# Mathematical Induction. Rosen Chapter 4.1 (6 th edition) Rosen Ch. 5.1 (7 th edition)

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1 Mathematical Induction Rosen Chapter 4.1 (6 th edition) Rosen Ch. 5.1 (7 th edition)

2 Mathmatical Induction Mathmatical induction can be used to prove statements that assert that P(n) is true for all positive integers n where P(n) is a propositional function. Propositional function in logic, is a sentence expressed in a way that would assume the value of true or false, except that within the sentence is a variable (x) that is not defined which leaves the statement undetermined.

3 Motivation Suppose we want to prove that for every value of n: n = n(n + 1)/2. Let P(n) be the predicate We observe P(1), P(2), P(3), P(4). Conjecture: nn, P(n). Mathematical induction is a proof technique for proving statements of the form nn, P(n).

4 Proving P(3) Suppose we know: P(1) n 1, P(n) P(n + 1). Prove: P(3) Proof: 1. P(1). [premise] 2. P(1) P(2). [specialization of premise] 3. P(2). [step 1, 2, & modus ponens] 4. P(2) P(3). [specialization of premise] 5. P(3). [step 3, 4, & modus ponens] We can construct a proof for every finite value of n Modus ponens: if p and pq then q

5 Example: n = n(n + 1)/2. Verify: F(1) = 1(1 + 1)/2 = 1. Assume: n = n(n + 1)/2 (Inductive hypothesis) Prove: if (P(k) is true, then P(k+1) is true n + (n + 1) = (n+1)(n+2)/2 c

6 Example: n = n(n + 1)/2. n(n+1)/2 + (n+1) = (n+1)(n+2)/2 n(n+1)/2 + 2(n+1)/2 = (n+1)(n+2)/2 (n(n+1)+2(n+1))/2 = (n+1)(n+2)/2 (n+1)(n+2)/2 = (n+1)(n+2)/2 Proven!

7 The Principle of Mathematical Induction Let P(n) be a statement that, for each natural number n, is either true or false. To prove that nn, P(n), it suffices to prove: P(1) is true. (base case) nn, P(n) P(n + 1). (inductive step) This is not magic. It is a recipe for constructing a proof for an arbitrary nn.

8 Mathematical Induction and the Domino Principle If the 1 st domino falls over and the nth domino falls over implies that domino (n + 1) then falls over domino n falls over for all n N. image from

9 Proof by induction 3 steps: Prove P(1). [the basis] Assume P(n) [the induction hypothesis] Using P(n) prove P(n + 1) [the inductive step]

10 Example Show that any postage of 8 can be obtained using 3 and 5 stamps. First check for a few values: 8 = = = = = How to generalize this?

11 Example Let P(n) be the sentence n cents postage can be obtained using 3 and 5 stamps. Want to show that P(k) is true implies P(k+1) is true for all k 8. 2 cases: 1) P(k) is true and the k cents contain at least one 5. 2) P(k) is true and the k cents do not contain any 5.

12 Example Case 1: k cents contain at least one 5 coin. k cents Replace 5 stamp by two 3 stamps k+1 cents Case 2: k cents do not contain any 5 coin. Then there are at least three 3 coins. k cents Replace three 3 stamps by two 5 stamps 5 5 k+1 cents 12

13 Examples Show that n = 2 n What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

14 Examples Show that for n 4, 2 n < n! What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

15 Examples Show that n 3 n is divisible by 3 for n>0 What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

16 Examples Show that (2n+1) = (n+1) 2 What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

17 Examples Show that: a + ar + ar ar n = ar n+1 a when r <>1 r-1 What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

18 Examples Show that 7 n n+1 is divisible by 57. What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

19 Wednesday s class DO: #3 and #5 from the Exercises from section 1 covering mathematical induction from Rosen Bring them to class (but do not turn them in).

20 Examples #3 Show that n 2 = n(n+1)(2n+1)/6 for n > 0 What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

21 Examples #5 Show that (2n+1) 2 = (n+1)(2n+3)(2n+1)/3 for n => 0 What is the basis statement? (Note, we want the equation, not a definition) Show that the basis statement is true, completing the base of the induction. What is the inductive hypothesis? (Note, we want the equation, not a definition) What do you need to prove in the inductive step? (Note, we want the equation, not a description). Complete the inductive step.

22 Be careful! Errors in assumptions can lead you to the dark side.

23 All horses have the same color Base case: If there is only one horse, there is only one color. Induction step: Assume as induction hypothesis that within any set of n horses, there is only one color. Now look at any set of n + 1 horses. Number them: 1, 2, 3,..., n, n + 1. Consider the sets {1, 2, 3,..., n} and {2, 3, 4,..., n + 1}. Each is a set of only n horses, therefore within each there is only one color. But the two sets overlap, so there must be only one color among all n + 1 horses. What's wrong here?

24 All horses have the same color Base case: If there is only one horse, there is only one color. Induction step: Assume as induction hypothesis that within any set of n horses, there is only one color. Now look at any set of n + 1 horses. Number them: 1, 2, 3,..., n, n + 1. Consider the sets {1, 2, 3,..., n} and {2, 3, 4,..., n + 1}. Each is a set of only n horses, therefore within each there is only one color. But the two sets overlap, so there must be only one color among all n + 1 horses. Why must they overlap? That was not one of the assumptions, they are disjoint.

25 Theorem For every positive integer n, if x and y are positive integers with max (x,y) = n, then x=y. Basis: if n = 1, then x = y = 1 Inductive step: let k be a positive integer. Assume whenever max(x,y) = k and x and y are positive integers, then x=y. Prove max(x,y) = k+1 where x and y are positive integers. max (x-1, y-1) = k, so by inductive hypothesis x-1 = y-1 and x=y. What is wrong?

26 Theorem For every positive integer n, if x and y are positive integers with max (x,y) = n, then x=y. Basis: if n = 1, then x = y = 1 Inductive step: let k be a positive integer. Assume whenever max(x,y) = k and x and y are positive integers, then x=y. Prove max(x,y) = k+1 where x and y are positive integers. max (x-1, y-1) = k, so by inductive hypothesis x-1 = y-1 and x=y. Nothing says x-1 and y-1 are positive integers. X-1 could = 0.

27 Example Show that n+1can be represented as a product of primes. Case n+1 is a prime: It can be represented as a product of 1 prime, itself. Case n+1 is composite: Then, n + 1 = ab, for some a,b < n + 1. Therefore, a = p 1 p 2... p k & b = q 1 q 2... q l, where the p i s & q i s are primes. Represent n+1 = p 1 p 2... p k q 1 q 2... q l.

28 Induction and Recursion Induction is useful for proving correctness/design of recursive algorithms Example // Returns base ^ exponent. // Precondition: exponent >= 0 public static int pow(int x, int n) { if (n == 0) { // base case; any number to 0th power is 1 return 1; } else { // recursive case: x^n = x * x^(n-1) return x * pow(x, n-1); } }

29 Induction and Recursion n! of some integer n can be characterized as: n! = 1 for n = 0; otherwise n! = n (n - 1) (n - 2)... 1 Want to write a recursive method for computing it. We notice that n! = n (n 1)! This is all we need to put together the method: public static int factorial(int n) { if (n == 0) { return 1; } else { return n * factorial(n-1); } }

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