# Chapter 6. Number Theory. 6.1 The Division Algorithm

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1 Chapter 6 Number Theory The material in this chapter offers a small glimpse of why a lot of facts that you ve probably nown and used for a long time are true. It also offers some exposure to generalization, that is, of taing some specific examples that are true and finding a general statement that includes these as specific cases. 6.1 The Division Algorithm Something everyone learns in elementary school is that when one integer is divided by another there is a unique quotient and a unique remainder. For example, when 65 is divided by 17 the quotient is 3 and the remainder is 14. That is, 65 = What about when 65 is divided by 17? We have 65 = ( 3) ( 17) + 14, and we also have 65 = ( 4) ( 17) 3. Should the remainder be 14 or -3? The convention is that the remainder is always non-negative when dividing by a negative number. The fact that there is a unique quotient and a unique remainder is a theorem. It bears the name The Division Algorithm because the proof tells you how to find the quotient and the remainder when an integer a is divided by an integer b eep subtracting multiples of b from a until what s left is a number r between 0 and b 1 (inclusive). The total number of times b was subtracted from a is the quotient, and the number r is the remainder. That is, a = bq + r, 0 r < b. Theorem The Division Algorithm Let a, b Z, with b 0. Then 1

2 2 CHAPTER 6. NUMBER THEORY there exist unique integers q and r so that a = bq + r and 0 r < b. The integers q and r in The Division Algorithm are the quotient and remainder when a is divided by b, respectively. The integer b is the divisor, and (for completeness we will note that) the integer a is the dividend. Instead of proving the Division Algorithm, we illustrate the proof with the example below below where 2024 is divided by 75. First, ten 75s are subtracted, leaving Then, ten more 75s are subtracted, leaving 524. From this number five 75s are subtracted, leaving 149. And finally, one 75 is subtracted leaving 74 (the remainder). The quotient is the total number of 75s subtracted, which is 26. Thus 2024 = ) Suppose, for the moment, that a and b are both positive. The quotient when a is divided by b is the largest integer q such that bq < a. This is the floor of a/b: if a = bq + r with 0 r < b then a/b = (bq + r)/b = q + (r/b) = q. The same thing happens when a is negative (notice that the quotient is a negative number). Now suppose a is positive and b is negative. Again, the quotient when a is divided by b is the largest integer q such that bq > a. (Such a q is negative!) This is the ceiling of a/b: if a = bq + r with 0 r < b then a/b = (bq + r)/b = q + (r/b) = q, where the last equality follows because b is negative and so r/b is in the interval ( 1, 0]. The same thing happens when a is negative. We state the observations just made formally, and also indicate a different proof.

3 6.2. BASE B REPRESENTATIONS 3 Proposition Let a, b Z, with b 0. If q and r are integers such that a = bq + r, 0 r < b, then { a/b if b > 0, and q = a/b if b < 0. Proof. We give the proof when b > 0. The proof when b < 0 is similar. By definition of the floor function, (a/b) 1 < a/b a/b. Multiplying through by the positive number b and simplifying gives a b < b a/b a. Let r = a b a/b. Then a = b a/b + r. Rearranging the right hand inequality gives, 0 a b a/b = r. Rearranging the left hand inequality gives r = a b a/b < b. Therefore, by The Division Algorithm, r is the remainder when a is divided by b, and a/b is the quotient. 6.2 Base b representations When we use the symbol 5374 to represent the integer five thousand, three hundred and seventy four, we understand the notation to mean Every place in the notation has a value that is a power of ten, the base of the system. The number 5 is in the thousands place, 3 is in the hundreds place, 7 is in the tens place and 4 is in the ones place. (Aside: if there were a decimal point, then the places to the right of it would be the tenths, hundredths, and so on because 1/10 = 10 1, 1/100 = 10 2, etc.) If the digits 0, 1,... 9 are used in the various places, then every integer can be uniquely represented (in base 10). It turns out that there is nothing special about 10, any integer b > 1 can be used in its place, as can integers b < 1, though we will not consider this case. Let b > 1 be an integer. A base b digit is one of the numbers 0, 1,..., b 1. If d, d 1,..., d 0 are all base b digits, then the notation (d d 1... d 1 d 0 ) b is shorthand for d b + d 1 b d 1 b 1 + d 0 b 0. Notice that every place in the notation has a value that is a power of the base: the value of the i-place from the right is b i 1 (the value of the rightmost place is b 0 ). For an integer n 0, if n = d b +d 1 b 1 + +d 1 b 1 +d 0 b 0 and d 0, then (d d 1... d 1 d 0 ) b is called the base b representation of n. By analogy with what happens in base 10, if n 0 then the base b representation of n is (d d 1... d 1 d 0 ) b.

4 4 CHAPTER 6. NUMBER THEORY For example, (234) 5 is the base 5 representation of the integer = 69 (in base 10). Every integer has a unique base b representation. We illustrate the proof that a representation exists with an example. Suppose we want to find the base 5 representation of 69, that is, base 5 digits d 0, d 1,... so that 69 = d 5 + d d d Rearranging the right hand expression gives 69 = (d d d 1 ) 5 + d 0, so that the ones digit, d 0 is the remainder when 69 is divided by 5, that is, 4. Furthermore, the quotient is the number (69 4)/5 = 13, which equals d d d 1 ), and hence has base 5 representation (d d 1... d 1 ) 5. (Notice the absence of d 0 in this representation.) Repeating what was just done, the ones digit of this number, that is d 1, is the remainder on dividing the 13 by 5. Hence d 1 = 2 and, continuing in this way, d 2 is the remainder on dividing the new quotient of (13 3)/5 = 2 by 5 (so d 2 = 2). Repeating again gives d j = 0 for all j > 2. Hence 69 = (234) 5, as above. Why is the representation unique? Because of The Division Algorithm. It says that the quotient and remainder are unique. Since the base 5 digits are remainders on division of a uniquely determined number by 5, there can be only one representation. The argument given above generalizes to give the proof of the following theorem. Theorem If b > 1, then every integer n has a unique base b representation. Proof. Since 0 = (0) b, and n has a representation if and only if n has a representation it is the negative of the representation of n it suffices to prove the statement when n 1. Use The Division Algorithm to define the numbers q 0, d 0, q 1, d 1,... as follows: n = b q 0 + d 0 0 d 0 < b q 0 = b q 1 + d 1 0 d 1 < b q 1 = b q 2 + d 2 0 d 2 < b... We claim that this process terminates, that is, eventually some quotient q = 0, and then q j = d j = 0 for all j >. To see this, notice that b > 1.

5 6.2. BASE B REPRESENTATIONS 5 and n > 0 implies n/b < n, so 0 q 0 = n/b < n. If q 1 > 0 then the same argument applies to give n > q 0 > q 1 0, and so on. The process must reach zero in at most n steps, which proves the claim. Now, each number d i is a base b digit, and n = q 0 b + d 0 = (q 1 b + d 1 ) b + d 0 = q 1 b 2 + d 1 b + d 0 = (q 2 b + d 2 ) b 2 + d 1 b + d 0 = q 2 b 3 + d 2 b 2 + d 1 b + d 0. = d b + d 1 b d 0 Hence n = (d d 1... d 1 d 0 ) b. Uniqueness of the representation follows from The Division Algorithm. The digit d 0 is the remainder when n is divided by b, and is uniquely determined. The digit d 1 is the remainder when the integer (n d 0 )/b is divided by b, and is uniquely determined. Continuing in this way, the entire base b representation is uniquely determined. The proof of Theorem tells us how to find the base b representation of a natural number n. The ones digit is the remainder when n is divided by b. When the quotient resulting from the previous division is divided by b, the remainder is the next digit to the left. The process stops when the quotient and remainders from a division by b are 0. If the base is bigger than 10, then we need to use other symbols to represent the digits. For example, in hexadecimal (base 16), the letters A, B, C, D, E, and F stand for 10 through 15, respectively. The hexadecimal number (A3F ) 16 = To convert between bases, say to convert (A3F ) 16 to binary (base 2), one could first convert it to base 10 using the meaning of the notation, and then to base 2 as above. However, the fact that 2 4 = 16 offers a shortcut. We now (A3F ) 16 = = Now replace each multiplier by its base 2 representation, inserting leading zeros if needed to get

6 6 CHAPTER 6. NUMBER THEORY 4 binary digits, to get (A3F ) 16 = ( ) 2 8 +( ) 2 4 +( ). Multiplying everything out gives the binary representatinon: (A3F ) 16 = ( ) 2. The representation on the right is obtained by replacing each hexadecimal digit by its representation as a 4-digit binary number (possibly having leading zeros). One can similarly use the fact that 2 4 = 16 to convert from binary to hexadecimal. Consider the number: ( ) 2 = = ( ) ( ) ( ) = = = (2BD) 16. This is the number obtained by adding leading zeros so the number of binary digits is a multiple of 4 and then starting at the right and replacing each sequence of 4 binary digits by its hexadecimal equivalent. (We use 4 digits at a time because 16 = 2 4.) The same sort of argument applies to convert between any two bases b and b t, for example between binary and octal (base 8). 6.3 The number of digits How many digits are there in the base 10 representation of n N? Numbers from 1 to 9 have one digits, numbers from 10 to 99 have 1 digits, numbers from 100 to 999 have three digits, and so on. The number of digits in the base 10 representation of n is one greater than in the base ten representation of n 1 whenever n is a power of 10. That is, the smallest base ten number with +1 digits is 10, and the largest base 10 number with digits is Thus, if 10 1 n < 10, then the base 10 representation of n has digits. For every n such that 10 1 n < 10 we have 1 log 10 (n) < so that (the number of digits in the base 10 representation of n) equals 1 + log 10 (n) + 1. There is nothing special about 10 in this discussion. In any base b > 1, if

7 6.4. DIVISIBILITY 7 b 1 n < b, then the base b representation of n has digits. It follows as above that (the number of digits in the base b representation of n) equals 1 + log b (n) + 1 (recall that log b (n) = x b x = n). As an aside, we note that logarithms to different bases differ only by multiplication by a constant. To see that, start with the fact that, for a, b > 0 and real number x we have a log a (x) = x = b log b (x). Therefore, since the function a y and log a (z) are inverses, log a (x) = log a (a log a (x) ) = log a (b log b (x) ) = log b (x) log a (b). Since log a (b) is a constant that is does not depend on x, we have that, for any x, log b (x) is a constant multiple of log a (x). The same statement is true with a and b reversed: either repeat the calculation with a and b exchanged, or just divide through by the non-zero number log a (b). 6.4 Divisibility If a and b are integers, we say that a divides b, and write a b, if there is an integer such that a = b. When this happens, we also say a is a divisor of b, b is divisible by a, or b is a multiple of a. Equivalently, a divides b when the remainder when b is divided by a equals 0, so that b is a times some other integer. Although it is true that if a divides b then b/a is an integer, notice that there is no discussion of fractions. Everything taing place involves only integers and multiplication. According to the definition, 5 30 because 5 6 = 30, 7 28 because ( 7) ( 4) = 28, because 10 ( 10) = 100 and 4 12 because ( 4) 3 = 12.

8 8 CHAPTER 6. NUMBER THEORY Which numbers divide zero? If a is any integer, then a 0 = 0 so a 0. In particular, 0 0. (However, zero does not divide any other integer.) It is clear from the definition that, for any integer b, we have 1 b (because 1 b = b), and also b b (because b 1 = b). The second of these say that the relation divides on Z is reflexive. It also turns out to be a transitive relation on Z (see below), but not an anti-symmetric relation on Z. (However, it is an anti-symmetric relation on N.) Divisibility is defined in terms of the existential quantifier there exists (the definition requires that there exists such that...), so proofs of divisibility involve demonstrating how such an integer can be found. This is what was happening in the previous paragraph, and also what will happen in the proofs of the propositions below. We now, for example, that 6 12 (because 6 2 = 12). Hence any multiple of 12, say 12, is also a multiple of 6 because 12 = 6 (2). The same factoring argument wors when 6 and 12 are replaced by any two numbers a and b such that a b. This fact, and its proof, are the next proposition. Proposition Let a, b, c Z. If a b and b c, then a c Proof. Suppose a b and b c. The goal is to find an integer so that a = c. Since a b, there is an integer m so that am = b. Since b c, there is an integer n so that bn = c. Therefore, c = bn = amn = a(mn). Since mn Z, we have that a c. In a manner similar to what s above, we now that since 6 12 and 6 18, then for any integers x and y we have 6 12x + 18y = 6 (2x) + 6 3y = 6 (2x + 3y). As before, this factoring argument wors whenever 6, 12 and 18 are replaced by numbers a, b and c such that a b and a c. Proposition Let a, b, c Z. If a b and a c, then a bx + cy for any integers x and y. Proof. Suppose a b and a c. Let x, y Z. The goal is to find an integer so that a = bx + cy. Since a b, there is an integer m so that am = b. Since a c, there is an integer n so that an = c. Therefore, bx + cy = amx + any = a(mx + ny). Since mx + ny Z, we have that a bx + cy.

9 6.5. PRIME NUMBERS AND UNIQUE FACTORIZATION 9 By taing y = 0 in the proposition above, we obtain the result that if a divides b, then it divides any integer multiple of b. The previous proposition can be generalized so that bx + cy is replaced by a sum involving more than two terms, each of which has a factor divisible by a. What could we say about the integers a and b if we have both a b and b a? Let s loo at an example. Tae b = 6. If a 6, then a is one of the numbers ±1, ±2, ±3, ±6. The only numbers in this collection that are also divisible by 6 are 6 and 6. After checing out a couple more examples, one is led to the next proposition. Proposition Let a, b, c Z. If a b and b a, then a = ±b. Proof. Since a b, there is an integer m so that am = b. Since b a, there is an integer n so that bn = a. Therefore, a = bn = amn, so that mn = 1. Since m and n are integers, either m = n = 1 or m = n = 1. If n = 1 then a = b and if n = 1 then a = b. 6.5 Prime numbers and unique factorization An integer p > 1 is called prime if its only positive divisors are 1 and itself. An integer n > 1 which is not prime is called composite. That is, an integer n is composite if there are integers a and b with 1 < a b < n such that n = ab. The integer 1 is neither prime nor composite. It is just a unit. The Grees thought of numbers as lengths. Every length n is made up of n unit lengths. A length was regarded as prime if it was not a multiple of some length other than the unit, and composite if it was. Theorem (Fundamental Theorem of Arithmetic). Every integer n > 1 can be written as a product of primes in exactly one way, up to the order of the factors. The Fundamental Theorem of Arithmetic is also nown as the Unique Factorization Theorem. It implies that every integer n > 1 has exactly one

10 10 CHAPTER 6. NUMBER THEORY prime factorization (or prime power decomposition) as n = p e 1 1 p e p e, where p 1 < p 2 < < p are different primes and e i 1, 1 i. In turn, this has some very basic consequences that one does not realize until thining about them that s why it is the fundamental theorem. Here are some examples. Every integer n > 1 has a prime divisor (which could be itself). By the Fundamental Theorem of Arithmetic, we can write n = p e 1 1 p e p e, where p 1 < p 2 < < p are different primes and e i 1, 1 i. Then p 1 p e p e p e and the term on the right hand side is at least one, so the prime p 1 n. (If p e p e p e = 1, that is, if = 1 and e 1 = 1, then n = p 1 is prime.) If a = b, then the two numbers a and b have exactly the same prime factorization. By the Fundamental Theorem of Arithmetic, if the prime p appears to the power e in the prime factorization of b, then p must appear a total of exactly e times in the prime factorizations of a and. Hence, if n has the prime factorization p e 1 1 p e p e, then the divisors of n are the numbers p d 1 1 p d p d where 0 d i e i. If a prime number p a 2, then p a. The prime factorization of a 2 has the same primes as the prime factorization of a the exponents are doubled. Hence, in fact, p 2 a 2. The Grees new that there were infinitely many prime numbers. There is a remarable proof in Euclid s Elements, published about 320BC. There is a temptation to thin of this boo only as a classic treatise on geometry (which it is). But, it also contains some very nice results in number theory. Euclid s argument uses proof by contradiction to show that there are more than n primes for any integer n; hence there are infinitely many primes. Theorem [Euclid, 300BC] There are infinitely many prime numbers. Proof. Suppose not, and let p 1, p 2,..., p n be the collection of all prime numbers. Consider the number N = p 1 p 2... p n + 1. The number N has a prime divisor (possibly itself). But none of p 1, p 2,..., p n divide N: each leaves a remainder of 1 when divided into N. Therefore there is a prime number not in the collection, a contradiction.

11 6.5. PRIME NUMBERS AND UNIQUE FACTORIZATION 11 While it is tempting to thing that the number N in Euclid s proof is prime, this is not always true. Try the first few possible values and see for yourself. It does not tae too long to get to an N that s composite. The Fundamental Theorem of Arithmetic can be used in proofs of irrationality. For example, let s show that log 10 (7) is irrational. The proof is by contradiction. Suppose there are integers a and b such that log 10 (7) = a/b. Then 10 a/b = 7 or 10 a = 7 b, contrary to the Fundamental Theorem of Arithmetic (a number can not be factored as 2 a 5 a and also as 7 b ). As a second example, we generalize our previous result that 2 is irrational by characterizing the non-negative integers n such that n is irrational. We first prove the useful lemma that if two numbers have a common divisor greater than one, then they have a common prime divisor. Lemma Let a, b Z. Suppose there is a number d > 1 such that d a and d b. Then a and b have a common prime factor. Proof. The integer d has a prime divisor, call it p (possibly p = d). Since p d and d a, we have p a. Similarly, p b. Theorem Let n 0 be an integer. Then n is irrational unless n is the square of an integer. Proof. We show that if n is rational, then n is the square of an integer. Suppose there are integers a and b so that n = a/b. Since n 0 we may assume that a 0 and b 1. Further, we may tae the fraction a/b to be in lowest terms, so that a and b have no common prime factor. Squaring both sides gives n = a 2 /b 2, so that nb 2 = a 2. If b > 1 then it has a prime divisor, say p. Then p b 2, so p must be a divisor of a 2 and hence of a. But a and b have no common prime factors. Therefore b = 1 and n = a 2, the square of an integer. The Sieve of Eratosthenes.is a method for generating all of the prime numbers less than or equal to n. 1. Write the numbers 2, 3, 4,..., n in a line. Circle the number Cross out all multiples of the number just circled. Circle the first number in the list which is neither circled nor crossed out. If the

12 12 CHAPTER 6. NUMBER THEORY number just circled is less than n, repeat this step using the newly circled number. 3. If the number just circled is greater than n, then circle all remaining numbers that are neither already circled nor crossed out. 4. The set of circled numbers is the set of primes less than or equal to n. So why does this wor? The first number to be circled is prime, and each subsequent number circled in step 2 is not a multiple of a smaller prime, hence it must be prime. It remains to explain why the process can be short circuited after a number greater than n has been circled. That s because of the proposition below. Proposition If n > 1 is composite, then it has a prime divisor p such that 2 p n. Proof. Suppose n > 1 is composite. Then there are integers a and b such that 1 < a b < n and n = ab. If both a and b are greater than n, then n = ab > n n = n, a contradiction. Thus a n. Any prime divisor of a is both less than or equal to a (and hence n) and a divisor of n (because p a and a n). Thus n has a prime divisor p such that 2 p n. 6.6 The gcd and the lcm If a and b are integers which are not both zero, the greatest common divisor of a and b is the largest integer d such that d a and d b. It is denoted by gcd(a, b). A number that divides both a and b is a common divisor of a and b. The number gcd(a, b) is the greatest number in this collection. Since 1 is always a common divisor of a and b, the collection isn t empty and gcd(a, b) 1. Why is there a greatest number in this collection? Since the numbers are not both zero, one of them is largest in absolute value, say b. No divisor of b can be greater that b, hence no common divisor of a and b can be larger than b. So gcd(a, b) is the largest integer d with 1 d b that divides both a and b.

13 6.6. THE GCD AND THE LCM 13 Why is there the restriction that a and b can not both be zero? Any number n is a divisor of zero (because n 0 = 0). Hence any integer n is a common divisor of 0 and 0, and there is no greatest such integer. Since a number and its negative have the same divisors gcd(a, b) = gcd( a, b ). For this reason it is common to let a and b be non-negative numbers when computing gcd(a, b). The following two facts are useful. Since any number divides zero, if a > 0 then gcd(a, 0) = a. Since the only positive divisor of 1 is 1 we have gcd(a, 1) = 1 for any integer a. If a > 1 and b > 1, then gcd(a, b) can be found using the prime factorizations of a and b. We do an example first. Suppose a = and b = First, rewrite these in modified form so that the same primes appear in each decomposition. To do this, we need to allow exponents to be zero. Written in this modified form, a = and b = The positive divisors of a are the numbers 2 a 5 b 7 c 11 d with 0 a 3, 0 b 4, 0 c 0, 0 d 1. The positive divisors of b are the numbers 2 a 5 b 7 c 11 d with 0 a 2, 0 b 0, 0 c 8, 0 d 4. Hence the positive common divisors of a and b are the numbers 2 a 5 b 7 c 11 d with 0 a min{3, 2}, 0 b min{4, 0}, 0 c min{0, 8}, 0 d min{1, 4}. The largest number in this collection is , so it must be the gcd. Theorem If a = p e 1 1 p e 2 2 p e and b = p f 1 1 p f 2 2 p f, where e i 0 and f i 0 for i = 1, 2,...,, then gcd(a, b) = p min{e 1,f 1 } 1 p min{e 2,f 2 } 2 p min{e,f }. Proof. The positive divisors of a are the numbers p d 1 1 p d 2 2 p d where 0 d i e i for i = 1, 2,...,. The positive divisors of b are the numbers p d 1 1 p d 2 2 p d where 0 d i f i for i = 1, 2,...,. Hence the positive common divisors of a and b are the numbers p d 1 1 p d 2 2 p d where 0 d i min{e i, f i } for i = 1, 2,...,. The largest number in this collection is the

14 14 CHAPTER 6. NUMBER THEORY greatest common divisor, hence gcd(a, b) = p min{e 1,f 1 } 1 p min{e 2,f 2 } 2 p min{e,f }. If a and b are integers, neither of which is 0, then the least common multiple of a and b is the smallest positive integer l such that a l and b l. It is denoted by lcm(a, b). That is, lcm(a, b) is the smallest l 1 number which is a common multiple of a and b. Why can t a or b be zero? Because there are no positive multiples of zero. Why do we require that the least common multiple be positive instead of just non-negative? (We could then allow a or b to be zero) Because zero is a common multiple of any two numbers, so the least common multiple would always be zero. Since a b is a positive common multiple of a and b, there is a smallest positive integer among the collection of common multiples of a and b. Since lcm(a, b) = lcm( a, b ), it is customary to tae a and b to be positive when computing the lcm. The least common multiple of a and b can be computed using the modified prime factorizations of a and b. The method is similar to finding the greatest common divisor. We first illustrate it with an example. Suppose a = and b = By the Unique factorization Theorem, the prime factorization of any positive multiple of a contains 2 a 5 b 7 c 11 d with a 3, b 4, c 0, d 1. Similarly, the prime factorization of any positive multiple of b contains 2 a 5 b 7 c 11 d with a 2, b 0, c 8, d 4. Hence, the prime factorization of any positive common multiple of a and b contains 2 a 5 b 7 c 11 d with 0 a max{3, 2}, 0 b max{4, 0}, 0 c max{0, 8}, 0 d max{1, 4}. The smallest number in this collection is , so it must be the lcm. The following theorem is proved similarly to the corresponding result for the gcd. Theorem If a = p e 1 1 p e 2 2 p e and b = p f 1 1 p f 2 2 p f, where e i 0 and f i 0 for i = 1, 2,...,, then gcd(a, b) = p max{e 1,f 1 } 1 p max{e 2,f 2 } 2 p max{e,f }. Together, Theorems and give the following.

15 6.7. THE EUCLIDEAN ALGORITHM 15 Corollary Let a and b be positive integers. Then gcd(a, b)lcm(a, b) = ab. 6.7 The Euclidean Algorithm In the previous section we saw that the gcd and lcm of two numbers can be computed using their prime factorizations. While the prime factorization is relatively easy to find for fairly small numbers, large numbers are notoriously difficult to factor, that is, a lot of computation is required. It is precisely this difficulty that lies at the heart of many cryptosystems. By 300BC, the Grees had an efficient algorithm for computing the gcd: the Euclidean Algorithm. Efficient? While finding the prime factorization of a and b requires a number of arithmetic operations that is proportional to the larger of these numbers, Lamé proved in 1844 that the Euclidean Algorithm requires a number of arithmetic operations proportional to the logarithm of the smaller of the two numbers. To put that into perspective, imagine a and b each have 200 digits, that is, are approximately Using the method based on the prime factorization requires roughly operations. If it were possible to do 1, 000, 000, 000 of these per second, then about seconds would be required for the computation (there are 31, 536, 000 seconds in a year, so the computation would tae about 24 years). Using the Euclidean Algorithm, about 200 operations are required. The main idea behind the Euclidean Algorithm is to successively replace the two number a and b by two smaller numbers in such a way that both pairs of numbers have the same gcd. The following proposition is the ey to the method. Proposition If a, b Z, not both zero, and a = bq+r, then gcd(a, b) = gcd(b, r). Proof. We will show that gcd(a, b) gcd(b, r) and gcd(a, b) gcd(b, r). Since gcd(a, b) divides a and b, it divides a bq = r. Hence gcd(a, b) is a common divisor of b and r, so gcd(b, r) gcd(a, b). On the other hand, gcd(b, r) divides both b and r, so it divides bq +r = a. Hence gcd(b, r) is a common divisor of a and b, gcd(a, b) gcd(b, r).

16 16 CHAPTER 6. NUMBER THEORY Therefore, gcd(a, b) = gcd(b, r). Before describing the Euclidean Algorithm, we illustrate it with an example. We will compute gcd(834, 384). The idea is to repeatedly apply Proposition until arriving at a situation where one of the numbers involved is zero. 834 = gcd(834, 384) = gcd(384, 66) 384 = = gcd(66, 54) 66 = = gcd(54, 12) 54 = = gcd(12, 6) = 6 12 = = gcd(6, 0) = 6 gcd(384, 66) = 6 Suppose a and b are non-negative integers with a b. The Euclidean Algorithm is the following process. 1. Let a 0 = a and b 0 = b. Set i = Use the division algorithm to write a i = bq i + r i, 0 r i < b i. 3. If r i 0 then Let a i+1 = b i and b i+1 = r i. Replace i by i + 1 and go bac to step (2). Otherwise gcd(a, b) = b i (the last non-zero remainder). Why does this wor? By Proposition 6.7.1, gcd(a 0, b 0 ) = gcd(a 1, b 1 ) = = gcd(a i, b i ) = gcd(b i, 0) = b i. Hence, if the process terminates, then the gcd is found. The process terminates because, for any i we have b = b 0 > r 0 = b 1 > r 1 = b 2 > r 2 = b 3 > > r i 0. That is, the sequence of remainders is a decreasing sequence of non-negative integers. Any such sequence must eventually reach zero. If x and y are integers, what integers arise as values of the sum 6x + 4y? Certainly only even numbers can arise because 2 4x + 6x for all x, y Z. Can we get them all? 0 by taing x = y = 0,

17 6.7. THE EUCLIDEAN ALGORITHM 17 2 by taing x = 1 and y = 1; 2 by taing x = 1 and y = 1, 4 by taing x = 0 and y = 1; 4 by taing x = 0 and y = 1, 6 by taing x = 1 and y = 0; 6 by taing x = 1 and y = 0, and so on. It seems reasonable to believe that all multiples of 2 arise. That this is true actually follows from the second bullet point. Since 2 = ( 1), multiplying through by an integer gives 2 = ( ). Thus all even integers can arise (remember that can be negative). We have just done an example of the next theorem, which says that for any integers a and b, not both zero, the numbers that can arise as an integer linear combination ax + by are precisely the multiples of gcd(a, b). We now generalize the previous paragraph, replacing 4 and 6 by any integers a and b. The question is which integers arise as values of the sum 6x+4y?. It turn out that for any integers a and b, not both zero, the numbers that can arise as an integer linear combination ax + by are precisely the multiples of gcd(a, b). Since gcd(a, b) a and gcd(a, b) b, we have gcd(a, b) ax+ by for any x and y. Hence only multiples of gcd(a, b) can arise. In order to show that every multiple of the gcd can arise, it is enough to show that there are integers x 0 and y 0 such that ax 0 + by 0 = gcd(a, b); then, the multiple gcd(a, b) = a(x 0 ) + b(y 0 ). To find x 0 and y 0, use the computation arising from the Euclidean Algorithm from bottom to top: the second to last line tells you you to write the gcd, r i 1, as a difference a i 1 q i 1 b i 1. That is, as a difference involving the smallest pair of numbers in the gcd computation. Each line above gives a substitution that allows the gcd to be expresses in terms of the next largest pair, until finally it is expressed in terms of a and b. We illustrate by writing 6 = gcd(834, 384) as an integer linear combination of 834 and = because = 6 = 54 ( ) 4 because = 12 = = ( ) because = 54 = = ( ) 29 because = 66 = = 834 ( 29) (63) x 0 = 6 and y 0 = 13

18 18 CHAPTER 6. NUMBER THEORY 6.8 Relatively prime integers Integers a and b are called relatively prime if gcd(a, b) = 1. The name can be thought of originating from prime relative to each other if the gcd equals 1 then a and b have no common prime factors. When there are integers x and y such that ax + by = d, then (from the discussion at the end of the last section) we now that d is a multiple of gcd(a, b), so that gcd(a, b) is one of the positive divisors of d. The case d = 1 is exceptional because there is only one possible positive divisor, namely 1, so it must be the gcd. Proposition Let a, b Z, not both zero. If there are integers x and y so that ax + by = 1, then gcd(a, b) = 1. Proof. Since gcd(a, b) a and gcd(a, b) b, we have gcd(a, b) ax + by = 1. Thus gcd(a, b) 1. Since gcd(a, b) is always at least 1, it follows that gcd(a, b) = 1. It is sometimes said that anything that can be proves about relatively prime integers follows from the above proposition. Here are three examples: For any integer a, gcd(a, a + 1) = 1. Tae x = 1 and y = 1 in the proposition. If gcd(a, b) = d, then gcd(a/d, b/d) = 1. There are integers x and y so that ax + by = d. Divide both sides by d. If a bc and gcd(a, b) = 1, then a c. There are integers x and y so that ax + by = 1. Multiply both sides by c to get acx + bcy = c. Since a ac and a bc, the integer a acx + bcy = c. Notice that the statement in the last bullet point is not true without the hypothesis that gcd(a, b) = 1. That is, it is not true that if a bc then a b or a c. For example but 6 4 and By the Fundamental Theorem of Arithmetic, if p is a prime number, then the possibilities for gcd(p, a) are 1 and p: either p appears in the prime factorization of a or it doesn t. Both the Fundamental Theorem of Arithmetic, and the third bullet point above, can be used to show the following (compare the comment in the previous paragraph it is important that p is prime).

19 6.9. ARITHMETIC (MOD M) 19 Proposition If p is prime and p ab, then p a or p b. Proof. Suppose p ab. If p a there is nothing to prove. Otherwise gcd(p, a) = 1, and hence p b. 6.9 Arithmetic (mod m) Most of us can tell time on a 24 hour cloc. The time 15:00 is 3PM, 22:00 is 10PM and so on. Forgetting about AM and PM for a moment, the time on a 12 hour cloc is obtained by subtracting 12 from 24 hour times that are greater than 12. That is, it is the remainder on division by 12. We can extend this idea. When it is 38:00 the 12 hour cloc time should be 2 (the remainder when 38 is divided by 12), and when it is 45:00 the time should be 3. The following arises from deeming two integers to be the same with respect to division by m (we will use the term modulo m ) when they have the same remainder on division by m. Notice that if a = mq 1 + r and b = mq 2 + r (the same r), then a b = (mq 1 + r) (mq 2 + r) = m(q 1 q 2 ). The converse is true too, that is, if m (a b) then a and b leave the same remainder on division by m. We will prove it later. If a, b Z, and m N, we say that a is congruent to b modulo m, and write a b (mod m) if a and b leave the same remainder on division by m. For example, (mod 7) (and ), (mod 9) (and ) (mod 10) (and ). Any multiple of m is congruent to zero (mod m). Proposition Let m N. For any integers a and b, a b (mod m) m a b

20 20 CHAPTER 6. NUMBER THEORY Proof. ( ) Suppose a b (mod m.) Then there exist integers 1, 2 and r such that a = 1 m + r and b = 2 m + r. Therefore a b = m( 1 2 ). Since 1 2 is an integer, m a b. ( ) Suppose m a b. Then a b = m for some integer, or equivalently a = b + m. By the Division Algorithm, we can write b = qm + r, 0 r m 1. Then a = b + m = qm + r + m = (q + )m + r. Since the quotient and remainder guaranteed by the division algorithm are unique, a and b leave the same remainder on division my m. That is, a b (mod m.) Although the definition of congruence is in terms of remainders, it is sometimes easier to use Proposition when proving statements about congruences. In many programming languages there is a function mod. If m N, then a (mod m) is the unique number among 0, 1, 2,..., m 1 to which a is congruent modulo m. That is, it is the remainder (as in the division algorithm - that s why its unique) when a is divided by m. You can thin of the integers (mod m) as the hours on a circular cloc with m hours, where noon is 0. The number of times you go around the circle and return to your starting point maes no difference to where you end up. What matters is the number of places you move when it is no longer possible to mae it around the circle any more, and this number is one of 0, 1, 2,..., m 1. These are the possible remainders on division by m. The universe of integers (mod m) really only consists of the numbers 0, 1, 2,..., m 1; modulo m, any other integer is just one of these with another name (in the same way that 3/6 is another name for 1/2). The expression a b (mod m) says that a and b are two names for the same position on the number circle in the paragraph above. A point that is worth special attention is any multiple of m leaves remainder 0 when divided by m, so m n if and only if n 0 (mod m). We will show that congruence (mod m) has a lot in common with our usual notion of equality of integers. In particular (leaving out the (mod m) in an attempt to mae the similarity more obvious): every number is congruent to itself; that is, congruence reflexive. (mod m) is if a is congruent to b then b is congruent to a; that is congruence

21 6.9. ARITHMETIC (MOD M) 21 (mod m) is symmetric. if a is congruent to b and b is congruent to c, then a is congruent to c (numbers congruent to the same thing are congruent to each other); that is that is congruence (mod m) is transitive. Proposition Let m N. Then, (1) For all integers a we have a a (mod m.) (2) For all integers a and b we have a b (mod m) b a (mod m.) (3) For all integers a, b, and c, if a b (mod m) and b c (mod m,) then a c (mod m.) Proof. We prove (3). The proofs of (1) and (2) are similar. Suppose a b (mod m) and b c (mod m.) Then a and b leave the same remainder on division by m, and so do b and c. Thus, a and c leave the same remainder on division by m. That is, a c (mod m.) This completes the proof of (3). Let n = (d d 1 d 0 ) 10. We saw before that d 0 is the remainder on division by 10. The same sort of argument shows that (d 1 d 0 ) 10 is the remainder on division by 100, that (d 2 d 1 d 0 ) 10 is the remainder on division by 1000, and so on. That is, n d 0 (mod 10), n (d 1 d 0 ) 10 (mod 100) and so on. In general the last t digits of the base 10 representation of n are the remainder when n is divided by 10 t, so for t 1, every integer n = (d d 1 d 0 ) 10 (d t d t 1 d 0 ) 10 (mod 10 t ). The following theorem is important because it tells you how to calculate (mod m.) It says that at any point in any calculation you can replace any number by one it is congruent to and not change the answer. Theorem Let m N. For any integers a, b, c and d, if a b (mod m) and c d (mod m), then, (1) a + c b + d (mod m), (2) a c b d (mod m),

22 22 CHAPTER 6. NUMBER THEORY (3) ac bd (mod m). Proof. We will use Proposition Suppose a b (mod m) and c d (mod m). Then m (a b) and m (c d), so that a = b+m and c = d+lm for some integers and l. Thus, a + c = b + m + d + lm = b + d + ( + l)m, so m [(a + c) (b + d)] and we have a+c b+d (mod m). Similarly, a c b d (mod m). Finally, ac = (b + m)(d + lm) = bd + blm + dm + lm 2 = bd + m(bl + d + lm). Therefore, m ac bd, so that ac bd (mod m). We illustrate the use of Theorem with two examples. Find the integer n such that 7 n < 14 and n (mod 7). By Theorem 6.9.3, (mod 7). The integer n such that 7 n < 14 and n 5 (mod 7) is 12. Find the last digit of We need to evaluate (mod 10). Since 7 2 = 49 1 (mod 10) we have (7 2 ) 50 ( 1) 50 1 (mod 10). Thus the last digit is Testing for divisibility by 3 and by 9 We can use properties of congruence to prove the (familiar) rule that an integer is divisible by 3 if and only if the sum of its decimal digits is divisible by 3. The ey is to observe that 10 1 (mod 3) and so by Theorem you can change 10 to 1 wherever it occurs. Remember that 3 n if and only if n 0 (mod 3). Suppose n = (d d 1... d 1 d 0 ) 10. Then n 0 (mod 3) d 10 +d d d (mod 3) d 1 + d d d (mod 3) which is what we wanted. One interesting thing about this criterion is that it can be applied recursively. For example, if and only if 3 ( ) = 45 if and only if 3 (4 + 5) = 9. If the criterion is applied enough times, then it always ends with testing whether some one digit number is divisible by 3. So, in some sense it is not necessary to now the multiplication table for 3 beyond 3 3.

23 6.10. TESTING FOR DIVISIBILITY BY 3 AND BY 9 23 Since 10 1 (mod 9), almost exactly the same argument shows that an integer is divisible by 9 if and only if the sum of its decimal digits is divisible by 9. Only a small change to the argument needed to show that (d d 1... d 1 d 0 ) 10 is divisible by 11 if and only if d d 1 + d 2 ± d 0 is divisible by 11. That is, 11 n if and only if 11 divides the alternating sum of the decimal digits of n. The alternating sum arises because 10 1 (mod 11). More complicated, but similar, tests for divisibility can be similarly devised for any divisor that is relatively prime to 10.

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