# Elementary Number Theory and Methods of Proof. CSE 215, Foundations of Computer Science Stony Brook University

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1 Elementary Number Theory and Methods of Proof CSE 215, Foundations of Computer Science Stony Brook University 1

2 Number theory Properties: 2 Properties of integers (whole numbers), rational numbers (integer fractions), and real numbers. Truth of mathematical statements. Example: Definition: For any real number x, the floor of x, x, is the largest integer that is less than or equal to x 2.3 = 2; = 12; 1.5 = 2 For any real number x, is x 1 = x 1? yes (true) For any real numbers x and y, is x y = x y? no (false) o = 0.9 = 0 o = 2 1 = 1

3 Number theory Proof example: If x is a number with 5x + 3 = 33, then x = 6 Proof: If 5x + 3 = 33, then 5x = 33 3 since subtracting the same number from two equal quantities gives equal results. 5x = 5x because adding 3 to 5x and then subtracting 3 just leaves 5x, and also, 33 3 = 30. Hence 5x = 30. That is, x is a number which when multiplied by 5 equals 30. The only number with this property is 6. Therefore, if 5x + 3 = 33 then x = 6. 3

4 Number theory introduction Properties of equality: (1) A = A (2) if A = B then B = A (3) if A = B and B = C, then A = C The set of all integers is closed under addition, subtraction, and multiplication 4

5 Number theory introduction 5 An integer n is even if, and only if, n equals twice some integer: n is even an integer k such that n = 2k An integer n is odd if, and only if, n equals twice some integer plus 1: n is odd an integer k such that n = 2k + 1 Reasoning examples: Is 0 even? Yes, 0 = 2 0 Is 301 odd? Yes, 301 = 2( 151) + 1. If a and b are integers, is 6a 2 b even? Yes, 6a 2 b = 2(3a 2 b) and 3a 2 b is an integer being a product of integers: 3, a, a and b.

6 Number theory introduction 6 An integer n is prime if, and only if, n>1 and for all positive integers r and s, if n=r s, then either r or s equals n: n is prime positive integers r and s, if n = r s then either r = 1 and s = n or r = n and s = 1 An integer n is composite if, and only if, n>1 and n=r s for some integers r and s with 1<r<n and 1<s<n: n is composite positive integers r and s such that n = r s and 1 < r < n and 1 < s < n Example: Is every integer greater than 1 either prime or composite? Yes. Let n be an integer greater than 1. There exist at least two pairs of integers r=n and s=1, and r=1 and s=n, s.t. n=rs. If there exists a pair of positive integers r and s such that n = rs and neither r nor s equals either 1 or n (1 < r < n and 1 < s < n), then n is composite. Otherwise, it s prime.

7 Proving Existential Statements 7 x D such that Q(x) is true if, and only if, Q(x) is true for at least one x in D Constructive proofs of existence: find an x in D that makes Q(x) true OR give a set of directions for finding such x Examples: an even integer n that can be written in two ways as a sum of two prime numbers Proof: n=10=5+5=3+7 where 5, 3 and 7 are prime numbers an integer k such that 22r + 18s = 2k where r and s are integers Proof: Let k = 11r + 9s. k is an integer because it is a sum of products of integers. By distributivity of multiplication the equality is proved.

8 Proving Existential Statements Nonconstructive proofs of existence: the evidence for the existence of a value of x is guaranteed by an axiom or theorem the assumption that there is no such x leads to a contradiction Problems: gives no idea of what x is 8

9 Disproving Universal Statements by Counterexample Disprove x in D, if P(x) then Q(x) The statement is false is equivalent to its negation is true by giving an example The negation is: x in D such that P(x) ~Q(x) Disproof by Counterexample: x in D, if P(x) then Q(x) is false if we find a value of x in D for which the hypothesis P(x) is true and the conclusion Q(x) is false x is called a counterexample Example: Disprove real numbers a and b, if a 2 = b 2 then a = b Counterexample: Let a = 1 and b = 1. a 2 = b 2 = 1, but a = b 9

10 Proving Universal Statements Universal statement: x D, if P(x) then Q(x) The method of exhaustion: if D is finite or only a finite number of elements satisfy P(x), then we can try all possibilities Example: Prove n Z, if n is even and 4 n 7, then n can be written as a sum of two prime numbers. Proof: 4 = 2 +2 and 6=

11 Proving Universal Statements Method of Generalizing from the Generic Particular suppose x is a particular but arbitrarily chosen element of the set, and show that x satisfies the property no special assumptions about x that are not also true of all other elements of the domain Method of Direct Proof: 1. Statement: x D, if P(x) then Q(x) 2. Let x is a particular but arbitrarily chosen element of D for which the hypothesis P(x) is true 3. Show that the conclusion Q(x) is true 11

12 Method of Direct Proof Example: prove that the sum of any two even integers is even 1. Formalize: integers m, n, if m and n are even then m + n is even 2. Suppose m and n are any even integers Existential Instantiation: If the existence of a certain kind of object is assumed or has been deduced then it can be given a name Since m and n equal twice some integers, we can give those integers names m = 2r, for some integer r and n = 2s, for some integer s m + n = 2r + 2s = 2(r + s) However, r + s is an integer because the sum of any two integers is an integer, so m + n is an even number The example can be formalized as a proved theorem 12

13 Common Mistakes 1. Arguing from examples: it is true because it s true in one particular case NO 2. Using the same letter to mean two different things 3. Jumping to a conclusion NO,we need complete proofs! 4. Circular reasoning: x is true because y is true since x is true 5. Confusion between what is known and what is still to be shown: What is known? Premises, axioms and proved theorems. 6. Use of any rather than some 7. Misuse of if 13

14 Showing That an Existential Statement Is False The negation of an existential statement is universal: To prove that an existential statement is false, we must prove that its negation (a universal statement) is true. Example - prove falsity of the existential statement: There is a positive integer n such that n 2 + 3n + 2 is prime. The negation is: For all positive integers n, n 2 + 3n + 2 is not prime. Let n be any positive integer n 2 + 3n + 2 = (n + 1)(n +2) where n + 1 > 1 and n + 2 > 1 because n 1 Thus n 2 + 3n + 2 is a product of two integers each greater than 1, and so it is not prime. 14

15 Rational Numbers A real number r is rational if, and only if, it can be expressed as a quotient of two integers with a nonzero denominator r is rational integers a and b such that r = a / b and b 0 Examples: 10/3, 5 /39, = 281/1000, 7 = 7/1, 0 = 0/1, = 12/99 Every integer is a rational number: n = n/1 15

16 A Sum of Rationals Is Rational real numbers r and s, if r and s are rational then r + s is rational Suppose r and s are particular but arbitrarily chosen real numbers such that r and s are rational r = a / b and s = c / d for some integers a, b, c, and d, where b = 0 and d = 0 r + s = a / b + c / d = ad / bd + bc / bd (rewriting the fraction with a common denominator) = (ad + bc)/ bd (by adding fractions with a common denominator) 16

17 Deriving Additional Results about Even and Odd Integers Prove: if a is any even integer and b is any odd integer, then (a 2 +b 2 +1)/2 is an integer Using the properties: 1. The sum, product, and difference of any two even integers are even. 2. The sum and difference of any two odd integers are even. 3. The product of any two odd integers is odd. 4. The product of any even integer and any odd integer is even. 5. The sum of any odd integer and any even integer is odd. 6. The difference of any odd integer minus any even integer is odd. 7. The difference of any even integer minus any odd integer is odd. 17

18 Suppose a is any even integer and b is any odd integer. By property 3, b 2 is odd. By property 1, a 2 is even. By property 5, a 2 + b 2 is odd. By property 2, a 2 + b is even. By definition of even, there exists an integer k such that a 2 + b = 2k. (a 2 +b 2 +1)/2 = k, which is an integer. 18

19 Divisibility If n and d are integers and d 0 then n is divisible by d if, and only if, n equals d times some integer d n an integer k such that n = dk n is a multiple of d d is a factor of n d is a divisor of n d divides n Notation: d n (read d divides n ) Examples: 21 is divisible by 3, 32 is a multiple of 16, 5 divides 40, 6 is a factor of 54, 7 is a factor of 7 Any nonzero integer k divides 0 as 0 = k 0 19

20 A Positive Divisor of a Positive Integer For all integers a and b, if a and b are positive and a divides b, then a b Suppose a and b are positive integers and a divides b Then there exists an integer k so that b = ak 1 k because every positive integer is greater than or equal to 1 Multiplying both sides by a gives a ka = b, since a is a positive number 20

21 Transitivity of Divisibility For all integers a, b, and c, if a b and b c, then a c Since a b, b = ar for some integer r. Since b c, c = bs for some integer s. Hence, c = bs= (ar)s = a(rs) by the associative law for multiplication rs is an integer, so a c 21

22 Counterexamples and Divisibility For all integers a and b, if a b and b a then a = b. Counterexample: Let a = 2 and b = 2 Then a b since 2 ( 2) and b a since ( 2) 2, but a b since 2 2 Therefore, the statement is false. 22

23 Unique Factorization of Integers Theorem Given any integer n > 1, there exist a positive integer k, distinct prime numbers p 1, p 2,..., p k, and positive integers e 1, e 2,..., e k such that: n = p e1 1 p e2 2 p e3 3...p ek k and any other expression for n as a product of prime numbers is identical to this except for the order in which the factors are written. Standard factored form: p 1 < p 2 < < p k 23

24 The Quotient-Remainder Theorem Given any integer n and positive integer d, there exist unique integers q and r such that n = dq + r and 0 r < d. n div d = the integer quotient obtained when n is divided by d n mod d = the nonnegative integer remainder obtained when n is divided by d. n div d = q and n mod d = r n = dq + r n mod d = n d (n div d ) Examples: 54 = 52+2 = ; hence q = 13 and r = 2 54 = = 4 ( 14) + 2; hence q = 14 and r = 2 24

25 Parity The parity of an integer refers to whether the integer is even or odd Consecutive Integers Have Opposite Parity Case 1: The smaller of the two integers is even Case 2: The smaller of the two integers is odd 25

26 Method of Proof by Division into Cases To prove: If A 1 or A 2 or... or A n, then C prove all of the following: If A 1, then C, If A 2, then C,... If A n, then C. C is true regardless of which of A 1, A 2,..., A n happens to be the case 26

27 Method of Proof by Division into Cases Example: any integer can be written in one of the four forms: n = 4q or n = 4q +1 or n = 4q +2 or n = 4q + 3 Proof: By the quotient-remainder theorem to n with d =4: n = 4q + r and 0 r < 4 the only nonnegative remainders r that are less than 4 are 0, 1, 2, and 3 Hence: n = 4q or n = 4q +1 or n = 4q +2 or n = 4q + 3 for some integer q 27

28 Absolute Value and the Triangle Inequality The absolute value of x is: x = x if x 0 x if x < 0 For all real numbers r, r r r Case 1 (r 0): r = r Case 2 (r<0): r = r, so, r = r 28

29 Floor and Ceiling The floor of a real number x, x, is a unique integer n such that n x < n+1: x = n n x < n+1 The ceiling of a real number x, x, is a unique integer n such that n 1 < x n: x = n n 1 < x n 29

30 Floor and Ceiling Examples: 25/4 = 6.25, where 6 < 6.25 < 7 25/4 = 6 25/4 = , where 0 < < = = , where 3 < 2.01 < = = 2 30

31 Disproving A Property of Floor Disproving: For all real numbers x and y, x + y = x + y Counterexample: x = y = 1/2 x + y = 1 = 1 x + y = 1/2 + 1/2 = = 0 Hence, x + y x + y 31

32 Hints on how to reason about & x = x + fractional part of x x + y = x + y + the sum of the fractional parts of x and y x + y = x+y + the fractional part of (x + y) Counterexample: x = y = 1/2 the sum of the fractional parts of x and y = 1 the fractional part of (x + y) = 0 32

33 Proving a Property of Floor For all real numbers x and for all integers m, x+m = x +m Suppose x is a particular but arbitrarily chosen real number m is a particular but arbitrarily chosen integer n = x n is an integer and n x < n + 1 add m: n + m x + m < n + m + 1 x+m = n + m = x + m, since n = x 33

34 The Floor of n/2 For any integer n, n/2 = n/2, if n is even (n 1)/2, if n is odd Suppose n is a particular but arbitrarily chosen integer Case 1 (n is odd): n = 2k + 1 for some integer k n/2 = (2k + 1)/2 = 2k/2 + 1/2 = k+1/2 = k = k (n 1)/2 = (2k + 1 1)/2 = 2k/2 = k Case 2 (n is even): n = 2k for some integer k n/2 = n/2 = k 34

35 Division quotient and remainder If n is any integer and d is a positive integer, if q= n/d and r=n d n/d, then n = dq + r and 0 r < d Proof: Suppose n is any integer, d is a positive integer dq + r = d n/d +(n d n/d ) = n q n/d < q + 1 * d dq n < dq + d dq 0 n dq < d 0 r < d [This is what was to be shown.] 35

36 Indirect Argument: Contradiction and Contraposition Proof by Contradiction (reductio ad impossible or reductio ad absurdum) A statement is true or it is false but not both Assume the statement is false If the assumption that the statement is false leads logically to a contradiction, impossibility, or absurdity, then that assumption must be false Hence, the given statement must be true 36

37 There Is No Greatest Integer Assumption: there is a greatest integer N N n for every integer n If there were a greatest integer, we could add 1 to it to obtain an integer that is greater N + 1 N This is a contradiction, no greatest integer can exist (our initial assumption) 37

38 No Integer Can Be Both Even and Odd Suppose there is at least one integer n that is both even and odd n = 2a for some integer a, by definition of even n = 2b+1 for some integer b, by definition of odd 2a = 2b + 1 2a 2b = 1 a b = 1/2 Since a and b are integers, the difference a b must also be an integer, contradiction! 38

39 The Sum of a Rational Number and an Irrational Number 39 The sum of any rational number and any irrational number is irrational real numbers r and s, if r is rational and s is irrational, then r + s is irrational Assume its negation is true: a rational number r and an irrational number s such that r + s is rational r = a/b for some integers a and b with b 0 r + s = c/d for some integers c and d with d 0 s = c/d a/b = (bc ad)/bd with bd 0 This contradicts the supposition that it is irrational

40 Argument by Contraposition Logical equivalence between a statement and its contrapositive We prove the contrapositive by a direct proof and conclude that the original statement is true x in D, if P(x) then Q(x) Contrapositive: x in D, if Q(x) is false then P(x) is false Prove the contrapositive by a direct proof 1. Suppose x is a (particular but arbitrarily chosen) element of D such that Q(x) is false 2. Show P(x) is false Contraposition: the original statement is true 40

41 Contraposition Example If the Square of an Integer Is Even, Then the Integer Is Even Contrapositive: For all integers n, if n is odd then n 2 is odd Suppose n is any odd integer n = 2k + 1 for some integer k, by definition of odd n 2 = (2k + 1) 2 = 4k 2 + 4k + 1 = 2(2k 2 + 2k) + 1 2(2k 2 + 2k) is an integer n 2 is odd [This was to be shown] 41

42 Contradiction Example If the Square of an Integer Is Even, Then the Integer Is Even Suppose the negation of the theorem: There is an integer n such that n 2 is even and n is not even Any integer is odd or even, by the quotient-remainder theorem with d = 2 since n is not even it is odd n = 2k + 1 for some integer k n 2 = (2k + 1) 2 = 4k 2 + 4k + 1 = 2(2k 2 + 2k) + 1 n 2 is odd Contradiction: n 2 is both odd and even (by hypothesis) 42

43 The Irrationality of 2 c 2 = = 2 c = 2 Suppose the negation: 2 is rational there exist 2 integers m and n with no common factors such that 43 Contradiction

44 The set of all prime numbers is infinite Proof (by contradiction): Suppose the set of prime numbers is finite: some prime number p is the largest of all the prime numbers: 2, 3, 5, 7, 11,...,p N = ( p) + 1 N > 1 N is divisible by some prime number q in 2, 3,...,p q divides p, but not ( p) + 1 = N (also proved by contradiction) Contradiction! 44

45 Application: Algorithms A variable refers to a specific storage location in a computer s memory The data type of a variable indicates the set in which the variable takes its values: integers, reals, characters, strings, boolean (the set {0, 1}) Assignment statement: x := e Conditional statements: if (condition) then s 1 else s 2 The condition is evaluated by substituting the current values of all algorithm variables appearing in it and evaluating the truth or falsity of the resulting statement 45

46 Application: Algorithms x := 5 if x >2 then y := x + 1 else do x := x 1 y := 3 x end do the condition x > 2 is true, then y := x + 1 := 6 46

47 Iterative statements while (condition) [statements that make up the body of the loop] end while i := 1, s := 0 while (i 2) s := s + i i := i + 1 end while 47

48 Iterative statements for variable := initial expression to final expression [statements that make up the body of the loop] next (same) variable for i := 1 to 4 x := i 2 next i 48

49 The Division Algorithm Given a nonnegative integer a and a positive integer d, find integers q and r that satisfy the conditions a = dq + r and 0 r < d Input: a [a nonnegative integer], d [a positive integer] Algorithm Body: r := a, q := 0 while (r d) r := r d q := q + 1 end while 49

50 The greatest common divisor The greatest common divisor of two integers a and b (that are not both zero), gcd(a, b), is that integer d with the following properties: 1. d is a common divisor of both a and b: d a and d b 2. For all integers c, if c is a common divisor of both a and b, then c is less than or equal to d: for all integers c, if c a and c b, then c d Examples: gcd(72, 63) = gcd(9 8, 9 7)= 9 If r is a positive integer, then gcd(r, 0) = r. 50

51 Euclidean Algorithm If a and b are any integers not both zero, and if q and r are any integers such that a = bq + r, then gcd(a, b) = gcd(b, r ). 51

52 Euclidean Algorithm 52 Given two integers A and B with A > B 0, this algorithm computes gcd(a, B) Input: A, B [integers with A > B 0] Algorithm Body: a := A, b := B, r := B while (b = 0) r := a mod b a := b b := r end while gcd := a Output: gcd [a positive integer]

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