(x) or, equivalently x, if P(x) then Q(x).

41 Modified: January 22 (3) x, P(x) is a necessary condition for Q(x) means x, if P(x) then Q(x) or equivalently x, if Q(x) then P(x). Ex. vote. people p, if p is at least 18 years old then p is eligible to The condition of p is at least 18 years old" is necessary for the condition p is eligible to vote." If p were younger than 18, then p would not be eligible to vote.

42 Modified: January 22

43 Ex. Rewrite each as a universal conditional statement: (a) Squareness is a sufficient condition for rectangularity. x, if x is a square, then x is a rectangle. (b) A product of two integers is odd only if both of them are odd. m, n Z, if mn is odd, then m, n are both odd

44 2.3 - Statements Containing Multiple Quantifiers

45 Consider the statement x R, y R, such that x < y What it means: For every real number x, there is a larger real number y. (TRUE) Consider the statement y R, such that x R, x < y What it means: There exists a real number y such that it is larger than any real number x. (FALSE) Conclusion: By reversing the order of quantifiers in a statement, that statement may change considerably.

46 Question: Given a statement of the form x D, y E such that P(x, y) or x D such that y E, P(x, y) how do we verify whether such a statement is true or false?

47 To check the truth value of the statement we can think of it as a game: x D, y E such that P(x, y) allow someone else to pick any element x D. you must respond by finding an element y E that works for that particular x. if you have no response, then you lose and the statement must be false. If you can always respond with a good choice of y, then you win and the statement is true.

48 Ex.2.3, 9 Let D = E = { 2, 1, 0, 1, 2} Is the following true? x D, y E such that x + y = 0 For each element in D, we must be able to find an element in E that makes the sum of the two elements equal to 0: If x = 2, then we choose y = 2 If x = 1, then we choose y = 1 If x = 0, then we choose y = 0 If x = 1, then we choose y = 1 If x = 2, then we choose y = 2 Yes, the statement is true. [Alternately, for each integer x D, the integer x is in D as well, including 0 and for all integers x, x + ( x) = 0.]

49 To check the truth value of the statement we can think of it as a game: x D, such that y E, P(x, y) we choose one fixed element x D your opponent tries to find a value of y E for which P(x, y) fails if your opponent can find no such value for y, then you have won and the statement is true. If your opponent finds a y E which disproves P(x, y), then you have lost and the statement is false.

50 Ex. 2.3, 9 Let D = E = { 2, 1, 0, 1, 2}. Is the following true? x D such that y E, x + y = y Let us choose x = 0. Then 0 + y = y for any y E because the elements of E are real numbers.

51 The reciprocal of a real number a is a real number b such that ab = 1. Ex. Rewrite these true statements using quantifiers and variables: (a) Every nonzero real number has a reciprocal nonzero real number x, y R such that xy = 1. (b) There is a real number with no reciprocal z R such that u R, zu 1. (c) There is a smallest positive integer m N such that n N, m n.

52 Suppose we want to negate the statement x D, y E such that P(x, y) Then, we have ( ) x D, y E such that P(x, y) x D such that ( ) y E such that P(x, y) x D such that y E, P(x, y) Similarly, ( we can show that ) x D such that y E, P(x, y) x D, y E such that P(x, y).

53 As we have seen, the order of quantifiers matters. However, if the quantifiers are identical, then the order is irrelevant. Ex. x R and y R, x y = y x is logically equivalent to y R and x R, x y = y x (similarly if there are two existential quantifiers). Note: The following is also true ( ) (1) x D, P(x) x x D P(x) ( ) (2) x D, P(x) x x D P(x).

54 Added: January 22 ( ) (1) x D, P(x) x x D P(x) Ex. For every x Z, 3x is positive. ( ) Equivalently, x, if x Z then 3x is positive ( ) (2) x D, P(x) x x D P(x). Ex. There exists a black dog d such that d has brown eyes. ( ) Equivalently, a dog d d D and d has brown eyes where D is the set of all black dogs.