 # MATH3962/4062 Rings, Fields and Galois Theory (Advanced)

## General Information

• Taught in Semester 1.
• Credit point value: 6.
• Classes per week: Three lectures and one tutorial.
• Lecturer(s): James Parkinson .

Please refer to the Senior Mathematics and Statistics Handbook for all questions relating to Senior Mathematics and Statistics. In particular, see the handbook entry for MATH3962 for further information relating to MATH3962.

You may also view the description of MATH3962 and the description of MATH4062 in the University's course search database.

Students have the right to appeal any academic decision made by the School or Faculty: see sydney.edu.au/students/academic-appeals.html.

## Class and consultation times

• Lectures will be held on Mondays, Wednesdays and Thursdays at 14:00.
• My consultation time is Wednesday 1-2pm in Carslaw 614.

## Unit outline

This unit of study investigates the modern mathematical theory that was originally developed for the purpose of studying polynomial equations. In a nutshell, the philosophy is that it should be possible to completely factorise any polynomial into a product of linear factors by working over a "large enough" field (such as the field of all complex numbers). Viewed like this, the problem of solving polynomial equations leads naturally to the problem of understanding extensions of fields. This in turn leads into the area of mathematics known as Galois theory.

The basic theoretical tools needed for this program include the fundamental concepts of groups, rings, and fields. The course begins with the definitions and examples of these concepts, as well as the associated structures such as subgroups, subrings, homomorphisms, ideals and quotients. These tools are then applied to study quotient rings of polynomial rings. The final part of the course deals with the basics of Galois theory, which links the process of solving polynomials with the process of taking field extensions, and then links this process to properties of the Galois group of the polynomial. Of course there is a lot to learn before understanding how this all works, but the point is that the problem of solving polynomial equations is converted into a much more accessible problem in finite group theory.

Along the way we will see some beautiful gems of mathematics, including Fermat's Theorem on primes expressible as a sum of two squares, solutions to the ancient greek problems of trisecting the angle, squaring the circle, and doubling the cube, and the crown of the course: Galois' proof that there is no analogue of the "quadratic formula" for the general quintic equation.

Here is a week-by-week plan of the topics that we will cover. However things might change, and the lectures are the definitive guide for the content of this course.

Week 1
Introduction and overview, definitions of groups and rings, examples
Week 2
Subgroups, cosets, Lagrange's Theorem, normal subgroups, quotient groups, the symmetric group
Week 3
Subrings, polynomial rings, homomorphisms, ideals, and the First Isomorphism Theorem for groups and rings
Week 4
The Correspondence Theorem, integral domains, field of fractions of an integral domain
Week 5
Principal ideal domains, Euclidean domains, greatest common divisors, prime and irreducible elements
Week 6
The Unique Factorisation Theorem, unique factorisation domains, case study: Gaussian integers.
Week 7
Unique factorisation in polynomial rings, irreducibility in polynomial rings
Week 8
Irreducibility in polynomial rings continued, ring and field extensions
Week 9
Minimal polynomials, degree of a field extension, constructible numbers
Week 10
Solution to constructibility problems, constructible polygons, splitting fields, separability
Week 11
Finite fields, Galois groups, statement of the Galois correspondence, the order of the Galois group
Week 12
Proof of the Galois correspondence, solving polynomial equations using radicals, insolubility of the general quintic
Week 13
Revision and tying off loose ends.
This is subject to change, depending on our progress and inspiration.

## Learning outcomes

The learning outcomes for this unit of study are as follows.

• be familiar with the basics of abstract ring and field theory;
• be familiar with the concepts of integral domains, principal ideal domains, Euclidean domains, and unique factorisation domains, and understand the relationships between these concepts;
• understand the concept of irreducibility in integral domains;
• be proficient at applying various irreducibility tests;
• be proficient at applying the Euclidean Algorithm in various contexts;
• have a solid working knowledge of the basic examples of rings and fields including the integers, Gaussian integers, polynomial rings, the rational numbers, and finite fields;
• be able to work with field extensions, including computing the degree of an extension and the minimal polynomial of a simple extension;
• understand the solutions to the three ancient greek geometric problems;
• know and be able to apply the basic concepts and definitions from Galois Theory;
• know the basic properties of the Galois group of a field extension;
• be able to compute Galois groups in simple examples;
• be able to construct proofs, including sophisticated proofs using a variety of concepts covered in the unit;
• be proficient in dealing in abstract concepts with an emphasis on the clear explanation of such concepts to others;
• be able to apply the theory and methods introduced in the unit to specific examples, both those encountered in lectures and tutorials, and to related examples.

## Assessment

• Assignment 1 is due Thursday 9th April. The questions are available here. Please read the instructions on the assignment carefully. In particular, your assignment must be typed using LaTeX. Here is a template tex file that you may use.
• Assignment 2 is due Thursday 21st May. The questions will be posted here later.

Your mark for MATH3962 will be calculated as follows (note that adjustments have been made as a result of moving online in response to covid19).

• Two assignments, worth 20% each. The assignments will give practice in investigating examples and constructing proofs, and feedback should help with your mathematical writing skills and exam preparation. The assignments are due (via LMS) by midnight on the following dates:
• Assignment 1 due on Thursday 9th April (Week 7)
• Assignment 2 due on Thursday 21st May (Week 12)
• Final exam, worth 60%, during examination period. Further details will be posted later.
• Tutorial participation is no longer an assessable component of thus unit. However we will move to weekly online zoom tutorials (Monday, 1pm). Mathematics is not a spectator sport. We will be working through the questions together. The tutorials are an integral part to the course, since the lectures are pretty dense and theory based. So it is very beneficial to attend our weekly meetings.

The tutorial sheets will be posted below. We won't get through all the questions in the tutorial. It is expected that you spend at least 3 or 4 hours of your own time each week finishing off as many of the questions as you can. This is key to success in this challenging course.