# Handout NUMBER THEORY

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1 Handout of NUMBER THEORY by Kus Prihantoso Krisnawan MATHEMATICS DEPARTMENT FACULTY OF MATHEMATICS AND NATURAL SCIENCES YOGYAKARTA STATE UNIVERSITY 2012

2 Contents Contents i 1 Some Preliminary Considerations Basic Axioms for Z The Principle of Mathematical Induction The Binomial Theorem Problems Recursion Concept The Concept of Recursion Discussion The Bright Graduate The Tower of Hanoi Problems Divisibility Elementary Divisibility Properties Floor and Ceiling of a Real Number The Division Algorithm Problems The Euclidean Algorithm GCD (Greatest Common Divisor) LCM (Least Common Multiple) The Euclidean Algorithm i

3 Contents ii 4.4 Discussion Bézout s Identity The Diophantine Equation Continued Fractions Counting in Arbitrary Base Positional Notation of Numbers Base 2 and Its Operations Addition Subtraction Multiplication Division Base 8 and 16 (Discussion) Base Base Prime Factorization Fundamental Theorem of Arithmetic Congruences Basic Properties Linear Congruences References 31

4 Topic 1 Some Preliminary Considerations 1.1 Basic Axioms for Z Some special sets are used throughout this handout. These sets are denoted by standard symbols, N = {1, 2, 3,...} = the set of natural numbers = Z +, Z = {..., 3, 2, 1, 0, 1, 2, 3,...} = the set of integers, Q = { n m n, m Z and m 0} = the set of rational numbers, and R = the set of real numbers. Note that N Z Q R. Below, a list of some particularly important properties of Z that will be needed. These are called axioms since we will not prove them in this course. Some basic axiom for Z: 1. If a, b Z, then a + b, a b, and ab Z. (Z is closed under addition, subtaction, and multiplication.) 2. If a Z then there is no x Z such that a < x < a Properties of inequalities: If a, b, c R then a. If a < b and b < c then a < c. b. If a < b then a + c < b + c. 1

5 TOPIC 1. SOME PRELIMINARY CONSIDERATIONS 2 c. If a < b and 0 < c then ac < bc. d. If a < b and c < 0 then bc < ac. e. Given a and b, then one and only one of the following holds: a = b, a < b, a > b 4. Laws of exponent: If n, m Z, a, b R and a and b are not zero then a. (a n ) m = a nm b. (ab) n = a n b n c. a n a m = a n+m 5. The Well Ordering Property for N: Every nonempty subset A of N contain a least element, that is to say, there exist an element m A N such that for each a A we have m a. 1.2 The Principle of Mathematical Induction Let P (n) be a statement concerning the integer variable n, and let n 0 be any fixed integer. P (n) is true for all integers n > n 0 if one can establish both of the following statements: i. P (n) is true for n = n 0, and ii. Whenever P (n) is true for n 0 n k then P (n) is true for n = k + 1. Example Prove that if n 5 then 2 n > 5n. Proof. We prove it by Mathematical Induction. i. If n = 5, we have 2 5 = 32 > 25 = 5 5 which is true. ii. Assume (the induction hypothesis) 2 n > 5n for n 0 n k. Taking n = k we have 2 k > 5k.

6 TOPIC 1. SOME PRELIMINARY CONSIDERATIONS 3 Multiplying both sides by 2 gives 2 k+1 > 10k. Now 10k = 5k + 5k and k 5 so k 1 and therefore 5k 5. Hence 10k = 5k + 5k 5k + 5 = 5(k + 1). It follows that 2 k+1 > 10k 5(k + 1). and therefore 2 k+1 > 5(k + 1). Hence we conclude that 2 n > 5n for n The Binomial Theorem For any n, k Z + satisfying 0 k n, the combination of k objects from n objects, denoted by n k, defined by n k = n! k!(n k)! Let x, y R and n Z +, the binomial theorem says that n (x + y) n = n x n k y k k It is easy to verify that k=0 (x + y) 2 = x 2 + 2xy + y 2 ; (x + y) 5 = x 5 + 5x 4 y + 10x 3 y x 2 y 3 + 5xy 4 + y 5 ; etc.

7 TOPIC 1. SOME PRELIMINARY CONSIDERATIONS Problems 1. We prove that every city is small, by induction on the number of its inhabitants. Clearly, if a city has just one inhabitant, it is small. Assume a city has n inhabitants. If this city is small, it is still so even if we add one inhabitant. So we deduce that all cities are small. Is everything right or is something amiss? a. Everything is right and it proves that mathematics cannot possibly be applied to concrete question. b. The basis of the induction is not right: if there is a single inhabitant, it is not a city. c. The basis of the induction is right, but the proof of the inductive step depends upon the definition of the small city. d. None of the above. 2. Assume we have defined a city to be small if it has less than inhabitants. Which of the statements (a) - (d) of problem 1 is correct? 3. Conjecture a formula for A n, if A = 1 1. Then by use of induction, prove your 0 1 formula. 4. Proof that for 1 k n, we have n + k n k 1 = n + 1 k Hint: Multiply the identity 1 k + 1 n k+1 = n+1 k(n k+1) by n! (k 1)!(n k)!. 5. Use the mathematical induction to prove the binomial theorem. 6. Prove that n n 1 = 2 n 1 (This problem is connected with the problem of Hanoi tower.)

8 Topic 2 Recursion Concept 2.1 The Concept of Recursion Definition 2.1. We say that the function f is defined recursively if the value of f at 1 is specified and for each n N, a rule is provided for determining f(n + 1) from f(n). Example The term n! can be recursively define by: f(1) = 1 and f(n + 1) = (n + 1)f(n). Example (Fibonacci sequence) Two newborn rabbits, a male and female, are left on a desert island on the 1 st of January. This couple becomes fertile after two months and, starting on the 1 st of March, they give birth to two more rabbits, a male and a female, the first day of each month. Each couple of newborn rabbits, analogously, become fertile after two months and, starting on the first day of their third month, gives birth to a new couple of rabbits. How many couples are there on the island after n months? Answer: Let f n denote the number of couples of rabbits, a male and a female, that are present in the island during the n th month. It is clear that f n is the sum of two numbers completely determined by the situation in the preceding months, that is: i. the number f n 1 of the couples of rabbits in the island in the (n 1) th month, as no rabbit dies; and ii. the number of the couples of rabbits born on the first day of n th month, which are as many as the couples of rabbits which are fertile on that day, and these in turn are as many as the f n 2 couples of rabbits that were in the island two months before. 5

9 TOPIC 2. RECURSION CONCEPT 6 As a consequence, we may write for the sequence f 1, f 2, f 3,..., f n recursively define by f 1 = 1, f 2 = 1, and f n = f n 1 + f n 2 for n 3. (2.1) We would like, in fact, to have a solution of the recurrence relation (2.1), that is closed formula giving the n th term of Fibonacci sequence, without having to compute all the preceding terms. Actually, the closed formula for the n th Fibonacci number is [( f n = n n )] 5. (2.2) 2 In order get the formula (2.2), we shall use matrix operations and some principles of linear algebra. Thus, we are not going to discuss about it yet. 2.2 Discussion Each group discuss one of the subsections below, subsection is for the groups: 1, 3, 4, and 7, and subsection is for the groups: 2, 5, 6, 8, and 9. Each subsection will presents by selected group The Bright Graduate Young Krisnawan, who was about to graduate at the head of his class at Yogyakarta State University, was in the pleasant position of having choice of two very attractive offers, both at \$5000 a year. Unable to make up his mind between them immediately, he wrote the two companies and asked his chances were for raises over the next ten years. Company A replied to the effect that it would guarantee a raise of \$300 every six months for the next ten years. And its raises will be started at the second sixth-month. Meanwhile, company B said it would guarantee a raise of \$1200 every twelve months for the next ten years since the second year. Krisnawan quite confused with this offer. Can you help him to make the decision? The Tower of Hanoi The game of the tower of Hanoi was invented by the mathematician E. Lucas in The tower of Hanoi consist of n circular holed discs, with a vertical peg A running through all

10 TOPIC 2. RECURSION CONCEPT 7 of them; the discs are stacked with their diameters decreasing from bottom up. The goal of the game is to transfer all discs, in the same order, that is to say, with their diameters decreasing from bottom up, on another peg C, by using a support peg B (see figure 2.1) and observing the following rules: i the discs must transferred one at a time from one peg to another one; ii never during the game, on any peg, a disc with a greater diameter may be located above a disc with a smaller diameter. Determine the number of moves necessary to conclude the game starting with n discs! Figure 2.1: The tower of Hanoi with n = 7 discs 2.3 Problems 1. What is the recursive function (recurrence relation) for the sum of the first n odd natural number? 2. What is the solution of the geometric progression a n = r a n 1, a 0 = k where r and k are fixed integer. 3. What is the recurrence relation for the world population if the population increase by 3% each year? 4. Iyan has opened a bank account for which there are no charges and yielding a yearly 4% interest which is computed and paid to his account every third month. Suppose Iyan deposited a certain amount of money when he opened the account and after that

11 TOPIC 2. RECURSION CONCEPT 8 he neither withdrew nor deposited money from the account. What is the recurrence relation determining amount of money Iyan has in his account after n years?

12 Topic 3 Divisibility 3.1 Elementary Divisibility Properties Definition 3.1. The term d n, d 0 divides n, means that there is an integer k such that n = dk. The term d n means that d does not divides n. In other words, we can say that d n iff d 0 and n = dk for some k, and d n iff there is no k such that n = dk. Theorem 3.2. (Divisibility Properties): If n, m, d, a, b Z then the following statements hold: 1. 1 n and if n 0 then n n and n 0 2. If d n and n m then d m (transitivity) 3. If d n and d m then d (an + bm) (linearity) 4. If d n then ad an (multiplication property) 5. If ad an then d 0 and d n (cancellation property) 6. If n 1 then n = ±1 7. If d, n Z + and d n then d n. (comparison property) Example Prove that if d k and d l then d (k l) 9

13 TOPIC 3. DIVISIBILITY 10 Proof. By use of Theorem 3.2 (3), and pick up a = 1, b = 1, k = n, and l = m we get the desired term. 3.2 Floor and Ceiling of a Real Number Definition 3.3. If x is any real number we define x = the greatest integer less than or equal to x, and x = the least integer greater than or equal to x The term x is called the floor of x and x is called the ceiling of x. Example , 2 = 3, 4, 7 = 5, 8, 1 = 9, and 7, 8 = 7. Note that if n is an integer, by definition we have; n = x n x < n + 1. Lemma 3.4. For all x R, we have x 1 < x x. Proof. Let n = x, this give immediately that x x and x < n + 1 which implies x 1 < n = x. 3.3 The Division Algorithm Theorem 3.5. (The Division Algorithm): If a, b Z and b > 0 then there exist unique integers q, r Z satisfying the two conditions: a = bq + r and 0 r < b. In this situation, the term a will be called dividend, b the divisor, q the quotient and r the remainder when a is divided by b. Note that there are two parts to this result, existence and uniqueness of the integers q and r.

14 TOPIC 3. DIVISIBILITY 11 Proof. First, we will prove the existence of q and r. Given b > 0 and any a, define q = a b r = a bq. Clearly, we have a = bq + r, but we need to prove that 0 r < b. By Lemma 3.4 we have a b 1 < a b a b. Multiply the terms by b, since b > 0, it gives b a > b a b a. Add all sides by a and replace a b by q, we obtain b > a bq 0. Since r = a bq, this gives us the desired result 0 r < b. We still have to prove that q and r are uniquely determined. Assume that a = bq 1 + r 1 and 0 r 1 < b, and a = bq 2 + r 2 and 0 r 2 < b. We must show that r 1 = r 2 and q 1 = q 2. Now if r 1 r 2, without loss of generality we can assume that r 2 > r 1. Subtracting the two equations above, we obtain b(q 1 q 2 ) + (r 1 r 2 ) = 0 b(q 1 q 2 ) = (r 2 r 1 ). This implies that b (r 2 r 1 ), by theorem 3.2 (7) this implies that b r 2 r 1. On the other hand, we see that 0 r 1 < r 2 < b. We have b > r 2 r 1, but this contradicts b r 2 r 1. So, we must conclude that r 2 = r 1. Now, we see that b(q 1 q 2 ) = r 2 r 1 = 0, since b > 0 then q 1 = q 2.

15 TOPIC 3. DIVISIBILITY 12 Example Show that the expression a(a2 +2) 3 is an integer for all a 1. Proof. According to the Division Algorithm, every a is of the form 3q, 3q + 1, or 3q + 2. For if a = 3q then which clearly is an integer. For if a = 3q + 1 then and a(a2 +2) 3 is an integer also. Finally, for if a = 3q + 2 then a(a 2 + 2) 3 a(a 2 + 2) 3 = q(9q 2 + 2) = (3q + 1)(3q 2 + 2q + 1) a(a 2 + 2) 3 = (3q + 2)(3q 2 + 4q + 2) an integer once more. Consequently, our result is established in all cases. 3.4 Problems 1. Show that any integer of the form 6k + 5 is also the form 3j + 2, but not conversely. 2. Use the Division Algorithm to establish the following: a. The square of any integer is either the form 3k or 3k + 1. b. The cube of any integer has one of the forms: 9k, 9k + 1, or 9k + 8. c. The cube of any integer has one of the forms: 7k, 7k + 1 or 7k 1. d. The fourth power of any integer is either the form 5k or 5k For n 1, prove that n(n+1)(2n+1) 6 is an integer. 4. For n 1, establish that the integer n(7n 2 + 5) is of the form 6k. 5. If n is an odd integer, show that n 4 + 4n is of the form 16k. 6. Verify that if an integer is simultaneously a square and a cube (as is the case with 64 = 8 2 = 4 3 ), then it must be either of the form 7k or 7k + 1.

16 Topic 4 The Euclidean Algorithm 4.1 GCD (Greatest Common Divisor) Definition 4.1. Let a and b be given integers, with at least one of them different from zero. The greatest common divisor of a and b, denoted by gcd (a, b), is the positive integer d satisfying the following: i. d a and d b ii. If c a and c b then c d. For which a = 0 and b = 0, we define gcd(0, 0) = 0. If e a and e b then we call e as a common divisor of a and b. Now let C(a, b) = {e : e a and e b}, (4.1) that is, C(a, b) is the set of all common divisor of a and b. Note that, since everything divides 0 then C(0, 0) = Z, so there is no largest common divisor of 0 with 0. This is why we must define gcd(0, 0). Example C(18, 27) = { 1, 1, 3, 3, 9, 9}, so gcd(18, 27) = 9. Example C( 15, 36) = { 1, 1, 3, 3}, so gcd( 15, 36) = 3. Example C(16, 25) = { 1, 1}, so gcd(16, 25) = 1. Theorem 4.2. Let a, b, c Z with gcd(a, b) = d, then 13

17 TOPIC 4. THE EUCLIDEAN ALGORITHM 14 i. gcd( a d, b d ) = 1 ii. gcd(a + cb, b) = d. Proof. There are two item to be prove, gcd( a d, b d ) = 1 and gcd(a + cb, b) = d. i. Let a, b Z with gcd(a, b) = d, we will show that gcd( a d, b d ) = 1. Let e C( a d, b d ) then e a d and e b d. Based on the definition 3.1, there are integers k and l such that a d = ek and b d = el, so that a = dek and b = del. Hence, de a and de b, so that de C(a, b). But, since gcd(a, b) = d, then d is the greatest element of C(a, b), hence de d, and its only happen if e = ±1. Thus, gcd( a d, b d ) = 1. ii. Let a, b, c Z with gcd(a, b) = d, we have to show that gcd(a + cb, b) = d. (4.2) Let p be any element of C(a, b) then p a and p b. By theorem 3.2 (3) we see that p (a + cb), so that p C(a + cb, b), hence C(a, b) C(a + cb, b). Now let q be any element of C(a + cb, b). Then again by theorem 3.2 (3), we see that q divides (a+cb) cb = a, so that q C(a, b). Hence C(a+cb, b) C(a, b). Thus C(a+cb, b) = C(a, b), and it means that gcd(a + cb, b) = gcd(a, b) = d. Hence gcd( a d, b d ) = 1 and gcd(a + cb, b) = d. In some cases we are more interested in pairs if integers than sharing no common divisor than ±1. Such pairs of integers are called relatively prime. Definition 4.3. The integers a and b are called relatively prime if gcd(a, b) = LCM (Least Common Multiple) Definition 4.4. Let a, b Z, a 0, and b 0. The least common multiple of a and b, denoted by lcm(a, b), is the positive integer m satisfying the following: i. a m and b m ii. If a c and b c, with c > 0, then m c.

18 TOPIC 4. THE EUCLIDEAN ALGORITHM 15 Example The positif common multiples of the integers 12 and 30 are 60, 120, 180,...; hence lcm( 12, 30) = 60. Theorem 4.5. For positive integers a and b gcd(a, b).lcm(a, b) = ab. (4.3) Proof. To begin, put d = gcd(a, b) and write a = dr, b = ds for integers r and s. If m = ab d, then m = as = rb, the effect of which is to make m a (positif) common multiple of a and b. Now let c be any positif integer that is a common multiple of a and b; say, for definitness, c = au = bv. As we know, there exist integers x and y satisfying d = ax + by. In consequence, c m = cd ab c(ax + by) = = ( ab c x + b) (4.4) (4.5) ( c y (4.6) a) = vx + uy (4.7) The equation states that m c, allowing us to conclude that m c. Thus, in accordance with definition (above), m = lcm(a, b); that is, which is what we started out to prove. lcm(a, b) = ab d = ab gcd(a, b) (4.8) Corollary 4.6. For any choice of positif integers a and b, lcm(a, b) = ab iff gcd(a, b) = The Euclidean Algorithm The greatest common divisor of two integers can be found by listing all their positive divisors and choosing the largest one common to each; but this is cumbersome for large numbers. The division algorithm provides a more efficient method to compute gcd(a, b). Prove that if a > 0 then gcd(a, 0) = a and gcd(a, a) = a Lemma 4.7. Let a > b > 0. If a = bq + r then gcd(a, b) = gcd(b, r). (4.9)

19 TOPIC 4. THE EUCLIDEAN ALGORITHM 16 Proof. It suffices to show that C(a, b) = C(b, r), that is, the common divisors of a and b are the same as the common divisors of b and r. To show this, first let d a and d b. Note that r = a bq, which is a linear comination of a and b. So, by theorem 3.2.(3) d r. Thus d b and d r. Using theorem 3.2.(3) again and using the fact that a = bq + r is a linear combination of b and r, we have d a. So d a and d b. We have thus shown that C(a, b) = C(b, r). So gcd(a, b) = gcd(b, r). By use of Division Algorithm and lemma 4.7 we may described The Euclidean Algorithm as follows: i. Apply the Division Algorithm to a and b to get a = q 1 b + r 1, 0 r 1 < b ii. If r 1 = 0 then b a and gcd(a, b) = b. If r 1 0, repeat step 1 (divide b by r 1 ) to get q 2 and r 2 satisfying b = q 2 r 1 + r 2, 0 r 2 < r 1 iii. If r 2 = 0 then gcd(a, b) = r 1. If r 2 0, repeat step 1 (divide r n 1 by r n ) to get q n+1 and r n+1 satisfying r n 1 = q n+1 r n + r n+1, 0 r n+1 < r n iv. The division prcess continues until some zero reminder appears (r n+1 = 0), then gcd(a, b) = r n. 4.4 Discussion Each group discuss one of the sections below, subsection is for the groups: 1, 6, and 7, subsection is for the groups: 2, 5, and 8 and subsection is for the groups: 3, 4, and 9. Each section will presents by selected group Bézout s Identity The Euclidean Algorithm also provides a way of proofing the following theorem

20 TOPIC 4. THE EUCLIDEAN ALGORITHM 17 Theorem 4.8. For all integers a and b there exist integer x and y such that gcd(a, b) = ax + by. (4.10) Proof. See Clark, 2002: page 25, and Burton, 1998: page 22. Equation (4.10) is called Bézout s Identity and turns out to be very useful. Use the Euclidean Algorithm to answer the following problems. 1. Find x and y of Bézout s identity gcd(480, 245) = 480x + 245y 2. What are the coefficients of and of 1515 in Bézout s identity for gcd(28231, 1515)? 3. Determine the greatest common divisor of the real polynomials f(x) = x 3 +3x 2 x 3 and g(x) = x 2 + 3x + 2, and find a Bézout s Identity The Diophantine Equation The simplest type of Diophantine equation that we shall consider is the linear Diophantine equation in two unknowns: ax + by = c where a, b, c are given integers and a, b are not both zero. A solution of this equation is a pair of integers x 0 and y 0 which, when substitued into the equation, satisfy it; that is, we ask that ax 0 + by 0 = c. Theorem 4.9. The linear diophantine equation ax + by = c, with a, b Z, and a, b are not both zero, admits solution x, y Z iff d c, where d = gcd(a, b). If x 0, y 0 is any particular of this equation, then all other solutions are given by ( ) b ( a x = x 0 + t y = y 0 t d d) Proof. See Burton, 1998: page 34, and Baldoni, et al., 2009: page 20. Use the Euclidean Algorithm to answer the following problems. 1. Find x and y of the Diophantine equation 95 = 480x + 245y 2. Find the coefficients of and of 1515 in Diophantine equation, 13 = 28231x y.

21 TOPIC 4. THE EUCLIDEAN ALGORITHM A farmer purchased one hundred head of livestock for a total cost of \$4000. Prices were as follow: calves, \$120 each; lambs, \$50 each; and piglets, \$25 each. If the farmer obtained at least one animal of each type, how many did he buy? Continued Fractions Definition A finite continued fractions is a fraction of the form a a a a n an where a 1, a 2,, a n R, all positive with the possible exception of a 1. The number a 2,, a n are called partial denominators, or partial quotiens, of the fraction. A finite continued fraction is said to be simple if all of its partial quotiens are integer. Use the Euclidean Algorithm to answer the following problems. 1. Write as a continued fraction. 2. What is the expression of in a continued fraction? 3. What is the expression of the quotiens of two successive Fibonacci number, f n+1 f n, in a continued fraction?

22 Topic 5 Counting in Arbitrary Base 5.1 Positional Notation of Numbers When we write the number 2532 in base 10, we mean the following expression: 2562 = The writen above is the positional notation in base 10.Examining further this example, notice that dividing 2562 by 10 we get 2562 = , that is 2, the rightmost digit, gives the remainder of the division by 10 of the original number. Going on, we divide the quotient we found by 10 again, we get 256 = So the second digit from the right, that is 6, once more gives the remainder of the division, and is so uniquely determined. If we continue our algorithm then we found that the next remainder will be 5 and 2. In conclusion, three digits appearing in the decimal representasion of the number are uniquely determined by successive divisions. Theorem 5.1. Let β be an integer greather than 1. Then, for each n N there exist a non-negative integer k and k + 1 integers a 0, a 1,..., a k such that 0 a i < β, for each i = 0,..., k, these being the only such integers satisfying: n = a k β k + a k 1 β k a 1 β + a 0. (5.1) 19

23 TOPIC 5. COUNTING IN ARBITRARY BASE 20 Proof. Apply the Euclidean algorithm in the following way: divide n by β obtaining n = q 1 β + a 0, 0 a 0 < β. If q 1 0, divide it by β obtaining q 1 = q 2 β + a 1, 0 a 1 < β. Going on the same way we get q 2 = q 3 β + a 2, 0 a 2 < β q 3 = q 4 β + a 3, 0 a 3 < β. q k 1 = q k β + a k 1, 0 a k 1 < β. Since n > q 1 > q 2 > > 0 is a strictly decreasing sequence of positive integers, this process must eventually end, say, at the (k 1) stage, that is q k < β. It means that if we continue our step then we will get q k+1 0. Setting a k = q k, proceed backwards, and we will reach the representation n = a k β k + a k 1 β k a 1 β + a 0 (5.2) which was our aim. The uniqueness of this expression is clear, as the digits a i appearing in it are uniquely determined (Theorem 3.5) as the remainders of the successive divisions. Example Express the ten-base number, 2562, in the number of base 9. Answer: Apply the Euclidean algorithm in the following way: 2562 = = = Thus, based on the Theorem 5.1 above, we have that 2562 = Example By use of Theorem 5.1 above, we can find that 2562 =

24 TOPIC 5. COUNTING IN ARBITRARY BASE 21 The choice of 10 as a notational base is purely conventional; in fact, along the centuries different cultures used different bases in their numerical systems: Babylonians used the base 60, Mayans the base 20 and so on. Computers use the base 2 and that why we called it the binary system. Each digits in the binary system conveys one bit of information; the symbol 0 interpreted by the computer as the command off and the symbol 1 as the command on. Other bases used in computer science are 8 and Base 2 and Its Operations We describe now the rules to perform the four operation (+,,, ) when we represent the numbers in base Addition The addition tables we must learn in order to perform additions in base 2 are very simple: = 0, = 1, = 1, = 10. Let a = n i=0 a i 2 i and b = n i=0 b i 2 i be two positif integers. Then a + b = n (a i + b i ) 2 i. i=0 Now use the addition table and remember that = 10, this means that the term (a i + b i ) 2 i = 2 i+1, if a i = b i = 1, and so we must carry 1 to the next coefficient, so the coefficient of 2 i+1 becomes a i+1 + b i+1 + 1, while the coefficient of 2 i is 0. Example Suppose we have to sum the numbers: a.) and and 1011; b.)

25 TOPIC 5. COUNTING IN ARBITRARY BASE Subtraction In the same way, if a = n i=0 a i 2 i and b = n i=0 b i 2 i and a b, then n a b = (a i b i ) 2 i. i=0 Here we face the problem if for some i we have a i = 0 and b i = 1. In this case, in the row representing a, we borrow a 1 from the firs 1 appearing when we move leftwards starting form a i. Let us see how we actually perform the subtraction of two numbers: Example Suppose we want to subtract by 1010 Answer: We have here a = = and b = 1010 = , a b = = ( ) = = (2 1) = ( ) = = (2 1) = = ( ) = = (2 1) = = Multiplication If we are considering the number a = n i=0 a i 2 i, then 2 j a = n i=0 a i 2 i+j, that is, (2 j a) may written simply moving leftwards by j positions the digits of a and putting on their right the same number of zeros. On the other hand, suppose we have the numbers n m a = a i 2 i and b = b j 2 j. i=0 Suppose the non-zero digits of b are exactly the h digits b j1, b j2,, b jh, that is, let In this case, we have a b = b = h h 2 j l. l=1 l=1 i=0 j=0 n a i 2 i+j l. Example Suppose we deal with a multiplication of by 1101

26 TOPIC 5. COUNTING IN ARBITRARY BASE Division Finally, as regards division, we may proceed in the same way as in base 10. Example Consider a = and b = 101. What is the value of a b in base 2? 5.3 Base 8 and 16 (Discussion) Each group discuss one of the sections below, subsection is for the groups: 3, 4, 5, 7, and 8, and subsection is for the groups: 1, 2, 6, and 9. Each section will presents by selected group Base 8 Discuss within your group about: transforming a number in base 10 to a number in base 8 and vice versa, transforming a number in base 2 to a number in base 8 and vice versa, and explain the four operations (+,,, ) in base Base 16 Discuss within your group about: transforming a number in base 10 to a number in base 16 and vice versa, transforming a number in base 2 to a number in base 16 and vice versa, and explain the four operations (+,,, ) in base 16.

27 Topic 6 Prime Factorization 6.1 Fundamental Theorem of Arithmetic Theorem 6.1. Fundamental Theorem of Arithmetic. Every integer n > 1 can be writen uniquely in the form n = p 1 p 2 p 2 p s, where s is a positive integer and p 1, p 2,...p s are primes satisfying p 1 p 2 p s. To prove Theorem 6.1, we need to establish some lemmas. Lemma 6.2. If a bc and gcd(a, b) = 1 then a c Proof. See Clark, 2002: page 38 and Burton, 1998: page 24. Lemma 6.3. (Euclid s Lemma) If p is a prime and p ab then p a or p b Proof. See Clark, 2002: page 38 and Burton, 1998: page 40. Lemma 6.4. Let p be prime and let a 1, a 2,, a n be integers and n 1. If p a 1 a 2 a n, then p a i for at least one i {1, 2,, n}. Proof. See Clark, 2002: page 39. So, now is the time to prove Theorem

28 TOPIC 6. PRIME FACTORIZATION 25 Proof. existence If n is a prime, then there is nothing more to prove. If n is composite, then there exists an integer d satisfying d n and d < n. Among all such integers d, we can choose p 1 the smallest by Well-Ordering Principle. Then p 1 must be prime, otherwise it would have a divisor q with 1 < q < p 1 ; but then q p 1 and p 1 n imply that q n, which contradicts the choice of p 1 as the smallest positive divisor, not equal to 1, of n. We therefore may write n = p 1 n 1, where p 1 is prime and 1 < n 1 < n. If n 1 happens to be a prime, then we have our representation. In the contrary case, the argument is repeated to produce a second prime number p 2 such that n 1 = p 2 n 2 ; that is n = p 1 p 2 n 2, 1 < n 2 < n 1. (6.1) If n 2 is a prime, then it is not necessary to go further. Otherwise n 2 = p 3 n 3, with p 3 a prime; n = p 1 p 2 p 3 n 3, 1 < n 3 < n 2. (6.2) The decreasing sequence n > n 1 > n 2 > n 3 > > 1 (6.3) cannot continue indefinitely, so after a finite number steps n k 1 is a prime, call it, p s. This lead to the prime factorization n = p 1 p 2 p s. (6.4) uniqueness Suppose that n = p 1 p 2 p s = q 1 q 2 q t where s 1, t 1, p 1, p 2,, p s, q 1, q 2,, q t are primes, p 1 p 2 p s, and q 1 q 2 q t. We will show by mathematical induction on s that s = t and p i = q i for i = 1, 2, t. Suppose s = 1, then n = p 1 is prime and we have p 1 = n = q 1 q 2 q t. If t > 1, this contradicts the fact that p 1 is prime, so t = 1 and we have p 1 = q 1 as desired. Now assume the result holds for all s such that 1 s k. We want to show that it holds for s = k + 1. So assume n = p 1 p 2 p k p k+1

29 TOPIC 6. PRIME FACTORIZATION 26 and n = q 1 q 2 q t where p 1 p 2 p k+1 and q 1 q 2 q t. Clearly that p k+1 n so p k+1 q 1 q 2 q t. By Lemma 6.4, p k+1 q i for some i {1, 2, t}. Since p k+1 and q i are primes then p k+1 = q i (Why?). Hence p k+1 = q i q t. By the similar argument q t n = p 1 p 2 p k+1 then q t = p j for some j {1, 2, k + 1}. Hence q t = p j p k+1. This shows that so p k+1 = q t. Note that p k+1 q t p k+1 p 1 p 2 p k p k+1 = q 1 q 2 q t 1 q t, since p k+1 = q t we can cancel this prime fromboth sides and we have p 1 p 2 p k = q 1 q 2 q t 1. By induction hypothesis, k = t 1 and p i = q i for i = 1, 2, t 1. Thus we have k +1 = t and p i = q i for i = 1, 2, t 1. So this theorem holds. By the Fundamental Theorem of Arithmetic, it is clear that if n > 1 then it can be written uniquely in the form n = p a 1 1 pa 2 2 pas s, (6.5) for some s 1. The term (6.5) called the canonical form of n. The Fundamental Theorem of Arithmetic also helps us to find gcd and lcm of 2 or more integers easily. Suppose there are k positive integers, n 1, n 2, n k, where n i > 1 for all i k. We can write these integers in the canonical form Then n 1 = p a1 1 1 p a1 2 2 ps a1s n 2 = p a2 1 1 p a2 2 2 ps a2s n k = p ak 1 1 p ak 2 2 p aks s. gcd(n 1, n 2,, n k ) = p min{a1 1,a2 1, ak 1 } 1 p min{a1 2,a2 2, ak 2 } 2 p min{a1s,a2s, aks} s

30 TOPIC 6. PRIME FACTORIZATION 27 and lcm(n 1, n 2,, n k ) = p max{a1 1,a2 1, ak 1 } 1 p max{a1 2,a2 2, ak 2 } 2 p max{a1s,a2s, aks} s. Example Find the gcd and lcm of 198, 216, and 252. Answer: It is clear that 198 = = = Thus gcd(198, 216, 252) = 2 min{1,2,3} 3 min{2,3} 7 min{0,1} 11 min{0,1} = = 18 lcm(198, 216, 252) = 2 max{1,2,3} 3 max{2,3} 7 max{0,1} 11 max{0,1} = = Theorem 6.5. If n > 1 is composite then it has a prime divisor p n Proof. See Clark, 2002: page 32.

31 Topic 7 Congruences 7.1 Basic Properties Definition 7.1. Let n be a fixed integer. Two integers a and b are said to be congruent modulo n, symbolized by a b( mod n) if n a b. Example Consider n = 7, prove that 3 24( mod 7)why??? 31 11( mod 7)why??? 15 64( mod 7)why??? yang ga kongruen??? Given an integer a, let q and r be its quotient and remainder upon division by n, so that a = qn + r 0 r < n. Then, by definition of congruence, a r( mod n). Since there are n choices for r, we see that every integer is congruent modulo n to exactly one of the values 0, 1,, n 1; in particular, a 0( mod n) iff n a. The set of n integers 0, 1,, n 1 is called the set of least positive residues modulo n. In general, a collection of n integers a 1, a 2,, a n is said to form a complete set of residues modulo n if a k is congruent modulo n to 0, 1,, or n 1. For instance, 12, 4, 11, 13, 22, 82, and 91 constitute a complete set of residues modulo 7. The next theorem shows that congruence is an equivalence relation. 28

32 TOPIC 7. CONGRUENCES 29 Theorem 7.2. (Congruence is an equivalence relation.) For all a, b, c Z and m > 0 we have i a a( mod m) {reflexivity} ii If a b( mod m) then b a( mod m) {symmetry} iii If a b( mod m) and b c( mod m) then a c( mod m) {transitivity} Proof. i a a = 0 = 0.m so m a a hence a a( mod m). ii If a b( mod m) then m a b it means that a b = qm hence b a = ( q)m so m b a hence b a( mod m) iii If a b( mod m) and b c( mod m) then m a b and m b c. By linearity property (Theorem 3.2), we have m (a b) + (b c) = a c so a c( mod m). hehehe... Congruence is an equivalence relation so we can make sets those we called equivalence classes of integers by congruence relation. Example We can make 5 equivalence classes of integers by congruence relation modulo 5, those are: = [0] 5 = {, 10, 5, 0, 5, 10, } ( 0 5 is a set of integers such that if a 0 5 then a 0( mod 5)) = [1] 5 = {, 9, 4, 1, 6, 11, } ( 1 5 is a set of integers such that if a 1 5 then a 1( mod 5)) = [2] 5 = {, 8, 3, 2, 7, 12, } ( 2 5 is a set of integers such that if a 2 5 then a 2( mod 5)) = [3] 5 = {, 7, 2, 3, 8, 13, } ( 3 5 is a set of integers such that if a 3 5 then a 3( mod 5)) = [4] 5 = {, 6, 1, 4, 9, 14, } ( 4 5 is a set of integers such that if a 4 5 then a 4( mod 5))

33 TOPIC 7. CONGRUENCES Linear Congruences An equation of the form ax b( mod n) is called linear congruence, and by a solution of such equation we mean an integer x 0 for which ax 0 b( mod n). By definition, ax 0 b( mod n) iff n ax 0 b iff ax 0 b = ny 0. Thus the problem of finding all integers which satisfy the linear congruence ax b( mod n) is identical with that of obtaining all solutions of the linear Diophantine equation ax ny = b (Topic 5.2.2). Theorem 7.3. The linear congruence ax b( mod n) has a solution iff d b where d = gcd(a, n).

34 References [1] Baldoni, M. W., Ciliberto, C., and Cattaneo, G. M. P Elementary Number Theory, Cryptography, and Codes. Springer-Verlag Berlin Heidelberg. [2] Burton, D. M Elementary Number Theory. Fourth Edition. The McGraw-Hill Companies, Inc. [3] Clark, W. E Elementary Number Theory. [4] Jacoby, O. and Benson, W.H Mathematics for Pleasure. Fawcett World Library: New York. 31

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