# Abstraction

#### Category: Algebraic number theory

In this post a proof of the following theorem is going to be sketched, following the treatment in Borevich and Shafarevich’s Number Theory. This sketch is by no means meant to be highly detailed and I am writing it mostly for my own purposes, so I avoid proving some things, even if they aren’t that straightforward.

Thue’s theorem: Suppose $$f(x,y)=a_0x^n+a_1x^{n-1}y+\dots+a_ny^n$$ is a binary form which has degree $$n\geq 3$$, is irreducible (i.e. $$f(x,1)$$ is an irreducible polynomial in $$x$$) and $$f(x,1)$$ has at least one nonreal root in $$\mathbb C$$. Then for any nonzero integer $$c$$ the equation $$f(x,y)=c$$ has only finitely many integral solutions.

Proof: Suppose otherwise…

The goal of this blog post is to provide an overview of the general theory of divisors, as described in a book Number Theory by Borevich and Shafarevich. The point of this post is for it to be somewhat expository, so it will avoid the longer proofs, sometimes just sketching the ideas.

The book “Number Fields” by D. Marcus is a very well-known introductory book on algebraic number theory. Its most memorable aspect is, without a doubt, the great number of exercises it contains. They vary from short(ish) computational exercises, through various technical results used later in the book, to series of exercises aimed to establish (sometimes very deep) results in number theory. They are structured in a way which allows even an unexperienced reader be able to solve most, if not all, exercises even on their first reading, thanks to (often very elaborate) hints provided.

However, even then a reader might want to refer some external source in order to see how the exercise can be solved, because otherwise it might be difficult to proceed any further (I myself would appreciate such a source at times). And, as they say, if you want something done right, do that yourself.

This post is based on Marcus’s Number Fields. More specifically, it is based on a series of exercises following chapter 4.

Recall the definition of the intertia group of a prime $$\frak P$$ in $$\mathcal O_L$$ lying over a prime $$\frak p$$ in $$\mathcal O_K$$ ($$L/K$$ is a Galois extension of number fields) – it’s the set of all $$\sigma\in G=\mathrm{Gal}(L/K)$$ such that, for all $$\alpha\in L$$, we have $$\sigma(\alpha)\equiv\alpha\pmod{\frak P}$$. We now generalize this group.

Definition: In setting as above, we define the $$n$$th ramification group $$E_n$$ to be the set of all $$\sigma\in G$$ such that $$\sigma(\alpha)\equiv\alpha\pmod{\frak P^{n+1}}$$. The groups $$E_n,n>1$$ are called the higher ramification groups.

Discriminant of a number field is arguably its most important numerical invariant. Quite closely connected to it is the different ideal. Here we discuss the most basic properties of these two concepts. The discussion mostly follows this expository paper by K. Conrad, but also takes from a series of exercises in chapter 3 of Number Fields.

The Dirichlet L-functions are an extremely important tool in studying primes in arithmetic progressions – their zeros “control” distribution of primes in arithmetic progressions in the same manner as zeros of Riemann zeta function control the overall distribution of primes. The first and the most elementary result involving these zeros, which is the key result in most proofs of Dirichlet’s theorem, is that there is never a zero at point $$s=1$$. This post will present a proof of this fact using results from algebraic number theory.

This post is based solely on the content of Marcus’s Number Fields. The prerequisities for it are basic results about ideals in number fields and a minute amount of complex analysis. No background in analytic number theory is necessary.

We shall establish relations between degrees, inertia degrees and ramification indices involved in decomposition and inertia field of a given prime in Galois extension. This is based on Marcus’s Number Fields and online notes by R. Ash. It follows a very similar appoach to, but is not based on, the one which can be found in this blog post by Sander Mack-Crane. For this blog post, understanding of Galois theory and basic facts about number fields is necessary.

Based on arguments in Marcus’s Number Fields and K. Conrad’s expository paper on ideal factorization. I assume familiarity with the concepts related to ideals and fractional ideals in a commutative ring.

Recall that an integral domain $$R$$ with the field of fractions $$K$$ is called a Dedekind domain if the following conditions hold:

• $$R$$ is Noetherian, so every nonempty set of ideals has a maximal element, or equivalently, every ideal is finitely generated,
• every prime ideal in $$R$$ is a maximal ideal, and
• $$R$$ is integrally closed in $$K$$, so that every root of a monic polynomial from $$R[x]$$ lying in $$K$$ lies in $$R$$.

Our goal is to prove the following proposition:

Proposition: Assume $$R$$ is a Dedekind domain. For any nonzero (i.e. containing a nonzero element) fractional ideal $$I$$ in $$K$$ there exists a fractional ideal $$J$$ such that $$IJ=R$$.