# B3.4 Algebraic Number Theory - Material for the year 2020-2021

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Rings and Modules and Number Theory. B3.1 Galois Theory is an essential pre-requisite. All second-year algebra and arithmetic. Students who have not taken Part A Number Theory should read about quadratic residues in, for example, the appendix to Stewart and Tall. This will help with the examples.

16 lectures

### Assessment type:

- Written Examination

An introduction to algebraic number theory. The aim is to describe the properties of number fields, but particular emphasis in examples will be placed on quadratic fields, where it is easy to calculate explicitly the properties of some of the objects being considered. In such fields the familiar unique factorisation enjoyed by the integers may fail, and a key objective of the course is to introduce the class group which measures the failure of this property.

Students will learn about the arithmetic of algebraic number fields. They will learn to prove theorems about integral bases, and about unique factorisation into ideals. They will learn to calculate class numbers, and to use the theory to solve simple Diophantine equations.

Field extensions, minimum polynomial, algebraic numbers, conjugates, discriminants, Gaussian integers, algebraic integers, integral basis

Examples: quadratic fields

Norm of an algebraic number

Existence of factorisation

Factorisation in $\mathbb{Q}(\sqrt{d})$

Ideals, $\mathbb{Z}$-basis, maximal ideals, prime ideals

Unique factorisation theorem of ideals

Relationship between factorisation of number and of ideals

Norm of an ideal

Ideal classes

Statement of Minkowski convex body theorem

Finiteness of class number

Computations of class number to go on example sheets

I. Stewart and D. Tall, *Algebraic Number Theory and Fermat's Last Theorem* (Third Edition, Peters, 2002).

D. Marcus, *Number Fields* (Springer-Verlag, New York-Heidelberg, 1977). ISBN 0-387-90279-1.