# 1.2 Properties of Integers

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1 10 CHAPTER 1. PRELIMINARIES 1.2 Properties of Integers The Well Ordering Principle and the Division Algorithm We now focus on a special set, the integers, denoted Z, as this set plays an important role in abstract algebra. We begin with an important result we will use often, the Well Ordering Principle. We will state it as an axiom, as it cannot be proven using the usual properties of arithmetic. Axiom 23 (Well Ordering Principle) Every nonempty set of positive integers contains a smallest element. Example 24 S = {1, 3, 5, 7, 9} is a set of positive integers. Its smallest element is 1. Can we say the same about sets of positive real numbers? Next, we look at the concept of divisibility, an important concept in number theory. Definition 25 A nonzero integer t is a divisor of an integer s and we write t s (read " t divides s") if there exists an integer k such that s = kt. In this case, we also say that s is a multiple of t. When t is not a divisor of s, we write t s. Definition 26 A prime is a positive integer greater than 1 whose only positive divisors are 1 and itself. Example 27 The divisors of 8 are ±1, ±2, ±4 and ±8. Example 28 The divisors of 7 are ±1 and ±7. So, 7 is prime. Example 29 What are the divisors of 0? A fundamental property of the integers is the division algorithm. Its proof involves the Well Ordering Principle. We now state and prove this result which we will use often. Theorem 30 (Division Algorithm) Let a and b be integers with b > 0. Then, there exists unique integers q and r with the property that where 0 r < b Proof. First we prove existence, then uniqueness. a = bq + r (1.1) Existence. With a and b as given, consider the set S = {a bk k is an integer and a bk 0}. Either 0 S or 0 / S.

2 1.2. PROPERTIES OF INTEGERS 11 Case 1 0 S, then there exists an integer k, call it q, such that a qb = 0 or a = qb. hence, we have existence with q = a and r = 0. b Case 2 0 / S. We want to use the Well Ordering Principle. For this, we have to show S is a nonempty set of positive integers. Since a, b and k are integers, a bk is also an integer. Since a bk 0 and 0 / S, we know that a bk > 0. We need to show S. If a > 0 then a 0 b = a > 0 S. If a < 0, then a b (2a) = a (1 2b) > 0 S. By the Well Ordering Principle, S has a smallest member. Call q the value of k for which a bk is the smallest member of S and r = a bq. Then, r 0 and a = bq + r. We still need to prove r < b. We do a proof by contradiction. Suppose r b. Then a b (q + 1) = a bq b = r b 0. However, a b (q + 1) < a bq and a bq is the smallest member of S, so we can t have a b (q + 1) 0. Thus r < b. Uniqueness. We assume there are at least two integers satisfying the condition and we show they are equal. Suppose that there exist integers q and q and r and r such that a = bq + r and a = bq + r with 0 r < b and 0 r < b. Without loss of generality, we may assume r r. Then, bq + r = bq + r or b (q q) = r r. so b r r but 0 r r < r < b. So, we must have r r = 0 which also implies q q = 0. Remark 31 This is the division you have been doing since you were little. r is the remainder. If r = 0, then b a. Example 32 For a = 20 and b = 3, the algorithm gives 20 = Greatest Common Divisor, Relatively Prime Integers Definition 33 (Greatest Common Divisor) Let m and n be two nonzero integers. The greatest common divisor of m and n, denoted gcd (m, n), is defined to be the largest of all common divisors of m and n. Definition 34 (Relatively Prime) Two nonzero integers m and n are said to be relatively prime if gcd (m, n) = 1. There are different techniques to find gcd (a, b). We illustrate one known as the Euclidean algorithm. We begin with some remarks. Suppose we are given integers a and b, assume a b. Then, by the division algorithm, there exists unique integers q 1 and r 1, 0 r 1 < b such that a = bq 1 +r 1. Any integer dividing a and b will also divide b and r 1 (see problems) hence gcd (a, b) = gcd (b, r 1 ). We repeat the process with b and r 1. There exists unique integers q 2 and r 2, 0 r 2 < r 1 such that b = r 1 q 2 + r 2. As before, gcd (b, r 1 ) = gcd (r 1, r 2 ). We

3 12 CHAPTER 1. PRELIMINARIES repeat the process with r 1 and r 2 to obtain unique integers q 3 and r 3, 0 r 3 < r 2 such that r 1 = r 2 q 3 + r 3. As before gcd (r 1, r 2 ) = gcd (r 2, r 3 ). Continuing this way, we obtain a sequence of remainders r 1 > r 2 > r 3... where each r i 0. By the well ordering principle, this process has to stop and some r i must eventually be 0. We have gcd (a, b) = gcd (b, r 1 ) = gcd (r 1, r 2 ) =... = gcd (r i, 0) = r i See the problems for the last equality. Thus, gcd (a, b) is the last nonzero remainder arising from our repeated division. We illustrate this with an example. Example 35 Let s find gcd (100, 64). Thus gcd (100, 64) = = = = = = Remark 36 If a number is written as a product of prime factors, then finding the greatest common divisor is simply a matter of gathering the common prime factors which are common to both. More specifically, if a = p h1 1 ph2 2...phn n and b = p k1 1 pk2 2...pkn n where p i are primes, with the understanding that some of the k i or h i might be 0 (if the corresponding prime is not present) and if we define s i = min (h i, k i ) for each i = 1..n, then gcd (a, b) = p s1 1 ps2 2...psn n Example 37 Find gcd ( , ) gcd ( , ) = = 70 Theorem 38 For any nonzero integers a and b, there exists integers s and t such that gcd (a, b) = as + bt Moreover, gcd (a, b) is the smallest positive integer of the form as + bt. Proof. There are diff erent ways to prove the existence part. We outline them here. Method 1: Here, we only show that gcd (a, b) = as + bt. We use the Euclidean algorithm developed above. So, this is a constructive proof. It will show

4 1.2. PROPERTIES OF INTEGERS 13 us how to find s and t. Let us assume that the Euclidean algorithm gave us the following: a = bq 1 + r 1 b = r 1 q 2 + r 2 r 1 = r 2 q 3 + r 3. r i 3 = r i 2 q i 1 + r i 1 r i 2 = r i 1 q i + r i r i 1 = r i q i Thus gcd (a, b) = r i. But using r i 2 = r i 1 q i + r i, we can write r i = r i 2 r i 1 q i thus writing r i as a linear combination of r i 1 and r i 2.Using the previous equation, we can write r i 1 = r i 3 r i 2 q i 1 thus we can express r i as a linear combination of r i 2 and r i 3. Using all the equations in the Euclidean algorithm in reverse order, we can express r i as a linear combination of a and b. Method 2: This proof involves the division algorithm and the Well Ordering Principle. It is similar in some ways to the proof of the division algorithm. Here, we will prove everything stated in the theorem. But this proof is not constructive, it does not tell us how to find s and t. Consider the set S = {am + bn am + bn > 0 and m and n are integers}. Since a, b, m, n are all integers, am+bn is an integer. So, S is a set of positive integers. It is not empty because if am + bn < 0 then a ( m) + b ( n) > 0. Thus, by the Well Ordering Principle, S has a smallest member, say d = as+bt. We now prove that d = gcd (a, b). For this, we first prove that d is a divisor of both a and b and then that it is in fact, the largest common divisor. d is a divisor of both a and b. First, we establish d divides a. Using the division algorithm, we can write a = dq +r with 0 r < d. If we had r > 0, then r = a dq = a (as + bt) q = a asq btq = a (1 sq) + b ( tq) Thus we would have a (1 sq) + b ( tq) S contradicting d is the smallest member of S. Thus r = 0 hence a = dq or d a. We can carry a similar argument for d b. Hence, d is a common divisor to both a and b. d is the largest common divisor to both a and b. Suppose d is another common divisor to both a and b that is a = d h and b = d k.

5 14 CHAPTER 1. PRELIMINARIES Then, d = as + bt = d hs + d kt = d (hs + kt) so d is a divisor of d, hence d > d. Thus d is the greatest common divisor. There is an important corollary of this theorem, which we will often use. Remark 39 The proof shown in method 1: also gives a way to find what that combination is. Example 40 Find s and t such that gcd (100, 64) = 100s + 64t. Recall from above that gcd (100, 64) = 4. Using our computations above, we have 4 = = 28 3 ( ) = = 4 (64 36) 3 36 = = (100 64) Corollary 41 If a and b are relatively prime, then there exists integers s and t such that as + bt = 1. The next lemma is very important and often used. Lemma 42 (Euclid s Lemma) Let a and b be integers. If p is prime and p ab then p a or p b. Proof. It is enough to prove that if p a then we must have p b. So, suppose p ab and p a. Then a and p are relatively prime and therefore there exist integers s and t such that 1 = as + pt. Then, b = abs + bpt. Since p divides the right hand side, it must also divide b. This fact is used in particular when proving that p is irrational when p is prime. Prime numbers are extremely important when dealing with integers. They are their building blocks, as the next result shows. Theorem 43 (Fundamental Theorem of Arithmetic) Every integer greater than 1 is either prime or a product of primes. This product is unique, except for the order of the factors. Proof. We will prove this result later in the chapter. Example = Example =

6 1.2. PROPERTIES OF INTEGERS 15 We finish this section with the notion of least common multiple. Definition 46 (Least Common Multiple) The least common multiple of two nonzero integers is the smallest positive integer that is a multiple of both integers. It is denoted lcm (a, b). Remark 47 One way to compute lcm (a, b) is to factor a and b as a product of primes and gather all the primes with the highest power which appear. More specifically, if a = p h1 1 ph2 2...phn n and b = p k1 1 pk2 2...pkn n where p i are primes, with the understanding that some of the k i or h i might be 0 (if the corresponding prime is not present) and if we define m i = max (h i, k i ) for each i = 1..n, then lcm (a, b) = p m1 1 pm2 2...pmn n Example 48 Find lcm ( , ) lcm ( , ) = = Example 49 Find lcm (100, 64) First, write 100 = and 64 = 2 6. Therefore, lcm (100, 64) = = Modular Arithmetic Before we give a formal definition let us look at simple examples we all have done in the past, without being aware we were performing modular arithmetic. Loosely speaking, modular arithmetic is a system of arithmetic for integers, where integers wrap around after they reach a certain value, called the modulus. Example 50 Suppose your electricity was out for 27 hours and all your clocks are off by that amount. Assume you have analogue clocks. To reset your clocks, you could of course, advance them by 27 hours. But you know it would be a waste of time. After you would have advanced it 27 hours, you would be at the same place. Hence, it would have been enough to advance them by 3 hours. Mathematically, this amounts to noticing that 27 = Example 51 Suppose you need to know which day of the week will be 23 days from Wednesday. It is not necessary to count 23 days. Knowing that days repeat themselves every 7 days, and the fact that 23 = , we see it is enough to add 2 days to Wednesday and we get Friday. Thus, in both cases, the answer was obtained by finding the remainder of a division. We can now state a more formal definition Definition 52 Let a and n be integers. Suppose that when we perform the division algorithm we get a = nq +r where q is the quotient and r the remainder with 0 r < n. Then, we write: a mod n = r

7 16 CHAPTER 1. PRELIMINARIES we also write a r (mod n) and read "a is congruent to r mod n". We will use the latter notation in the very near future. Remark 53 In other words, a mod n = r a = nq + r for some integer q. Also, a mod n is the remainder of the division of a by n. Example mod 12 = 1 since 25 = Example mod 64 = 36 since 100 = Example mod 45 = 0 since 32 = Example 57 2 mod 15 = 13 since 2 = 15 ( 1) + 13 In other words, when doing modular arithmetic (mod n), the following happens. Suppose we start counting from 0. Until we reach n 1, it is the same as regular arithmetic. However, with modular arithmetic, mod n, when we reach n, we restart at 0. The table below illustrates this with n = 5. i i mod We see that i mod 5 (a mod n in general) is simply the remainder of the division by 5 (n in the general case). The same idea applies when we do modular arithmetic. We use normal arithmetic, then apply modular arithmetic to the result. With this in mind, we can define addition and multiplication modulo n as follows: Definition 58 (Addition modulo n) Let a and b be two integers. Addition modulo n is defined to be regular addition mod n that is (a + b) mod n. If a+b < n then (a + b) mod n = a + b. If a + b > n then (a + b) mod n is the remainder of the division of a + b by n. Definition 59 (multiplication modulo n) This is defined similarly. Let a and b be two integers. Multiplication modulo n is defined to be regular multiplication mod n that is (ab) mod n. If ab < n then (ab) mod n = ab. If ab > n then (ab) mod n is the remainder of the division of ab by n. Example 60 Suppose that n = 5. Then But (2 + 1) mod 5 = = 3 (3 + 4) mod 5 = 7 mod n = 2

8 1.2. PROPERTIES OF INTEGERS 17 Similarly (4 + 1) mod 5 = 5 mod 5 Example 61 Suppose that n = 7.Then, and = mod 7 = 6 mod 7 = mod 7 = 12 mod 7 = 5 Here are some properties of modular arithmetic. Proposition 62 Suppose that a, b, a, b are integers and n is a positive integer. Then, the following results are true: 1. If a mod n = a and b mod n = b then (a + b) mod n = (a + b ) mod n. 2. If a mod n = a and b mod n = b then (a b) mod n = (a b ) mod n. 3. If a mod n = a and b mod n = b then (ab) mod n = (a b ) mod n. 4. a mod n = b mod n n (a b) Proof. See problems Important things to Remember We list the important results students must know. These results come from the notes and the problems assigned. Well Ordering Principle. Division algorithm. If a and b are integers, b > 0, then there exist unique integers q and r with 0 r < b such that a = bq + r. If d = gcd (a, b) then there exists integers s and t such that d = sa + bt and the greatest common divisor of a and b is the smallest positive integer of this form. a and b are relatively prime if and only if there exist integers s and t such that 1 = as + bt. Fundamental theorem of arithmetic. p prime and p ab then p a or p b.

9 18 CHAPTER 1. PRELIMINARIES Properties of modular arithmetic. If a and b are positive integers then ab = gcd (ab) lcm (a, b). If a and b are relatively prime integers which divide another integer c then ab also divides c. Let a and n be positive integers. ax mod n = 1 has a solution gcd (a, n) = 1. gcd (a, bc) = 1 gcd (a, b) = 1 and gcd (a, c) = Exercises 1. Prove that if a = bq + r and k is an integer then k divides a and b if and only if k divides b and r. Conclude that gcd (a, b) = gcd (b, r). 2. Prove that gcd (m, 0) = m. 3. Prove that p is irrational if p is prime (hint: assume it is not). 4. Prove proposition In your book, at the end of chapter 0, do # 1, 2, 4, 7, 8, 9, 10, 12, 13, 19.

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