DOCSLIB.ORG

Discrete Mathematics Number Theory and Cryptography

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Discrete Mathematics Number Theory and Cryptography Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Discrete Mathematics Number Theory and Cryptography Prof. Steven Evans Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors 4.1: Divisibility and Modular Arithmetic Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Divisibility Definition If a and b are integers with a 6= 0, we say that a divides b if there b is an integer c such that b = ac (equivalently: if a is an integer), and we write a j b. In this case, we say that a is a factor or divisor of b and that b is a multiple of a. Prof. Steven Evans Discrete Mathematics Corollary If a, b, and c are integers with a 6= 0 such that a j b and a j c, then a j (mb + nc) whenever m and n are integers. Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Divisibility Theorem Let a, b, and c be integers with a 6= 0. 1 If a j b and a j c, then a j (b + c). 2 If a j b, then a j bc for all integers c. 3 If a j b and b j c, then a j c. Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Divisibility Theorem Let a, b, and c be integers with a 6= 0. 1 If a j b and a j c, then a j (b + c). 2 If a j b, then a j bc for all integers c. 3 If a j b and b j c, then a j c. Corollary If a, b, and c are integers with a 6= 0 such that a j b and a j c, then a j (mb + nc) whenever m and n are integers. Prof. Steven Evans Discrete Mathematics Notation d is called the divisor. a is called the dividend. q is called the quotient. r is called the remainder. Sometimes q = a div d and r = a mod d are used to denote these relationships. Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors The division algorithm Theorem: The division algorithm Let a be an integer and d a positive integer. Then there are unique integers q and r, with 0 ≤ r < d, such that a = dq + r. Prof. Steven Evans Discrete Mathematics Sometimes q = a div d and r = a mod d are used to denote these relationships. Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors The division algorithm Theorem: The division algorithm Let a be an integer and d a positive integer. Then there are unique integers q and r, with 0 ≤ r < d, such that a = dq + r. Notation d is called the divisor. a is called the dividend. q is called the quotient. r is called the remainder. Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors The division algorithm Theorem: The division algorithm Let a be an integer and d a positive integer. Then there are unique integers q and r, with 0 ≤ r < d, such that a = dq + r. Notation d is called the divisor. a is called the dividend. q is called the quotient. r is called the remainder. Sometimes q = a div d and r = a mod d are used to denote these relationships. Prof. Steven Evans Discrete Mathematics Theorem Let a and b be integers, and let m be a positive integer. Then a ≡ b (mod m) if and only if a mod m = b mod m. Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Definition If a and b are integers and m is a positive integer, then a is congruent to b modulo m if m j a − b. We denote this by a ≡ b (mod m): This sentence is called a congruence and that m is its modulus (pl.: moduli). Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Definition If a and b are integers and m is a positive integer, then a is congruent to b modulo m if m j a − b. We denote this by a ≡ b (mod m): This sentence is called a congruence and that m is its modulus (pl.: moduli). Theorem Let a and b be integers, and let m be a positive integer. Then a ≡ b (mod m) if and only if a mod m = b mod m. Prof. Steven Evans Discrete Mathematics If a ≡ b (mod m), by the definition of congruence we know that m j (a − b). This means there's an integer k with a − b = km, or equivalently that a = b + km. Conversely, if there is an integer k with a = b + km, then km = a − b. Hence, m divides a − b, so that a ≡ b (mod m). Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Theorem Let m be a positive integer. The integers a and b are congruent modulo m if and only if there is an integer k such that a = b + km. Proof: Prof. Steven Evans Discrete Mathematics Conversely, if there is an integer k with a = b + km, then km = a − b. Hence, m divides a − b, so that a ≡ b (mod m). Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Theorem Let m be a positive integer. The integers a and b are congruent modulo m if and only if there is an integer k such that a = b + km. Proof: If a ≡ b (mod m), by the definition of congruence we know that m j (a − b). This means there's an integer k with a − b = km, or equivalently that a = b + km. Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Theorem Let m be a positive integer. The integers a and b are congruent modulo m if and only if there is an integer k such that a = b + km. Proof: If a ≡ b (mod m), by the definition of congruence we know that m j (a − b). This means there's an integer k with a − b = km, or equivalently that a = b + km. Conversely, if there is an integer k with a = b + km, then km = a − b. Hence, m divides a − b, so that a ≡ b (mod m). Prof. Steven Evans Discrete Mathematics Theorem Let m be a positive integer. If a ≡ b (mod m) and c ≡ d (mod m), then a + c ≡ b + d (mod m) and ac ≡ bd (mod m): Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Definition The set of all integers congruent to an integer a modulo m is called the congruence class of a modulo m. There are m pairwise disjoint equivalence classes modulo m, and the union of these equivalence classes is the entire set of integers. Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Definition The set of all integers congruent to an integer a modulo m is called the congruence class of a modulo m. There are m pairwise disjoint equivalence classes modulo m, and the union of these equivalence classes is the entire set of integers. Theorem Let m be a positive integer. If a ≡ b (mod m) and c ≡ d (mod m), then a + c ≡ b + d (mod m) and ac ≡ bd (mod m): Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Modular arithmetic Corollary Let m be a positive integer and let a and b be integers. Then: (a + b) mod m = ((a mod m) + (b mod m)) mod m; and ab mod m = ((a mod m)(b mod m)) mod m: Prof. Steven Evans Discrete Mathematics Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Arithmetic modulo m We can define modular arithmetic operations on Zm, the set of nonnegative integers less than m (i.e., the set f0; 1;:::; m − 1g). The operation of addition modulo m is given by a +m b = (a + b) mod m; where the addition on the right-hand side of this equation is the ordinary addition of integers. The operation of multiplication modulo m is given by a ·m b = (a · b) mod m; where again the multiplication on the right-hand side is ordinary multiplication of integers. The operations +m and ·m are called addition and multiplication modulo m, respectively. Prof. Steven Evans Discrete Mathematics Associativity: If a, b, and c belong to Zm, then (a +m b) +m c = a +m (b +m c). Commutativity: If a and b belong to Zm, then a +m b = b +m a and a ·m b = b ·m a. Identity elements: For a 2 Zm, we have a +m 0 = a and a ·m 1 = a. Additive inverses: If a 6= 0 belongs to Zm, then m − a is an additive inverse of a modulo m and 0 is its own additive inverse. That is: a +m (m − a) = 0 and 0 +m 0 = 0. Distributivity: If a, b, and c belong to Zm, then a ·m (b +m c) = (a ·m b) +m (a ·m c): Divisibility and Modular Arithmetic Integer Representations and Algorithms Primes and Greatest Common Divisors Arithmetic modulo m Closure: Let a; b 2 Zm, then a +m b and a ·m b are also in Zm.
Load more

Recommended publications
  • By Sieving, Primality Testing, Legendre's Formula and Meissel's
    Computation of π(n) by Sieving, Primality Testing, Legendre’s Formula and Meissel’s Formula Jason Eisner, Spring 1993 This was one of several optional small computational projects assigned to undergraduate mathematics students at Cambridge University in 1993. I’m releasing my code and writeup in 2002 in case they are helpful to anyone—someone doing research in this area wrote to me asking for them. My linear-time version of the Sieve of Eratosthenes may be original; I have not seen that algorithm anywhere else. But the rest of this work is straightforward implementation and exposition of well-known methods. A good reference is H. Riesel, Prime Numbers and Computer Methods for Factorization. My Common Lisp implementation is in the file primes.lisp. The standard language reference (now available online for free) is Guy L. Steele, Jr., Common Lisp: The Language, 2nd ed., Digital Press, 1990. Note: In my discussion of running time, I have adopted the usual ideal- ization of a machine that can perform addition and multiplication operations in constant time. Real computers obviously fall short of this ideal; for exam- ple, when n and m are represented in base 2 by arbitrary length bitstrings, it takes time O(log n log m) to compute nm. Introduction: In this project we’ll look at several approaches for find- ing π(n), the numberof primes less than n. Each approach has its advan- tages. • Sieving produces a complete list of primes that can be further analyzed. For instance, after sieving, we may easily identify the 8169 pairs of twin primes below 106.
    [Show full text]
  • Primes and Primality Testing
    Primes and Primality Testing A Technological/Historical Perspective Jennifer Ellis Department of Mathematics and Computer Science What is a prime number? A number p greater than one is prime if and only if the only divisors of p are 1 and p. Examples: 2, 3, 5, and 7 A few larger examples: 71887 524287 65537 2127 1 Primality Testing: Origins Eratosthenes: Developed “sieve” method 276-194 B.C. Nicknamed Beta – “second place” in many different academic disciplines Also made contributions to www-history.mcs.st- geometry, approximation of andrews.ac.uk/PictDisplay/Eratosthenes.html the Earth’s circumference Sieve of Eratosthenes 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 Sieve of Eratosthenes We only need to “sieve” the multiples of numbers less than 10. Why? (10)(10)=100 (p)(q)<=100 Consider pq where p>10. Then for pq <=100, q must be less than 10. By sieving all the multiples of numbers less than 10 (here, multiples of q), we have removed all composite numbers less than 100.
    [Show full text]
  • Number Theory Learning Module 3 — the Greatest Common Divisor 1
    Number Theory Learning Module 3 — The Greatest Common Divisor 1 1 Objectives. • Understand the definition of greatest common divisor (gcd). • Learn the basic the properties of the gcd. • Understand Euclid’s algorithm. • Learn basic proofing techniques for greatest common divisors. 2 The Greatest Common Divisor Classical Greek mathematics concerned itself mostly with geometry. The notion of measurement is fundamental to ge- ometry, and the Greeks were the first to provide a formal foundation for this concept. Surprisingly, however, they never used fractions to express measurements (and never developed an arithmetic of fractions). They expressed geometrical measurements as relations between ratios. In numerical terms, these are statements like: 168 is to 120 as 7 is to 4, (2.1) which we would write today as 168{120 7{5. Statements such as (2.1) were natural to greek mathematicians because they viewed measuring as the process of finding a “common integral measure”. For example, we have: 168 24 ¤ 7 120 24 ¤ 5; so that we can use the integer 24 as a “common unit” to measure the numbers 168 and 120. Going back to our example, notice that 24 is not the only common integral measure for the integers 168 and 120, since we also have, for example, 168 6 ¤ 28 and 120 6 ¤ 20. The number 24, however, is the largest integer that can be used to “measure” both 168 and 120, and gives the representation in lowest terms for their ratio. This motivates the following definition: Definition 2.1. Let a and b be integers that are not both zero.
    [Show full text]
  • Primality Testing for Beginners
    STUDENT MATHEMATICAL LIBRARY Volume 70 Primality Testing for Beginners Lasse Rempe-Gillen Rebecca Waldecker http://dx.doi.org/10.1090/stml/070 Primality Testing for Beginners STUDENT MATHEMATICAL LIBRARY Volume 70 Primality Testing for Beginners Lasse Rempe-Gillen Rebecca Waldecker American Mathematical Society Providence, Rhode Island Editorial Board Satyan L. Devadoss John Stillwell Gerald B. Folland (Chair) Serge Tabachnikov The cover illustration is a variant of the Sieve of Eratosthenes (Sec- tion 1.5), showing the integers from 1 to 2704 colored by the number of their prime factors, including repeats. The illustration was created us- ing MATLAB. The back cover shows a phase plot of the Riemann zeta function (see Appendix A), which appears courtesy of Elias Wegert (www.visual.wegert.com). 2010 Mathematics Subject Classification. Primary 11-01, 11-02, 11Axx, 11Y11, 11Y16. For additional information and updates on this book, visit www.ams.org/bookpages/stml-70 Library of Congress Cataloging-in-Publication Data Rempe-Gillen, Lasse, 1978– author. [Primzahltests f¨ur Einsteiger. English] Primality testing for beginners / Lasse Rempe-Gillen, Rebecca Waldecker. pages cm. — (Student mathematical library ; volume 70) Translation of: Primzahltests f¨ur Einsteiger : Zahlentheorie - Algorithmik - Kryptographie. Includes bibliographical references and index. ISBN 978-0-8218-9883-3 (alk. paper) 1. Number theory. I. Waldecker, Rebecca, 1979– author. II. Title. QA241.R45813 2014 512.72—dc23 2013032423 Copying and reprinting. Individual readers of this publication, and nonprofit libraries acting for them, are permitted to make fair use of the material, such as to copy a chapter for use in teaching or research. Permission is granted to quote brief passages from this publication in reviews, provided the customary acknowledgment of the source is given.
    [Show full text]
  • 1. the Euclidean Algorithm the Common Divisors of Two Numbers Are the Numbers That Are Divisors of Both of Them
    1. The Euclidean Algorithm The common divisors of two numbers are the numbers that are divisors of both of them. For example, the divisors of 12 are 1, 2, 3, 4, 6, 12. The divisors of 18 are 1, 2, 3, 6, 9, 18. Thus, the common divisors of 12 and 18 are 1, 2, 3, 6. The greatest among these is, perhaps unsurprisingly, called the of 12 and 18. The usual mathematical notation for the greatest common divisor of two integers a and b are denoted by (a, b). Hence, (12, 18) = 6. The greatest common divisor is important for many reasons. For example, it can be used to calculate the of two numbers, i.e., the smallest positive integer that is a multiple of these numbers. The least common multiple of the numbers a and b can be calculated as ab . (a, b) For example, the least common multiple of 12 and 18 is 12 · 18 12 · 18 = . (12, 18) 6 Note that, in order to calculate the right-hand side here it would be counter- productive to multiply 12 and 18 together. It is much easier to do the calculation as follows: 12 · 18 12 = = 2 · 18 = 36. 6 6 · 18 That is, the least common multiple of 12 and 18 is 36. It is important to know the least common multiple when adding two fractions. For example, noting that 12 · 3 = 36 and 18 · 2 = 36, we have 5 7 5 · 3 7 · 2 15 14 15 + 14 29 + = + = + = = . 12 18 12 · 3 18 · 2 36 36 36 36 Note that this way of calculating the least common multiple works only for two numbers.
    [Show full text]
  • The Pollard's Rho Method for Factoring Numbers
    Foote 1 Corey Foote Dr. Thomas Kent Honors Number Theory Date: 11-30-11 The Pollard’s Rho Method for Factoring Numbers We are all familiar with the concepts of prime and composite numbers. We also know that a number is either prime or a product of primes. The Fundamental Theorem of Arithmetic states that every integer n ≥ 2 is either a prime or a product of primes, and the product is unique apart from the order in which the factors appear (Long, 55). The number 7, for example, is a prime number. It has only two factors, itself and 1. On the other hand 24 has a prime factorization of 2 3 × 3. Because its factors are not just 24 and 1, 24 is considered a composite number. The numbers 7 and 24 are easier to factor than larger numbers. We will look at the Sieve of Eratosthenes, an efficient factoring method for dealing with smaller numbers, followed by Pollard’s rho, a method that allows us how to factor large numbers into their primes. The Sieve of Eratosthenes allows us to find the prime numbers up to and including a particular number, n. First, we find the prime numbers that are less than or equal to √͢. Then we use these primes to see which of the numbers √͢ ≤ n - k, ..., n - 2, n - 1 ≤ n these primes properly divide. The remaining numbers are the prime numbers that are greater than √͢ and less than or equal to n. This method works because these prime numbers clearly cannot have any prime factor less than or equal to √͢, as the number would then be composite.
    [Show full text]
  • RSA and Primality Testing
    RSA and Primality Testing Joan Boyar, IMADA, University of Southern Denmark Studieretningsprojekter 2010 1 / 81 Outline Outline Symmetric key ■ Symmetric key cryptography Public key Number theory ■ RSA Public key cryptography RSA Modular ■ exponentiation Introduction to number theory RSA RSA ■ RSA Greatest common divisor Primality testing ■ Modular exponentiation Correctness of RSA Digital signatures ■ Greatest common divisor ■ Primality testing ■ Correctness of RSA ■ Digital signatures with RSA 2 / 81 Caesar cipher Outline Symmetric key Public key Number theory A B C D E F G H I J K L M N O RSA 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 RSA Modular D E F G H I J K L M N O P Q R exponentiation RSA 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 RSA Greatest common divisor Primality testing P Q R S T U V W X Y Z Æ Ø Å Correctness of RSA Digital signatures 15 16 17 18 19 20 21 22 23 24 25 26 27 28 S T U V W X Y Z Æ Ø Å A B C 18 19 20 21 22 23 24 25 26 27 28 0 1 2 E(m)= m + 3(mod 29) 3 / 81 Symmetric key systems Outline Suppose the following was encrypted using a Caesar cipher and the Symmetric key Public key Danish alphabet. The key is unknown. What does it say? Number theory RSA RSA Modular exponentiation RSA ZQOØQOØ, RI. RSA Greatest common divisor Primality testing Correctness of RSA Digital signatures 4 / 81 Symmetric key systems Outline Suppose the following was encrypted using a Caesar cipher and the Symmetric key Public key Danish alphabet.
    [Show full text]
  • The I/O Complexity of Computing Prime Tables 1 Introduction
    The I/O Complexity of Computing Prime Tables Michael A. Bender1, Rezaul Chowdhury1, Alex Conway2, Mart´ın Farach-Colton2, Pramod Ganapathi1, Rob Johnson1, Samuel McCauley1, Bertrand Simon3, and Shikha Singh1 1 Stony Brook University, Stony Brook, NY 11794-2424, USA. fbender,rezaul,pganapathi,rob,smccauley,shiksinghg @cs.stonybrook.edu 2 Rutgers University, Piscataway, NJ 08854, USA. ffarach,[email protected] 3 LIP, ENS de Lyon, 46 allee´ d’Italie, Lyon, France. [email protected] Abstract. We revisit classical sieves for computing primes and analyze their performance in the external-memory model. Most prior sieves are analyzed in the RAM model, where the focus is on minimizing both the total number of operations and the size of the working set. The hope is that if the working set fits in RAM, then the sieve will have good I/O performance, though such an outcome is by no means guaranteed by a small working-set size. We analyze our algorithms directly in terms of I/Os and operations. In the external- memory model, permutation can be the most expensive aspect of sieving, in contrast to the RAM model, where permutations are trivial. We show how to implement classical sieves so that they have both good I/O performance and good RAM performance, even when the problem size N becomes huge—even superpolynomially larger than RAM. Towards this goal, we give two I/O-efficient priority queues that are optimized for the operations incurred by these sieves. Keywords: External-Memory Algorithms, Prime Tables, Sorting, Priority Queues 1 Introduction According to Fox News [21], “Prime numbers, which are divisible only by themselves and one, have little mathematical importance.
    [Show full text]
  • THE SIEVE of ERATOSTHENES an Ancient Greek Mathematician by the Name of Eratosthenes of Cyrene (C
    THE SIEVE OF ERATOSTHENES An ancient Greek mathematician by the name of Eratosthenes of Cyrene (c. 200 B.C.) developed an algorithm for finding prime numbers that has come to be known as the Sieve of Eratosthenes. Eratosthenes’s Sieve 1. Create a sieve containing all the integers from 2 through n. 2. Remove the nonprime integers from the sieve by removing the multiples of 2, of 3, of 5, and so on, until reaching n . 3. Only primes will remain in the sieve. Let’s design a Java program that uses this algorithm. We’ll follow the programming methodology How to Invent an Algorithm.1 How to Invent an Algorithm Step What? How? 1 Understand the problem Use pencil and paper to solve the problem by hand. by solving it Let n = 35. Create a sieve containing 2 through 35: 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 Remove the multiples of 2: 2 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 The multiples of 3: 2 3 5 7 11 13 17 19 23 25 29 31 35 The multiples of 5: 2 3 5 7 11 13 17 19 23 29 31 7 > 5.92 = 35 , so we’re done. The numbers left are all prime. 1 Programming 101 ‒ Algorithm Development → How to Invent an Algorithm, p. 1. And Programming 101 ‒ Algorithm Development → Help for Beginners, p.
    [Show full text]
  • Prime Number Sieves Utrecht University
    Prime number sieves Utrecht University Guido Reitsma Supervisor: Lasse Grimmelt January 17th 2020 Acknowledgements First I want to thank Lasse for being a great supervisor, especially given that it was his first bachelor's thesis that he supervised and I'm definitely not the easiest person. Secondly I want to thank all my friends, family and other people, who supported me and listened to my thoughts for the last three months and before. 2 Contents 1 Introduction 4 1.1 Sieve of Eratosthenes Historically . .4 1.2 The way many people know the Sieve of Eratosthenes . .4 1.3 Estimating primes via Eratosthenes' Sieve . .5 1.4 RSA . .8 2 Combinatorial Sieve 10 2.1 Notation . 10 2.2 Sieve weights . 10 2.3 Buchstab's Identity . 11 2.4 Brun's sieve . 12 3 The General Number Field Sieve 14 3.1 Difference of Squares Factorization method . 14 3.2 Free parameters in GNFS . 14 3.3 Z[θ]........................................ 15 3.4 Finding two squares . 15 3.5 Smoothness over a factor base . 16 3.6 Verifying squares in Z and Z[θ]......................... 17 3.7 Combine everything . 18 4 Discussion and Conclusion 20 4.1 How to go further with sieves . 20 4.2 Number of calculations using GNFS . 21 4.3 Is RSA really safe? . 21 4.4 Conclusion . 21 3 Chapter 1 Introduction In this thesis there are two applications from the Sieve of Eratosthenes The two sieves that are mentioned in this thesis are the combinatorial sieve and the General Number field Sieve. We will then compare both of the methods which each other and look how these methods coincide or differ.
    [Show full text]
  • Some Facts from Number Theory
    Computer Science 52 Some Facts from Number Theory Fall Semester, 2014 These notes are adapted from a document that was prepared for a different course several years ago. They may be helpful as a summary of the facts that we use in Computer Science 52 as we study RSA encryption. The exercises in this document are left over from the previous course; although they are a tremendous amount of fun, they are not part of the assigned work for Computer Science 52. 1 Quotients and Remainders If m and n are integers, and n 6= 0, then there is a unique pair of integers q and r satisfying m = qn + r , and 0 ≤ r < jnj. Not surprisingly, q is the quotient and r is the remainder when m is divided by n.1 We say that n divides m, or n is a divisor of m, when the remainder is zero. The notation n j m is used to signify that n divides m. An integer p is a prime number if 1 < jpj and the only divisors of p are ±1 and ±p. We write a ≡ b (mod n) if b − a is divisible by n.2 This notion, called congruence, is an equivalence relation. All the usual laws of arithmetic hold for congruence; for example, we have the distributivity of multiplication over addition and the associa- tivity of multiplication. a(b + c) ≡ ab + ac (mod n) a(bc) ≡ (ab)c (mod n) When computing arithmetic results for a congruence, it does not hurt replace a value by its remainder on division by n.
    [Show full text]
  • Divisibility and Greatest Common Divisors
    DIVISIBILITY AND GREATEST COMMON DIVISORS KEITH CONRAD 1. Introduction We will begin with a review of divisibility among integers, mostly to set some notation and to indicate its properties. Then we will look at two important theorems involving greatest common divisors: Euclid's algorithm and Bezout's identity. The set of integers is denoted Z (from the German word Zahl = number). 2. The Divisibility Relation Definition 2.1. When a and b are integers, we say a divides b if b = ak for some k 2 Z. We then write a j b (read as \a divides b"). Example 2.2. We have 2 j 6 (because 6 = 2 · 3), 4 j (−12), and 5 j 0. We have ±1 j b for every b 2 Z. However, 6 does not divide 2 and 0 does not divide 5. Divisibility is a relation, much like inequalities. In particular, the relation 2 j 6 is not the number 3, even though 6 = 2 · 3. Such an error would be similar to the mistake of confusing the relation 5 < 9 with the number 9 − 5. Notice divisibility is not symmetric: if a j b, it is usually not true that b j a, so you should not confuse the roles of a and b in this relation: 4 j 20 but 20 - 4. Remark 2.3. Learn the definition of a j b as given in Definition 2.1, and not in the b 0 form \ a is an integer." It essentially amounts to the same thing (exception: 0 j 0 but 0 is not defined), however thinking about divisibility in terms of ratios will screw up your understanding of divisibility in other settings in algebra.
    [Show full text]