# MA233: Elementary Number Theory

Unit 3: Congruence, with Applications   If you weren't already convinced of our claim in Unit 1 that the remainder is more interesting and useful for number theory than quotients, then this unit will overwhelm you with evidence. *Congruence** begins as an innocent-looking relationship based on divisibility, which itself is a property of the remainder. We quickly show that this superficial simplicity masks a tool of incredible power. It determines an equivalence relation, and captures the ring properties of the integers – but not some other properties of the integers! We call the concomitant equivalence classes residues, and show that it suffices to perform any arithmetic on the residues by... computing the remainders!*

With some theory out of the way, we turn immediately to applications. Many interesting problems in the real world can be stated in terms of linear congruence relations, a disguised form of our recently-acquired friend, linear Diophantine equations. This provides a method for solving not only linear congruence relation, but also systems of linear congruence relations. This latter problem is an ancient one, used in practical situations by both Chinese and Indian mathematicians, and enjoys a property called the Chinese Remainder Theorem. We prove this theorem two different ways; unlike the Fundamental Theorem of Arithmetic, each proof gives us a practical method for solving the problems.

Finally, we return to the question of primality, studying congruence in the context of a prime number. In this case, the residues enjoy the properties not only of a ring, but of a field, which leads to a tool for primality testing.

Unit3 Learning Outcomes
Upon successful completion of this unit, you will be able to:
- compute canonical residues modulo an integer; - identify properties of the integers that apply to congruence, and properties that do not; - explain rules of divisibility by the integers 2, 3, 4, 5, 6, 7, 8, 9, 11; - use the Chinese Remainder Theorem to solve a system of linear congruences; - determine when a choice of modulus and residue admits a multiplicative inverse for the residue with respect to that modulus, and compute the inverse; and - test for primality using Fermat’s Little Theorem

3.1 Congruence   3.1.1 Definition, Examples, and Ring Properties of Congruence   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.1: Introduction to Congruences” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.1: Introduction to Congruences” (PDF)

`````` Instructions: Read “Section 4.1: Introduction to Congruences” on
pages 77–81, through “Example 29.” Stop when you get to “Theorem
25.”

Reading this section, taking notes, and studying the examples
should take approximately 30 minutes.
``````
• Assessment: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Congruence Exercises” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Congruence Exercises” (PDF)

Instructions: Try to do Exercises 1-4 and 6-11 on page 83.

After attempting the exercises assigned above, discuss your solutions in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take approximately 1 hour.

3.1.2 Review of Equivalence Relations   - Reading: Elias Zakon’s “Mathematical Analysis: Volume I” Link: Elias Zakon’s “Mathematical Analysis: Volume I” (PDF)

`````` Instructions: Read pages 12-14, starting from the heading
“Definition 4.”

Reading this section, taking notes, and studying the examples
should take approximately 15 minutes.
``````
• Assessment: Elias Zakon’s “Mathematical Analysis: Volume I - Equivalence Relation Exercise” Link: Elias Zakon’s “Mathematical Analysis: Volume I - Equivalence Relation Exercise” (PDF)

Instructions: Try to do Exercise 8 on page 15.

After attempting the exercise assigned above, discuss your solution in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take from 15 minutes to 1 hour, depending on your comfort level with the material.

3.1.3 Congruence as an Equivalence Relation   - Assessment: The Saylor Foundation's “Justification that Congruence Is a Symmetric Relation” Link: The Saylor Foundation's “Justification that Congruence Is a Symmetric Relation” (PDF)

`````` Instructions: (1) Show that congruence is an equivalence relation,
by showing that it satisfies the reflexive, symmetric, and
transitive properties. (2) Explain what Theorem 2 of Elias Zakon’s
“Mathematical Analysis: Volume I” implies about congruence
classes.

Completing this assessment should take approximately 30 minutes.
``````

3.1.4 Familiar Properties Not Satisfied by Congruence   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory” (PDF)

`````` Instructions: Read pages 81-82, starting immediately after “Example
29.”

Reading this section, taking notes, and studying the examples
should take approximately 15 minutes.
``````

3.1.5 Residue Systems   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.2.1: Residue Systems” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.2.1: Residue Systems” (PDF)

`````` Instructions: Read “Section 4.2.1 Residue Systems on pages 84-85.

Reading this section, taking notes, and studying the examples
should take approximately 15 minutes.
``````
• Assessment: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Residue Systems Exercises” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Residue Systems Exercises” (PDF)

Instructions: Try to do Exercises 1 and 2 on page 86.

After attempting the exercises assigned above, discuss your solutions in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take approximately 15 minutes.

3.1.6 Sage Lab: “Residue Arithmetic”   - Web Media: The Saylor Foundation’s Sage Lab: “Residue Arithmetic” Link: The Saylor Foundation’s Sage Lab: “Residue Arithmetic” (Sage)

`````` Instructions: Download the linked set of labs. Upload the third one
(3.1.7-SageWS3.sws) to the Sage website where you created an account
(subunit 1.4.2). Work through the lab carefully.

Completing this assignment should take approximately 30 minutes.
``````

3.1.7 The Secret Life of Linear Congruence Relations (Linear Diophantine Equations)   A linear congruence relation has the form ax ≡ b (mod m), where aand bare known constants, while xis the unknown variable. By the definitions of congruence and divisibility, we must be able to find an integer qsuch that ax – b = mq. Let y = -q, and we can rewrite this equation as

ax + my = b

called a Diophantine equation. (The variables xand yare to the first degree, so we call it a linear Diophantine equation.) Diophantine equations have a long, storied history in number theory, and some of number theories most important problems have been Diophantine equations. We would like to solve these sorts of equations, in part because, for now, we should like to solve linear congruence relations, but later there will be other applications, as well. As you will see, you already knowhow to solve these equations!

• Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 2.6: Linear Diophantine Equations” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 2.6: Linear Diophantine Equations” (PDF)

Instructions: Reread “Section 2.6 Linear Diophantine Equations on pages 49–51. If you are still familiar with this material, you can skip to the next reading.

Rereading this section, taking notes, and studying the examples should take approximately 15 minutes.

• Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Linear Congruences” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Linear Congruences” (PDF)

Instructions: Read “Section 4.3 Linear Congruences” on pages 81-82.

Reading this section, taking notes, and studying the examples should take approximately 15 minutes.

• Assessment: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Linear Congruence Exercises” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Linear Congruence Exercises” (PDF)

Instructions: Try to do Exercises 1-4 on pages 88-89.

After attempting the exercises assigned above, discuss your solutions in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take approximately 1 hour.

3.2 The Other Sun Tzu’s “Art of War”   3.2.1 The Chinese Remainder Theorem   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Introduction to Section 4.4: The Chinese Remainder Theorem and Section 4.4.1: Direct Solution” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Introduction to Section 4.4: The Chinese Remainder Theorem and Section 4.4.1: Direct Solution” (PDF)

`````` Instructions: Read the introduction to “Section 4.4: The Chinese
Remainder Theorem” and all of “Section 4.4.1: Direct Solution” on
pages 89-91.

Reading these sections, taking notes, and studying the examples
should take approximately 30 minutes.
``````
• Assessment: Wissam Raji’s “An Introductory Coursein Elementary Number Theory - Chinese Remainder Theorem Direct Solution Exercises” Link: Wissam Raji’s “An Introductory Coursein Elementary Number Theory - Chinese Remainder Theorem Direct Solution Exercises” (PDF)

Instructions: Try to do Exercises 1-3 on page 91.

After attempting the exercises assigned above, discuss your solutions in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take approximately 30 minutes.

3.2.2 Incremental Solution   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.4.2: Incremental Solution” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.4.2: Incremental Solution” (PDF)

`````` Instructions: Read “Section 4.4.2: Incremental Solution” on pages
91-92.

Reading this section, taking notes, and studying the examples
should take approximately 15 minutes.
``````
• Assessment: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Chinese Remainder Theorem Incremental Solution Exercises” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Chinese Remainder Theorem Incremental Solution Exercises” (PDF)

Instructions: Try to do Exercises 1-3 on page 92.

After attempting the exercises assigned above, discuss your solutions in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take approximately 30 minutes.

3.2.3 Sage Lab: “Chinese Remainders and Polynomial Factorization”   - Web Media: The Saylor Foundation’s Sage Lab: “Chinese Remainders and Polynomial Factorization” Link: The Saylor Foundation’s Sage Lab: “Chinese Remainders and Polynomial Factorization” (Sage)

`````` Instructions: Download the linked lab. Upload the fourth one
(3.2.3-SageWS4.sws) it to the Sage website where you created an
account (subunit 1.4.2). Work through the lab carefully.

Completing this assignment should take approximately 30 minutes.
``````

3.3 Primality Testing and Fermat’s Little Theorem   3.3.1 Field Properties of Congruence Modulo *p*   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Introduction to Section 4.5: Field Properties of Residues Modulo a Prime, and a Primality Test and Section 4.5.1: When Is a System of Residues a Field?” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Introduction to Section 4.5: Field Properties of Residues Modulo a Prime, and a Primality Test and Section 4.5.1: When Is a System of Residues a Field?” (PDF)

`````` Instructions: Read the introduction to “Section 4.5: Field
Properties of Residues Modulo a Prime, and a Primality Test” and all
of “Section 4.5.1: When Is a System of Residues a Field? on pages
93-96.

Reading these sections, taking notes, and studying the examples
should take approximately 30 minutes.
``````

3.3.2 Fermat’s Little Theorem as a Test for Primality   - Reading: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.5.2: Fermat’s Little Theorem as a Primality Test” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Section 4.5.2: Fermat’s Little Theorem as a Primality Test” (PDF)

`````` Instructions: Read “Section 4.5.2: Fermat’s Little Theorem as a
Primality Test” on page 96.

Reading this section, taking notes, and studying the examples
should take less than 15 minutes.
``````
• Assessment: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Fermat’s Little Theorem Exercise” Link: Wissam Raji’s “An Introductory Course in Elementary Number Theory - Fermat’s Little Theorem Exercise” (PDF)

Instructions: Try to do Exercise 1 on page 96.

After attempting the exercise assigned above, discuss your solution in the course discussion forum. Feel free to respond to other students’ postings as well. If you haven’t already done so, you will need to create a free account at the link above to participate in the discussions.

Completing this assessment should take approximately 15 minutes.