# Mathematical Induction

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1 Mathematical S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

2 Mathematical S 0 S 1 S 2 S 3 S4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

3 S 4 Mathematical S 0 S 1 S 2 S 3 S5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

4 Mathematical S0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

5 S 0 Mathematical S1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

6 S 0 S 1 Mathematical S2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

7 S 0 S 1 S 2 Mathematical S3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

8 S 0 S 1 S 2 S 3 Mathematical S4 S 5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

9 S 0 S 1 S 2 S 3 S 4 Mathematical S5 S 6 S 7 S 8 S 9 S 10 Like dominoes!

10 S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 Mathematical S10 Like dominoes!

11 Mathematical S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 For any k N, if S k is true, then S k+1 is true. (This is called the inductive step. The assumption S k is true is called the inductive hypothesis. )

12 Mathematical S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 For any k N, if S k is true, then S k+1 is true. (This is called the inductive step. The assumption S k is true is called the inductive hypothesis. )

13 Mathematical S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 For any k N, if S k is true, then S k+1 is true. (This is called the inductive step. The assumption S k is true is called the inductive hypothesis. )

14 Mathematical S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 For any k N, if S k is true, then S k+1 is true. (This is called the inductive step. The assumption S k is true is called the inductive hypothesis. ) S a is true for some a N. (This is called the base case. )

15 Mathematical S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 For any k N, if S k is true, then S k+1 is true. (This is called the inductive step. The assumption S k is true is called the inductive hypothesis. ) S a is true for some a N. (This is called the base case. ) Therefore, S k is true for all k a.

16 Mathematical Proposition: Suppose n N. Then 5 (n 5 n)

17 Mathematical Proposition: Suppose n N. Then 5 (n 5 n) Proof: (by induction) Let k N, and suppose 5 (k 5 k). Then there exists some x Z such that k 5 k = 5x. Then: (k + 1) 5 (k + 1) = [k 5 + 5k k k 2 + 5k + 1] (k + 1) = [k 5 k] + [5k k k 2 + 5k] = 5x + [5k k k 2 + 5k] = 5[x + k 4 + 2k 3 + 2k 2 + k] ( ) So, 5 (k + 1) 5 (k + 1) When n = 1, n 5 n = 0, so 5 (n 5 n). It follow by induction that, for every n N, 5 (n 5 n).

18 Mathematical n 1 Proposition: Suppose n N. Then 2 k = 2 n 1. k=0

19 Mathematical n 1 Proposition: Suppose n N. Then 2 k = 2 n 1. k=0 Proof: (by induction) m 1 Let m N, and suppose 2 k = 2 m 1. Then: k=0 m k=0 m 1 2 k = 2 m + k=0 2 k = 2 m + (2 m 1) = 2(2 m ) 1 = 2 m+1 1 So, if the statement holds for n = m, then it also holds for n = m + 1. n 1 When n = 1, 2 k = 2 0 = 1, and 1 = 2 n 1, so the statement holds when n = 1. k=0 n 1 It follow by induction that, for every n N, 2 k = 2 n 1. k=0

20 Mathematical Proposition: Let f (x) = x ln x. Then for all n N such that n 2, (Recall 0! = 1.) f (n) (x) = ( 1) n (n 2)!x 1 n.

21 Mathematical Proposition: Let f (x) = x ln x. Then for all n N such that n 2, f (n) (x) = ( 1) n (n 2)!x 1 n. (Recall 0! = 1.) Proof: (by induction) Suppose k N and f (k) (x) = ( 1) k (k 2)!x 1 k. Then f (k+1) (x) = d { } f (k) (x) dx = d {( 1) k (k 2)!x 1 k} dx = (1 k)( 1) k (k 2)!x 1 k 1 = (k 1)( 1) k (k 2)!x 1 (k+1) = ( 1) k+1 (k 1)!x 1 k+1 So, if the statement holds for n = k, then is also holds for n = k + 1. Note f (x) = ln x + 1 (by the product rule), so f (2) (x) = 1 x = ( 1)2 0!x 1 2, so the statement holds for n = 2. By induction, we conclude the statement holds for all natural n 2.

22 Cautionary Tale Proposition: Let f (x) = e x Then f (n) (x) = 0 for all n N. Proof: Suppose f (n) (x) = 0. Then f (n+1) (x) = d {0} = 0. So, if the statement dx holds for n, then it also holds for n + 1. We conclude by induction that f (n) (x) = 0 for all n N.

23 Cautionary Tale Proposition: Let f (x) = e x Then f (n) (x) = 0 for all n N. Proof: Suppose f (n) (x) = 0. Then f (n+1) (x) = d {0} = 0. So, if the statement dx holds for n, then it also holds for n + 1. We conclude by induction that f (n) (x) = 0 for all n N. We need a base case! This is clearly false.

24 : Triominoes Triomino

25 : Triominoes Triomino 4 4 chessboard

26 : Triominoes Triomino 4 4 chessboard

27 : Triominoes Triomino 4 4 chessboard

28 : Triominoes Triomino 4 4 chessboard

29 : Triominoes Triomino 4 4 chessboard

30 : Triominoes Triomino

31 : Triominoes Proposition: For any n N, a chessboard of dimensions 2 n 2 n that is missing any single square can be tiled by triominoes.

32 : Triominoes Proposition: For any n N, a chessboard of dimensions 2 n 2 n that is missing any single square can be tiled by triominoes. Proof : (by induction) When n = 1, the board is exactly the shape of a single triomino, so it can be tiled.

33 : Triominoes Proposition: For any n N, a chessboard of dimensions 2 n 2 n that is missing any single square can be tiled by triominoes. Proof : (by induction) When n = 1, the board is exactly the shape of a single triomino, so it can be tiled. Suppose any board of dimensions 2 n 2 n missing one square can be tiled. Consider a board of dimension 2 n+1 2 n+1, missing any one square.

34 : Triominoes Proposition: For any n N, a chessboard of dimensions 2 n 2 n that is missing any single square can be tiled by triominoes. Proof : (by induction) When n = 1, the board is exactly the shape of a single triomino, so it can be tiled. Suppose any board of dimensions 2 n 2 n missing one square can be tiled. Consider a board of dimension 2 n+1 2 n+1, missing any one square. Then we can cut the board into four equal-sized boards, each of dimension 2 n 2 n. One of these contains the missing square; by the inductive hypothesis, this can be tiled. The other three smaller boards do not contain any missing pieces.

35 : Triominoes Proposition: For any n N, a chessboard of dimensions 2 n 2 n that is missing any single square can be tiled by triominoes. Proof : (by induction) When n = 1, the board is exactly the shape of a single triomino, so it can be tiled. Suppose any board of dimensions 2 n 2 n missing one square can be tiled. Consider a board of dimension 2 n+1 2 n+1, missing any one square. Then we can cut the board into four equal-sized boards, each of dimension 2 n 2 n. One of these contains the missing square; by the inductive hypothesis, this can be tiled. The other three smaller boards do not contain any missing pieces. Place a triomino on the three squares belonging to the three smaller boards that meet in the middle of the board (as in the picture below). Now the smaller boards are all of size 2 n 2 n, and we need to tile all but one of their squares. By the inductive hypothesis, we can finish the tiling. We conclude that the proposition holds.

36 Triominoes tiling

37 Triominoes tiling

38 Triominoes tiling

39 Triominoes tiling

40 Triominoes tiling

41 Triominoes tiling

42 Triominoes tiling

43 Triominoes tiling

44 Triominoes tiling

45 Triominoes tiling

46 Triomino Corollary Corollary: For any n N, 3 2 2n 1.

47 Triomino Corollary Corollary: For any n N, 3 2 2n 1. Proof: 2 2n 1 is the number of squares on a chessboard of dimension 2 n 2 n that s missing one piece. Since these squares can be covered completely by triomino pieces that have three squares each, 3 2 2n 1.

48 Prime Divisors Proposition: Let p be prime. For any integers a 1,..., a n, if p divides the product a 1 a n, then p divides some a i, 1 i n.

49 Prime Divisors Proposition: Let p be prime. For any integers a 1,..., a n, if p divides the product a 1 a n, then p divides some a i, 1 i n. Lemma: If a and b are integers with gcd(a, b) = 1, then there exist k, l Z such that ak + bl = 1.

50 Prime Divisors Proposition: Let p be prime. For any integers a 1,..., a n, if p divides the product a 1 a n, then p divides some a i, 1 i n. Lemma: If a and b are integers with gcd(a, b) = 1, then there exist k, l Z such that ak + bl = 1. Proof: We proceed by induction on the number of integers. For the base case, suppose n = 2, and p a 1a 2. If p a 1, we re done. Suppose p a 1. Since p is prime, gcd(p, a 1) = 1, so there exist k, l Z such that kp + a 1l = 1. Multiplying by a 2, we see a 2 = kpa 2 + a 1a 2l. Since p a 1a 2, we see p divides the right hand side of the equation, so p a 2. This finishes the base case. Suppose now that the corollary holds whenever n m for some m Z. Suppose p a 1 a ma m+1. Let A = a 1 a m. If p a m+1, we re done. If not, then since p A a m+1, by the base step, p A. Then by the inductive hypothesis, p divides some a i, 1 i m. This finishes the proof.

51 S 0 S 1 S 2 S 3 S 4 S 5 S 6 S 7 S 8 S 9 S 10

52 S 0 S 1 S 2 S3 S 4 S 5 S 6 S 7 S 8 S 9 S 10

53 S 0 S 1 S 2 S 3 S4 S 5 S 6 S 7 S 8 S 9 S 10

54 S 0 S 1 S 2 Outline for proof by strong induction: S3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Suppose S n is true for every n k. Then S k+1 is true as well. This part is the inductive step; the inductive hypothesis is that S n is true for every n k.

55 S 0 S 1 S 2 Outline for proof by strong induction: S3 S 4 S 5 S 6 S 7 S 8 S 9 S 10 Suppose S n is true for every n k. Then S k+1 is true as well. This part is the inductive step; the inductive hypothesis is that S n is true for every n k. S k is true for all k a. Base case(s)

56 S 0 S 1 S 2 S 3 Outline for proof by strong induction: S4 S 5 S 6 S 7 S 8 S 9 S 10 Suppose S n is true for every n k. Then S k+1 is true as well. This part is the inductive step; the inductive hypothesis is that S n is true for every n k. S k is true for all k a. Base case(s) We conclude by (strong) induction that S k is true for all k.

57 S 0 S 1 S 2 S 3 S 4 Outline for proof by strong induction: S5 S 6 S 7 S 8 S 9 S 10 Suppose S n is true for every n k. Then S k+1 is true as well. This part is the inductive step; the inductive hypothesis is that S n is true for every n k. S k is true for all k a. Base case(s) We conclude by (strong) induction that S k is true for all k.

58 S 0 S 1 S 2 S 3 S 4 S 5 Outline for proof by strong induction: Suppose S n is true for every n k. Then S k+1 is true as well. This part is the inductive step; the inductive hypothesis is that S n is true for every n k. S6 S 7 S 8 S 9 S 10 S k is true for all k a. Base case(s) We conclude by (strong) induction that S k is true for all k.

59 S 0 S 1 S 2 S 3 S 4 S 5 S 6 Outline for proof by strong induction: Suppose S n is true for every n k. Then S k+1 is true as well. This part is the inductive step; the inductive hypothesis is that S n is true for every n k. S7 S 8 S 9 S 10 S k is true for all k a. Base case(s) We conclude by (strong) induction that S k is true for all k.

60 Proposition: Let {a n} be the sequence of numbers defined as follows: a 1 = 2 a 2 = 4 a 3 = 8 a n = a n 1 + a n 2 + 2a n 3 for every natural n 4. Then a n = 2 n for every n N.

61 Proposition: Let {a n} be the sequence of numbers defined as follows: a 1 = 2 a 2 = 4 a 3 = 8 a n = a n 1 + a n 2 + 2a n 3 for every natural n 4. Then a n = 2 n for every n N. Proof: Suppose a n = 2 n for every natural n k, and k 3. Then k + 1 4, so: a k+1 = a k + a k 1 + 2a k 2 = 2 k + 2 k k 2 = 2 k k 1 = 2 2 k = 2 k+1 Then a k+1 = 2 k+1 for every natural n k + 1. Note a 1 = 2 1, a 2 = 2 2, and a 3 = 2 3. By strong induction, we conclude a n = 2 n for every n N.

62 : Nim Game of Nim There are two piles of matches, and two players. A turn consists of a player may take as many matches as they like (but at least one) from one of the piles. The player who takes the last match wins.

63 : Nim Game of Nim There are two piles of matches, and two players. A turn consists of a player may take as many matches as they like (but at least one) from one of the piles. The player who takes the last match wins. Proposition: In a game of Nim where both piles of matches are initially the same size, the second player can always win if they take an equal number of matches as their opponent did in their last term, but from the other pile.

64 : Nim Game of Nim There are two piles of matches, and two players. A turn consists of a player may take as many matches as they like (but at least one) from one of the piles. The player who takes the last match wins. Proposition: In a game of Nim where both piles of matches are initially the same size, the second player can always win if they take an equal number of matches as their opponent did in their last term, but from the other pile. Proof : (by induction on the initial size of the piles) Suppose we know Player 2 can win when the sizes of the piles are less than m for some m N. (Inductive Hypothesis) Player 1 takes k matches from one pile, for some k m. If k = m, then Player 2 finishes off the remaining pile and wins. If k m, then after Player 2 s move, each pile has size m k, and 1 m k < m, so by the IH, Player 2 can now win. When each pile has size 1, Player 2 wins on their first move. We conclude by induction that the proposition is true.

65 Proposition: For any convex polygon with n 3 edges, the sum of the interior angles is (n 2)180 degrees. Lemma: If you draw a line between any two (non-consecutive) points on a convex polygon, you divide that convex polygon into two (smaller) convex polygons. Lemma: The interior angles of any triangle sum to 180 degrees.

66 Proposition: For any convex polygon with n 3 edges, the sum of the interior angles is (n 2)180 degrees. Lemma: If you draw a line between any two (non-consecutive) points on a convex polygon, you divide that convex polygon into two (smaller) convex polygons. Lemma: The interior angles of any triangle sum to 180 degrees. First, note that a triangle has angles summing to 180 degrees. Suppose the proposition holds whenever n k 1. Then consider any polygon with k edges. Draw a point between two ponts that are separated by only one point, so the polygon is cut into a triangle and a convex polygon on k 1 points. Then the sum of the angles inside the triangle is 180, while the sum of the angles inside the other polygon is (by the inductive hypothesis) 180(k 3). Then the sum of the inside angles of the original polygon is (k 3) = 180(k 2), so the proposition also holds for n = k. By strong induction, we conclude the proposition holds for all natural n 3.

67 Proof by Proposition: The statements S 1, S 2,... are all true. Proof: S 1 is true

68 Proof by Proposition: The statements S 1, S 2,... are all true. Proof: S 1 is true Suppose (by way of contradiction) that not every S n is true

69 Proof by Proposition: The statements S 1, S 2,... are all true. Proof: S 1 is true Suppose (by way of contradiction) that not every S n is true Then there exists some smallest n N such that S n is false

70 Proof by Proposition: The statements S 1, S 2,... are all true. Proof: S 1 is true Suppose (by way of contradiction) that not every S n is true Then there exists some smallest n N such that S n is false Then S n is false and S n 1 is true

71 Proof by Proposition: The statements S 1, S 2,... are all true. Proof: S 1 is true Suppose (by way of contradiction) that not every S n is true Then there exists some smallest n N such that S n is false Then S n is false and S n 1 is true. Contradiction.

72 Proof by : Fibonnaci Numbers Fibonnaci Sequence F 0 = 1 F 1 = 1 F 2 = 2 For n 3 in N, F n = F n 1 + F n 2

73 Proof by : Fibonnaci Numbers Fibonnaci Sequence F 0 = 1 F 1 = 1 F 2 = 2 For n 3 in N, F n = F n 1 + F n 2 Let ϕ be the golden ratio, ϕ = Proposition: For every n N, F n < ϕ n.

74 Proof by : Fibonnaci Numbers Let ϕ be the golden ratio, ϕ = Proposition: For every n N, F n < ϕ n. Proof: Note that φ > 3 2, so F1 < φ1 and F 2 < 9 4 < φ2. (Base cases.) Suppose F n is the first Fibonnaci number such that F n ϕ n. Then n 3 (by the base cases) and so F n 1 < ϕ n 1 and F n 2 < ϕ n 2. Then: ϕ n F n = F n 1 + F n 2 < ϕ n 1 + ϕ n 2 ϕ 2 < ϕ + 1 ( ) < This is a contradiction. < < We conclude that there does not exist any n N such that F n ϕ. Therefore, the proposition is true.

75 Proof by : 4 5 n 1 Proposition: For every n N, 4 5 n 1.

76 Proof by : 4 5 n 1 Proposition: For every n N, 4 5 n 1. Proof: Note that when n = 1, Suppose n is the smallest natural number so that 4 5 n 1. Then n 2 (by the base case), so n 1 N (that is it isn t zero), so by hypothesis, 4 5 n 1 1. That is, there exists some x N such that 4x = 5 n 1 1. Now: 5 n 1 = 5 5 n 1 1 = 5 (4x + 1) 1 = 5(4x) = 4(5x) + 4 = 4(5x + 1) This contradicts the assumption that 4 5 n 1. We conclude that there is no natural n such that 4 5 n 1, so the proposition is true.

77 Proof by : Fundamental Theorem of Arithmetic Fundamental Theorem of Arithmetic Every n N greater than 1 has a unique prime factorization. That is, if n = p 1 p 2 p 3 p k and n = a 1 a 2 a 3 a l are two prime factorizations of n, then k = l, and the primes p i and a i are the same (but they might be in a different order).

78 Proof by : Fundamental Theorem of Arithmetic Fundamental Theorem of Arithmetic Every n N greater than 1 has a unique prime factorization. That is, if n = p 1 p 2 p 3 p k and n = a 1 a 2 a 3 a l are two prime factorizations of n, then k = l, and the primes p i and a i are the same (but they might be in a different order). This proof involves both existence and uniqueness.

79 Proof by : Fundamental Theorem of Arithmetic Fundamental Theorem of Arithmetic Every n N greater than 1 has a unique prime factorization. That is, if n = p 1 p 2 p 3 p k and n = a 1 a 2 a 3 a l are two prime factorizations of n, then k = l, and the primes p i and a i are the same (but they might be in a different order). This proof involves both existence and uniqueness. Proof: First, we will use strong induction to show that every integer n > 1 has a prime factorization. For the base case, suppose n = 2. Then 2 = 2 is the prime factorization of n. For the inductive step, suppose there exists an integer k 2 such that that every integer n in [2, k] has a prime factorization. We consider the integer k + 1. If k + 1 is prime, then it is its own prime factorization. If it is not prime, then k + 1 = ab for some positive integers a and b, both of which are greater than 1 and less than k + 1. By the inductive hypothesis, a = p 1 p k and b = a 1 a l for primes p i, a i. Then k + 1 = p 1 p k a 1 a l. So, k + 1 has a prime factorization. We conclude by induction that every integer greater than 1 has a prime factorization.

80 Fundamental Theorem of Arithmetic, Cont d Now we will prove uniqueness, using proof by smallest counterexample. For the base case, note that n = 2 has only one prime factorization, since the only prime divisor of 2 is itself. Suppose for the sake of contradiction that there is some integer n > 2 with different prime factorizations. Let n be, in particular, the smallest such integer, and label the factorizations n = p 1 p k and a 1 a l. Then p 1 n, so p 1 a 1 a l. Then by a previous theorem, p 1 a i for some 1 i l. Then, since a i is prime, a i = p 1. But now the integer n p 1 has two different prime factorizations, p 2 p k and a 1 a i 1 a i+1 a l. This contradicts that n was the smallest such integer, and finishes the proof.

### Induction. Margaret M. Fleck. 10 October These notes cover mathematical induction and recursive definition

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