# Sequences and Mathematical Induction. CSE 215, Foundations of Computer Science Stony Brook University

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1 Sequences and Mathematical Induction CSE 215, Foundations of Computer Science Stony Brook University

2 Sequences A sequence is a function whose domain is all the integers between two given integers a m, a m+1, a m+2,..., a n all the integers greater than or equal to a given integer a m, a m+1, a m+2,... a k is a term in the sequence kis the subscript or index m is the subscript of the initial term n is the subscript of the last term (m n) An explicit formula or general formula for a sequence is a rule that shows how the values of a k depend on k 2

3 Sequences Examples: a k = 2 k is the sequence 2, 4, 8, 16, 32, 64, 128,... Index Term a k = k/k + 1, for all integers k 1: b i = i-1/i, for all integers i 2: 3 a k for k 1 is the same sequence with b i for i 2

4 Sequences An Alternating Sequence: c j = ( 1) j for all integers j 0: c 0 = ( 1) 0 = 1 c 1 = ( 1) 1 = 1 c 2 = ( 1) 2 = 1 c 3 = ( 1) 3 = 1 c 4 = ( 1) 4 = 1 c 5 = ( 1) 5 = 1 4

5 Find an explicit formula for a sequence The initial terms of a sequence are: a k is the general term of the sequence, a 1 is the first element observe that the denominator of each term is a perfect square observe that the numerator equals ±1: alternating sequence with -1 when k is even: 5

6 Find an explicit formula for a sequence Result sequence: Alternative sequence: 6

7 Summation Notation If m and n are integers and m n, the summation from k equals m to n of a k,, is the sum of all the terms a m, a m+1, a m+2,..., a n k is the index of the summation m is the lower limit of the summation n is the upper limit of the summation 7

8 Summation Notation Example: a 1 = 2, a 2 = 1, a 3 = 0, a 4 = 1, a 5 = 2 8

9 Summation Notation Summation notation with formulas example: Changing from Summation Notation to Expanded Form: 9

10 Summation Notation Changing from Expanded Form to Summation Notation: The general term of this summation can be expressed as for integers k from 0 to n 10

11 Sequences Evaluating expression for given limits: 11

12 Sequences Recursive definitions: Use of recursion examples: 12

13 This image cannot currently be displayed. Sequences Telescoping Sums: 13

14 Product Notation The product from k equals m to n of a k,,for m and n integers and m n, is the product of all the terms a m, a m+1, a m+2,..., a n Example: 14

15 Product Notation Recursive definition: 15

16 Sequences If a m, a m+1, a m+2,... and b m, b m+1, b m+2,... are sequences of real numbers: Generalized distributive law: if c is any real number: 16

17 Sequences Using Properties of Summation and Product: a k = k + 1 b k = k 1 17

18 Sequences Using Properties of Summation and Product: a k = k + 1 b k = k 1 18

19 Sequences Change of variable examples: 19

20 Factorial Notation 20 The quantity n factorial, n!, is defined to be the product of all the integers from 1 to n: n! = n (n 1) ! is defined to be 1: 0! = 1 0! =1 1! = 1 2! = 2 1 =2 3! = = 6 4! = = 24 5! = = 120 6! = = 720 7! = = 5,040 8! = = 40,320 9! = = 362,880

21 Factorial Notation A recursive definition for factorial is: Computing with Factorials: 21

22 n choose r n choose r,, represents the number of subsets of size r that can be chosen from a set with n elements, for n and r integers with 0 r n Examples: 22

23 n choose r 4 choose 2 = 4! / (2!*2!) = 6 Example: Let S = {1,2,3,4} The 6 subsets of S with 2 elements are: {1,2} {1,3} {1,4} {2,3} {2,4} {3,4} 23

24 Sequences in Computer Programming Arrays: int[] a = new int[50]; 24

25 Example Algorithm with Arrays Convert from Base 10 to Base 2: 38 = = ( ) = = ( ) = = ( ) = = ( ) =

26 Convert from Base 10 to Base 2 Input: n [a nonnegative integer] Algorithm Body: q := n, i := 0 while (i = 0 or q = 0) r[i] := q mod 2 q := q div 2 i := i + 1 end while Output: r[0], r[1], r[2],..., r[i 1] [a sequence of integers] 26

27 Mathematical Induction The Principle of Mathematical Induction: Let P(n) be a property that is defined for integers n, and let a be a fixed integer. Suppose the following two statements are true: 1. P(a) is true. 2. For all integers k a, if P(k) is true then P(k + 1) is true. Then the statement for all integers n a, P(n) is true. That is: P(a) is true. P(k) P(k + 1), k a P(n) is true, n a 27

28 Mathematical Induction The Method of Proof by Mathematical Induction: To prove a statement of the form: For all integers n a, a property P(n) is true. Step 1 (base step): Show that P(a) is true. Step 2 (inductive step): Show that for all integers k a, if P(k) is true then P(k + 1) is true: Inductive hypothesis: suppose that P(k) is true, where k is any particular but arbitrarily chosen integer with k a. Then show that P(k + 1) is true. 28

29 Mathematical Induction Example: For all integers n 8, n can be obtained using 3 and 5 coins: Base step: P(8) is true because 8 can = one 3 coin and one 5 coin Inductive step: for all integers k 8, if P(k) is true then P(k+1) is also true Inductive hypothesis: suppose that k is any integer with k 8: P(k): k can be obtained using 3 and 5 coins We must show: P(k+1)is true:(k+1) can be obtained using 3 and 5 coins Case 1 (There is a 5 coin among those used to make up the k ): replace the 5 coin by two 3 coins; the result will be (k + 1). Case 2 (There is not a 5 coin among those used to make up the k ): because k 8, at least three 3 coins must have been used. Remove three 3 coins (9 ) and replace them by two 5 coins (10 ); the result will be (k + 1) 29

30 Mathematical Induction Example: The Sum of the First n Integers: Base step: P(1): Inductive step: P(k) is true, for a particular but arbitrarily chosen integer k 1: Prove P(k+1): 30

31 The Sum of the First n Integers A formula in closed form represents a sum with a variable number of terms without an ellipsis or a summation symbol. Applying the Formula for the Sum of the First n Integers: 31

32 Geometric sequence Each term is obtained from the preceding one by multiplying by a constant factor: if the first term is 1 and the constant factor is r: 1, r, r 2, r 3,..., r n,... Base step: Prove P(0): 1 = 1 (Proved) Inductive hypothesis:suppose P(k) is true:for k 0: Prove P(k + 1): 32

33 Geometric sequence 33

34 Geometric sequence Examples: 34

35 Mathematical Induction Proving a Divisibility Property: P(n): for all integers n 0, 2 2n 1 is divisible by 3 Basic step P(0): = 0 is divisible by 3 Induction hypothesis: Suppose P(k) is True: 2 2k 1 is divisible by 3 Prove: P(k+1): 2 2(k+1) 1 is divisible by 3? 35

36 Mathematical Induction Proving a Divisibility Property: 2 2k + r is an integer because integers are closed under multiplication and summation so, 2 2(k+1) 1 is divisible by 3 36

37 Mathematical Induction Proving an Inequality: P(n): for all integers n 3, 2n + 1 < 2 n Base step: Prove P(3): < < 8 (True) Inductive step: Suppose that for k 3, P(k) is True: 2k + 1 < 2 k Show: P(k+1): 2(k+1) + 1 < 2 k+1 That is: 2k + 3 < 2 k+1 2k + 3 = (2k + 1) + 2 < 2 k + 2 k = 2 k+1 because 2k + 1 < 2 k by the inductive hypothesis and because 2 < 2 k for all integers k 3 37

38 Mathematical Induction A sequence: a 1 = 2 and a k = 5a k-1 for all integers k 2 38 Prove: a n = 2 5 n 1 Proof by induction: P(n): a n = 2 5 n 1 for all integers n 1 Base step: P(1): a 1 = = = 2 1 = 2 Inductive hypothesis: assume P(k) is true: a k = 2 5 k 1 Show: P(k+1): a k+1 = 2 5 (k+1) 1 = 2 5 k? a k+1 = 5a (k+1)-1 by definition of a 1, a 2, a 3,... = 5 a k since (k + 1) 1 = k = k 1 by inductive hypothesis = 2 (5 5 k 1 ) by regrouping = 2 5 k by the laws of exponents

39 Mathematical Induction A Problem with Trominoes (Tetris): For any integer n 1, if one square is removed from a 2 n 2 n checkerboard, the remaining squares can be completely covered by L-shaped trominoes Base case: a 2 2 checkerboard can be covered by 1 L-shaped tromino 39

40 Mathematical Induction Inductive hypothesis for k 1: P(k): if one square is removed from a 2 k 2 k checkerboard, the remaining squares can be completely covered by L-shaped trominoes P(k+1): if one square is removed from a 2 k+1 2 k+1 checkerboard, the remaining squares can be completely covered by L-shaped trominoes 40

41 Strong Mathematical Induction 41 The Principle of Strong Mathematical Induction (or the principle of complete induction): P(n) is a property that is defined for integers n, and a and b are fixed integers with a b. Base step: P(a), P(a + 1),..., and P(b) are all true Inductive step: For any integer k b, if P(i) is true for all integers i from a through k (inductive hypothesis), then P(k + 1) is true Then the statement for all integers n a, P(n) is true. That is: P(a), P(a+1),, P(b-1), P(b) are true. k b, ( a i k, P(i)) P(k + 1) P(n) is true, n a

42 Strong Mathematical Induction Any statement that can be proved with ordinary mathematical induction can be proved with strong mathematical induction (and vice versa). 42

43 Strong Mathematical Induction Divisibility by a Prime: Any integer greater than 1 is divisible by a prime number P(n): n is divisible by a prime number Base case: P(2): 2 is divisible by a prime number 2 is divisible by 2 and 2 is a prime number Inductive hypothesis: Let k be any integer with k 2 P(i): i is divisible by a prime number for all integers P(i) is true for all integers i from 2 through k Show: P(k + 1): k + 1 is divisible by a prime number 43

44 Strong Mathematical Induction Case 1 (k + 1 is prime): In this case k + 1 is divisible by itself (a prime number): k+1 = 1*(k+1) Case 2 (k + 1 is not prime): k + 1 = a*b where a and b are integers with 1<a<k+1 and 1<b<k+1. From k + 1 = a*b, k + 1 is divisible by a By inductive hypothesis, a is divisible by a prime number p By transitivity of divisibility, k + 1 is divisible by the prime number p. Therefore, k+1 is divisible by a prime number p. 44

45 Strong Mathematical Induction A sequence s 0, s 1, s 2,... s 0 =0, s 1 =4, s k =6s k-1 5s k-2 for all integers k 2 s 2 = 6s 1 5s 0 = = 24, s 3 = 6s 2 5s 1 = = = 124 Prove: s n =5 n 1 Base step P(0) and P(1) are true: P(0): s 0 =5 0 1 = 1 1 = 0 P(1): s 1 = = 5 1 = 4 Inductive step: Let k be any integer with k 1, s i =5 i 1 for all integers i with 0 i k 45

46 Strong Mathematical Induction We must show P(k + 1) is true: s k+1 = 5 k+1 1 s k+1 = 6s k 5s k-1 by definition of s 0, s 1, s 2,... = 6(5 k 1) 5(5 k-1 1) by definition hypothesis = 6 5 k 6 5 k + 5 by multiplying out and applying a law of exponents = (6 1)5 k 1 by factoring out 6 and arithmetic = 5 5 k 1 by arithmetic = 5 k+1 1 by applying a law of exponents 46

47 Strong Mathematical Induction The Number of Multiplications Needed to Multiply n Numbers is (n-1) P(n): If x 1, x 2,..., x n are n numbers, then no matter how parentheses are inserted into their product, the number of multiplications used to compute the product is n 1. Base case P(1): The number of multiplications needed to compute the product of x 1 is 1 1 = 0 Inductive hypothesis: Let k by any integer with k 1 and for all integers i from 1 through k, if x 1, x 2,..., x i are numbers, then no matter how parentheses are inserted into their product, the number of multiplications used to compute the product is i 1. 47

48 Strong Mathematical Induction We must show: P(k + 1): If x 1, x 2,..., x k+1 are k + 1 numbers, then no matter how parentheses are inserted into their product, the number of multiplications used to compute the product is (k + 1) 1 = k When parentheses are inserted in order to compute the product x 1 x 2... x k+1, some multiplication is the final one: let L be the product of the left-hand l factors and R be the product of the right-hand r factors: l + r = k + 1 By inductive hypothesis, evaluating L takes l 1 multiplications and evaluating R takes r 1 multiplications (l 1) + (r 1) + 1 = (l + r ) 1 = (k + 1) 1 = k 48

49 Strong Mathematical Induction Existence and Uniqueness of Binary Integer Representations: any positive integer n has a unique representation in the form n = c r 2 r + c r-1 2 r-1 + +c c c 0 where r is a nonnegative integer, c r =1, and c j =0 or 1 for j=0,..., r 1 Proof of Existence: Base step: P(1): 1 = c where c 0 = 1, r=0. Inductive hypothesis: k 1 is an integer and for all integers i from 1 through k: P(i): i = c r 2 r + c r-1 2 r-1 + +c c c 0 We must show that k + 1 can be written in the required form. 49

50 Strong Mathematical Induction Case 1 (k + 1 is even): (k + 1)/2 is an integer By inductive hypothesis: (k + 1)/2 =c r 2 r + c r-1 2 r-1 + +c c c 0 k + 1 =c r 2 r+1 + c r-1 2 r + +c c c 0 2 =c r 2 r+1 + c r-1 2 r + +c c c Case 2 (k + 1 is odd): k is even, so k/2 is an integer By inductive hypothesis: 50 k/2 =c r 2 r + c r-1 2 r-1 + +c c c 0 k =c r 2 r+1 + c r-1 2 r + +c c c 0 2 k + 1 =c r 2 r+1 + c r-1 2 r + +c c c =c r 2 r+1 + c r-1 2 r + +c c c

51 Strong Mathematical Induction Proof of Uniqueness: Proof by contradiction: Suppose that there is an integer n with two different representations as a sum of nonnegative integer powers of 2: 2 r + c r-1 2 r c c 0 = 2 s + d s-1 2 s d d 0 r and s are nonnegative integers, and each c i and each d i equal 0 or 1 Assume: r < s By geometric sequence: 2 r + c r-1 2 r c c 0 2 r + 2 r = 2 r+1 1 < 2 s 2 r + c r-1 2 r c c 0 < 2 s + d s-1 2 s d d 0 Contradiction 51

52 Recursion A sequence can be defined in 3 ways: enumeration: -2,3,-4,5,... general pattern: a n =(-1) n (n+1), for all integers n 1 recursion: a 1 =-2 and a n =(-1) n-1 a n-1 +(-1) n define one or more initial values for the sequence AND define each later term in the sequence by reference to earlier terms A recurrence relation for a sequence a 0, a 1, a 2,... is a formula that relates each term a k to certain of its predecessors a k-1, a k-2,..., a k-i, where i is an integer with k i 0 The initial conditions for a recurrence relation specify the values of a 0, a 1, a 2,..., a i-1, if i is a fixed integer, OR a 0, a 1,..., a m, where m is an integer with m 0, if i depends on k. 52

53 Recursion Computing Terms of a Recursively Defined Sequence: Example: Initial conditions: c 0 = 1 and c 1 = 2 Recurrence relation: c k = c k-1 + k * c k-2 + 1, for all integers k 2 c 2 = c c by substituting k = 2 into the recurrence relation = since c 1 = 2 and c 0 = 1 by the initial conditions = 5 c 3 = c c by substituting k = 3 into the recurrence relation = since c 2 = 5 and c 1 = 2 = 12 c 4 = c c by substituting k = 4 into the recurrence relation = since c 3 = 12 and c 2 = 5 = 33 53

54 Recursion Writing a Recurrence Relation in More Than One Way: Example: Initial condition: s 0 = 1 Recurrence relation 1: s k = 3s k-1-1, for all integers k 1 Recurrence relation 2: s k+1 = 3s k - 1, for all integers k 0 54

55 Recursion Sequences That Satisfy the Same Recurrence Relation: Example: Initial conditions: a 1 = 2 and b 1 = 1 Recurrence relations: a k = 3a k-1 and b k = 3b k-1 for all integers k 2 a 2 = 3a 1 = 3 2 = 6 b 2 = 3b 1 = 3 1 = 3 a 3 = 3a 2 = 3 6 = 18 b 3 = 3b 2 = 3 3 = 9 a 4 = 3a 3 = 3 18 = 54 b 4 = 3b 3 = 3 9 = 27 55

56 Recursion Fibonacci numbers: 1. We have one pair of rabbits (male and female) at the beginning of a year. 2. Rabbit pairs are not fertile during their first month of life but thereafter give birth to one new male&female pair at the end of every month. 56

57 Recursion 57 Fibonacci numbers: The initial number of rabbit pairs: F 0 = 1 F n : the number of rabbit pairs at the end of month n, for each integer n 1 F n = F n-1 + F n-2, for all integers k 2 F 1 = 1, because the first pair of rabbits is not fertile until the second month How many rabbit pairs are at the end of one year? January 1 st : F 0 = 1 September 1 st : F 8 =F 7 +F 6 =21+13=34 February 1 st : F 1 = 1 October 1 st : F 9 =F 8 +F 7 =34+21=55 March 1 st : F 2 = F 1 + F 0 = 1+1= 2 November 1 st : F 10 =F 9 +F 8 =55+34=89 April 1 st : F 3 = F 2 + F 1 = 2+1= 3 December 1 st : F 11 =F 10 +F 9 =89+55=144 May 1 st : F 4 = F 3 + F 2 = 3+2= 5 January 1 st : F 12 =F 11 +F 10 =144+89=233 June 1 st : F 5 = F 4 + F 3 = 5+3= 8 July 1 st : F 6 = F 5 + F 4 = 8+5= 13 August 1 st : F 7 = F 6 + F 5 = 13+8= 21

58 Recursion Compound Interest: A deposit of \$100,000 in a bank account earning 4% interest compounded annually: the amount in the account at the end of any particular year = the amount in the account at the end of the previous year + the interest earned on the account during the year = the amount in the account at the end of the previous year the amount in the account at the end of the previous year A 0 = \$100,000 A k = A k-1 + (0.04) A k-1 = 1.04 A k-1, for each integer k 1 A 1 = 1.04 A 0 = \$104,000 A 2 = 1.04 A 1 = 1.04 \$104,000 = \$108,

59 Recursion Compound Interest with Compounding Several Times a Year: An annual interest rate of i is compounded m times per year: the interest rate paid per each period is i/m P k is the sum of the the amount at the end of the (k 1) period + the interest earned during k-th period P k = P k-1 + P k-1 i/m = P k-1 (1+ i/m) If 3% annual interest is compounded quarterly, then the interest rate paid per quarter is 0.03/4 =

60 Compound Interest Example: deposit of \$10,000 at 3% compounded quarterly For each integer n 1, P n = the amount on deposit after n consecutive quarters. P k = P k-1 P 0 = \$10,000 P 1 = P 0 = \$10,000 = \$10, P 2 = P 1 = (1.0075) \$10, = \$10, P 3 = P 2 ~ (1.0075) \$10, = \$10, P 4 = P 3 ~ (1.0075) \$10, = \$10, The annual percentage rate (APR) is the percentage increase in the value of the account over a one-year period: APR = ( )/ = = 3.034% 60

61 Recursive Definitions of Sum and Product The summation from i=1 to n of a sequence is defined using recursion: The product from i=1 to n of a sequence is defined using recursion: 61

62 Sum of Sums For any positive integer n, if a 1,a 2,...,a n and b 1,b 2,...,b n are real numbers, then Proof by induction base step: inductive hypothesis: 62

63 Sum of Sums Cont.: We must show that: Q.E.D. 63

64 Recursion Arithmetic sequence: there is a constant d such that a k = a k 1 + d for all integers k 1 It follows that, a n = a 0 + d*n for all integers n 0. Geometric sequence: there is a constant r such that a k = r * a k 1 for all integers k 1 It follows that, a n = r n * a 0 for all integers n 0. 64

65 Recursion A second-order linear homogeneous recurrence relation with constant coefficients is a recurrence relation of the form: a k = A * a k-1 + B * a k-2 for all integers k some fixed integer where A and B are fixed real numbers with B = 0. 65

66 Supplemental material on Sequences: Correctness of Algorithms A program is correct if it produces the output specified in its documentation for each set of inputs initial state (inputs): pre-condition for the algorithm final state (outputs): post-condition for the algorithm Example: Algorithm to compute a product of nonnegative integers Pre-condition: The input variables m and n are nonnegative integers Post-condition: The output variable p equals m*n 66

67 Correctness of Algorithms The steps of an algorithm are divided into sections with assertions about the current state of algorithm [Assertion 1: pre-condition for the algorithm] {Algorithm statements} [Assertion 2] {Algorithm statements}... [Assertion k 1] {Algorithm statements} [Assertion k: post-condition for the algorithm] 67

68 Correctness of Algorithms Loop Invariants: used to prove correctness of a loop with respect to pre- and post-conditions [Pre-condition for the loop] while (G) [Statements in the body of the loop] end while [Post-condition for the loop] A loop is correct with respect to its pre- and post-conditions if, and only if, whenever the algorithm variables satisfy the precondition for the loop and the loop terminates after a finite number of steps, the algorithm variables satisfy the postcondition for the loop 68

69 Loop Invariant 69 A loop invariant I(n) is a predicate with domain a set of integers, which for each iteration of the loop, (induction) if the predicate is true before the iteration, the it is true after the iteration If the loop invariant I(0) is true before the first iteration of the loop AND After a finite number of iterations of the loop, the guard G becomes false AND The truth of the loop invariant ensures the truth of the post-condition of the loop then the loop will be correct with respect to it preand post-conditions

70 Loop Invariant Correctness of a Loop to Compute a Product: A loop to compute the product m*x for a nonnegative integer m and a real number x, without using multiplication [Pre-condition: m is a nonnegative integer, x is a real number, i = 0, and product = 0] while (i m) product := product + x i:= i+ 1 end while [Post-condition: product = mx] Loop invariant I(n): [i = n and product = n*x] Guard G: i m 70

71 Base Property: I (0) is i = 0 and product = 0 x = 0 Inductive Property: [If G I (k) is true before a loop iteration (where k 0), then I (k+1) is true after the loop iteration.] Let k is a nonnegative integer such that G I (k) is true: i m i= n product = n*x Since i m, the guard is passed and product = product + x = k*x + x = (k + 1)*x i=i+ 1 = k + 1 I(k + 1): (i = k + 1 and product = (k + 1)*x) is true Eventual Falsity of Guard: [After a finite number of iterations of the loop, G becomes false] After m iterations of the loop: i = m and G becomes false 71

72 Correctness of the Post-Condition: [If N is the least number of iterations after which G is false and I (N) is true, then the value of the algorithm variables will be as specified in the post-condition of the loop.] I(N) is true at the end of the loop: i = N and product = N*x G becomes false after N iterations, i = m, so m = i = N The post-condition: the value of product after execution of the loop should be mx is true. 72

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