# Mathematical induction. Niloufar Shafiei

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1 Mathematical induction Niloufar Shafiei

2 Mathematical induction Mathematical induction is an extremely important proof technique. Mathematical induction can be used to prove results about complexity of algorithms correctness of certain types of computer programs theorem about graphs and trees Mathematical induction can be used only to prove results obtained in some other ways. 1

3 Mathematical induction Assume P(n) is a propositional function. Principle of mathematical induction: To prove that P(n) is true for all positive integers n we complete two steps 1. Basis step: Verify P(1) is true. 2. Inductive step: Show P(k) P(k+1) is true for all positive integers k. 2

4 Mathematical induction Basis step: P(1) Inductive step: k (P(k) P(k+1)) Result: n P(n) domain: positive integers 1. P(1) 2. k (P(k) P(k+1)) 3. P(1) P(2) 4. P(2) by Modus ponens 5. P(2) P(3) 6. P(3) by Modus ponens 3

5 Mathematical induction [P(1) k (P(k) P(k+1))) n P(n)] How to show P(1) is true? P(1): n is replaced by 1 in P(n) Then, show P(1) is true. How to show k (P(k) P(k+1))? Direct proof can be used Assume P(k) is true for some arbitrary k. Then, show P(k+1) is true. 4

6 Show that n = n(n+1)/2, where n is a positive integer. First define P(n) P(n) is n= n(n+1)/2 Basis step: (Show P(1) is true.) 1 = 1(2)/2 So, P(1) is true. 5

7 Show that n = n(n+1)/2, where n is a positive integer. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true k = k(k+1)/2 Show P(k+1) is true. P(k+1) is k+(k+1)=(k+1)(k+2)/ k+(k+1) = k(k+1)/2 + (k+1) = [k(k+1) + 2(k+1)] / 2 = [k 2 + k + 2k +2] / 2 = [k 2 + 3k +2]/2 = (k+1)(k+2)/2 We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction n = n(n+1)/2. 6

8 Show that (2n-1) = n 2, where n is a positive integer. First define P(n) P(n) is (2n-1) = n 2 Basis step: (Show P(1) is true.) 2-1 = 1 2 So, P(1) is true. 7

9 Show that (2n-1) = n 2, where n is a positive integer. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true (2k-1) = k 2 Show P(k+1) is true. P(k+1) is (2k-1)+(2(k+1)-1) = (k+1) (2k-1)+(2(k+1)-1) = k 2 + (2(k+1)-1) = k 2 + 2k + 1 = (k+1) 2 We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction (2n-1) = n 2. 8

10 Mathematical induction Sometimes we need to show that P(n) is true for n = b, b+1, b+2,, where b is an integer other than 1. Mathematical induction: Basis step: Show P(b) is true. Inductive step: Show k (P(k) P(k+1)) is true. 9

11 Use mathematical induction to show that n = 2 n+1-1 for all nonnegative integers n. First define P(n) P(n) is n = 2 n+1-1 Basis step: (Show P(0) is true.) 2 0 = So, P(0) is true. 10

12 Use mathematical induction to show that n = 2 n+1-1 for all nonnegative integers n. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true k = 2 k+1-1 Show P(k+1) is true. P(k+1) is k+1 = 2 k k +2 k+1 = 2 k k+1 = 2. 2 k+1-1 = 2 k+2-1 We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction n = 2 n

13 Use mathematical induction to prove the formula for the sum of a finite number of terms of a geometric n progression. ar k = a+ar+ar 2 + +ar n = (ar n+1 - a) / (r-1) when r 1 k=0 First define P(n) P(n) is a+ar+ar 2 + +ar n = (ar n+1 - a) / (r-1). Basis step: (Show P(0) is true.) ar 0 = (ar - a)/(r-1) = a So, P(0) is true. 12

14 Use mathematical induction to prove the formula for the sum of a finite number of terms of a geometric n progression. ar k = a+ar+ar 2 + +ar n = (ar n+1 - a) / (r-1) when r 1 k=0 Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. a+ar+ar 2 + +ar k = (ar k+1 - a) / (r-1) Show P(k+1) is true. P(k+1) is a+ar+ar 2 + +ar k+1 = (ar k+2 - a) / (r-1) 13

15 Use mathematical induction to prove the formula for the sum of a n finite number of terms of a geometric progression. ar k = a+ar+ar 2 + +ar n = (ar n+1 - a) / (r-1) when r 1 k=0 a+ar+ar 2 + +ar k +ar k+1 = (ar k+1 - a) / (r-1) + ar k+1 = (ar k+1 - a) / (r-1) + ar k+1 (r-1) / (r-1) = (ar k+1 - a + ar k+2 - ar k+1 ) / (r-1) = (ar k+2 - a) / (r-1) We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction a+ar+ar 2 + +ar n = (ar n+1 - a) / (r-1). 14

16 Use mathematical induction to prove that 2 n <n! for every positive integer n with n 4. First define P(n) P(n) is 2 n < n!. Basis step: (Show P(4) is true.) 2 4 < < 24 So, P(4) is true. 15

17 Use mathematical induction to prove that 2 n <n! for every positive integer n with n 4. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. 2 k < k! Show P(k+1) is true. P(k+1) is 2 (k+1) < (k+1)! 2. 2 k < 2. k! 2 (k+1) < 2. k! < (k+1). k! = (k+1)! We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction 2 n <n!. 16

18 Harmonic numbers H j, j=1,2,3, are defined by H j = 1+1/2 + 1/ /j. Use mathematical induction to show that H 2 n 1 + n/2, whenever n is a nonnegative integer. First define P(n) P(n) is H 2 n 1+n/2. Basis step: (Show P(0) is true.) H /2 1 1 So, P(0) is true. 17

19 Use mathematical induction to show that H 2 n 1 + n/2, whenever n is a nonnegative integer. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. H 2 k = 1+1/2+1/3+ +1/2 k 1 + k/2 Show P(k+1) is true. P(k+1) is 1+1/2+1/3+ +1/2 k + 1/(2 k +1) + + 1/2 k+1 1+(k+1)/2 1+1/2+1/3+ +1/2 k + 1/(2 k +1) + + 1/2 k+1 1+k/2+ 1/(2 k +1) + + 1/2 k+1 (1 + k/2) + 2 k. 1/2 k+1 (1 + k/2) + 1/2 = 1 + (k+1)/2 We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction H 2 n 1 + n/2. 18

20 Use mathematical induction to prove that n 3 -n is divisible by 3 whenever n is a positive integer. First define P(n) P(n) is n 3 -n is divisible by 3. Basis step: (Show P(1) is true.) = 0 is divisible by 3. So, P(1) is true. 19

21 Use mathematical induction to prove that n 3 -n is divisible by 3 whenever n is a positive integer. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. (k 3 -k is divisible by 3) Show P(k+1) is true. (P(k+1) is (k+1) 3 - (k+1) is divisible by 3.) (k+1) 3 - (k+1) = (k 3 + 3k 2 + 3k + 1) - (k+1) = (k 3 - k) + 3(k 2 + k) By inductive hypothesis (k 3 - k) is divisible by 3 and 3(k 2 + k) is divisible by 3 because it is 3 times an integer, so P(k+1) is divisible by 3 We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction n 3 -n is divisible by 3. 20

22 Let S be a set with n elements, where n is nonnegative integer. Use mathematical induction to show that S has 2 n subsets. First define P(n) P(n) is A set with n elements has 2 n subsets. Basis step: (Show P(0) is true.) The empty set has 2 0 =1 subset, namely, itself. So, P(0) is true. 21

23 Let S be a set with n elements, where n is nonnegative integer. Use mathematical induction to show that S has 2 n subsets. Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. (set S with k elements has 2 k subsets) Show P(k+1) is true. (set T (=S {a}) has 2 k+1 subsets.) For each subset X of S there are exactly two subsets of T, namely, X and X {a}. Since S has 2 k subsets, T has 2. 2 k = 2 k+1 subsets. We showed that P(k+1) is true under assumption that P(k) is true. So, by mathematical induction, any set with n elements, has 2 n subsets. 22

24 Use mathematical induction to prove the following generalization of one of De Morgan s laws: A j = A j J=1 J=1 when n 2. First define P(n) n n P(n) is A j = A j. J=1 J=1 Basis step: (Show P(2) is true.) A 1 A 2 = A 1 A 2 By De Morgan s law, P(2) is true. n n 23

25 Use mathematical induction to prove A j = A j when n 2. J=1 J=1 Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. n A j = A j Show P(k+1) is true. J=1 J=1 n k k+1 k k+1 A j = A j J=1 J=1 24

26 Use mathematical induction to prove n n A j = A j when n 2. J=1 J=1 k+1 k A j = ( A j ) A k+1 J=1 J=1 k = ( A j ) A k+1 (by De Morgan s law) J=1 k = ( A j ) A k+1 (by inductive hypothesis) J=1 k+1 = A J=1 j We showed that P(k+1) is true under assumption that P(k) is true. 25

27 An odd number of people stand in a yard at mutually distinct distances. At the same time each person throws a pie at their nearest neighbor, hitting this person. Use mathematical induction to show that there is at least one person who is not hit by a pie. First define P(n) P(n) is there is one survivor whenever 2n+1 people stand in a yard at distinct mutual distances and each person throws a pie at their nearest neighbor. 26

28 Basis step: (Show P(1) is true.) There are 2(1)+1=3 people (A,B and C) in the pie fight. Assume the closest pair is A and B. Since the distances between pairs of people are different, the distance between A and C and the distance between B and C are greater than the distance between A and B. So, A and B throw a pie at each other, while C throws a pie at either A or B, whichever is closer. So, C is not hit by a pie and P(1) is true. 27

29 Inductive step: (Show k (P(k) P(k+1)) is true.) Assume P(k) is true. There is at least one survivor whenever 2k+1 people stand in a yard at distinct mutual distances and each throws a pie at their nearest neighbor. Show P(k+1) is true. Assume there are 2(k+1)+1=2k+3 people in a yard with distinct distance between pairs of people. Let A and B be the closest pair of people among 2k+3 people. So, A and B throw pies at each other. 28

30 Case 1: If someone else throws a pie at either A or B. So, three pies are thrown at A and B. So, at most (2k+3-3) = 2k pies are thrown at the remaining 2k+1 people. This guarantees that at least one person is a survivor. Case 2: If no one else throws a pie at either A or B. Besides A and B, there are 2k+1 people. Since the distances between pairs of people are all different, by inductive hypothesis, there is at least one survivor when 2k+1 people throws pie at each other. So, by mathematical induction, P(n) is true. 29

31 Recommended exercises 3,6,7,11,13,16,21,27,29,33,35,39,41,43,45,49,59,61,70 30

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