18.785 - Number Theory I

# SYLLABUS

## Course Overview

Historically, number theory has often been separated into algebraic and analytic tracks, but we will not make such a sharp distinction. Indeed, one of the central themes of modern number theory is the intimate connection between its algebraic and analytic aspects. These connections lie at the heart of many of recent breakthoughs and current areas of research, including the modularity theorem, the Sato-Tate theorem, the Riemann hypothesis the Birch and Swinnerton-Dyer conjecture, and the Langlands program.

Having said that, number theory is, at its core, the study of numbers. Our starting point is thus the integer ring Z, its field of fractions Q, and the various completions and algebraic extensions of Q. This means we will initially cover many of the standard topics in algebraic number theory, including: Dedekind domains, decomposition of prime ideals, local fields, ramification, the discriminant and different, ideal class groups, and Dirichlet's unit theorm. We will spend roughly the first half of the semester on these topics, and then move on to some closely related analytic topics, including zeta functions and L-functions, the prime number theorem, primes in arithmetic progressions, the analytic class number formula, and the Chebotarev density theorem. We will also present the main theorems of local and global class field theory (one of the crowning achievements of early 20th century number theory), but we will only have time to cover a few parts of the proofs.

The lecture notes and problem sets from an earlier version of the course are available on the OpenCourseWare website. We will largely follow the same path, but there will be some changes (in particular, I plan to include more material on global function fields).

## Prerequisites

Die Mathematik ist die Königin der Wissenschaften und die Zahlentheorie ist die Königin der Mathematik -- Carl Friedrich Gauss

As suggested by this quote, number theory is supported by many subfields of mathematics, and we will not hesitate to call upon them as needed. In most cases these supporting subjects will play a minor role, but you should be aware that at various points in the course we will make reference to standard material from many other branches of mathematics, including topology, complex analysis, representation theory, and algebraic geometry. When this happens, I will include in the lecture notes a quick review of any terminology and theorems we need that fall outside of the official corequisite for this course, which is 18.705 (commutative algebra). Note that 18.705 transitively includes 18.100 (Real Analysis), as well as 18.701 (Algebra I) and 18.702 (Algebra II), as prerequsites. In past years, 18.112 (Complex Analysis) was also a formal prerequisite. This is no longer the case, but if you have never studied complex analysis you will need to be prepared to do some extra reading when we come to zeta functions and L-functions.

For graduate students in course 18 (the target audience of this course), none of the prerequisites should be an issue, but undegraduates and grduate students from other departments may need to spend some time acquainting (or reacquainting) themselves with supporting material as it arises. Undergraduates who wish to take this coures should be sure to also read the Undergraduates section below.

## Text Books

There is no required text; lecture notes will be provided (these will typically be posted the day after each lecture). You can find the lecture notes for last year's course here. We will cover much of the same material, but there will likely be some differences in emphasis and a few new topics.

(1) I encourage you to take notes in class that include definitions and statements of lemmas and theorems, but only a high level summary of the proofs (many of which I will only sketch in class in any case). After class, you should attempt to fill in the proofs on your own. This is a great way to learn and will help you absorb the material much more effectively than a purely passive approach. You can then consult the lecture notes I will provide and/or any of the texts below to fill in gaps and to compare your approach with mine.

(2) Number theory is a vast subject, and it is good to see it from many different perspectives. Below are a number of standard references that I can recommend. All but one of them can accessed online from MIT (see the MIT Libraries web page for information on offisite access). The classic text of Cassels and Frohlich is not "officially" available in online form, but googling "Cassels and Frohlich" will quickly lead you to several scanned versions; alternatively, you can purchase the 2010 reprint by the London Mathematical Society, which corrects most of the errata noted below and is available for under \$50.

Algebraic Number Theory, J.W.S. Cassels and A. Frohlich. (errata).
Multiplicative Number Theory, H. Davenport.
Algebraic Number Theory, J.S. Milne.
Class Field Theory, J.S. Milne.
Algebraic Number Theory, S. Lang.
An Invitation to Arithmetic Geometry, D. Lorenzini. (errata).
Introduction to Modern Theory,Yu. I. Manin and A. A. Panchishkin.
Algebraic Number Theory, J. Neukirch.
Number Theory in Function Fields, M. Rosen.
A Course in Arithmetic, J.-P. Serre.
Local Fields, J.-P. Serre.

As noted above, commutative algebra is a corequisite for this course. For those who want/need to brush up on their commutative algebra, the course notes for the 2013 edition of 18.705 are available online.

A Term of Commutative Algebra, A. Altman and S. Kleiman (errata).

I can also recommend the following texts, according to taste (Atiyah-MacDonald is an examplar of brevity, while Eisenbud is wonderfully discursive; Matsumura, my personal favorite, is somewhere in between, and I also recommend Milne's primer.):

Commutative Algebra, M.F. Atiyah and I.G. MacDonald.
Commutative Algebra with a View Toward Algebraic Geometry, D. Eisenbud.
Commutative Ring Theory, H. Matsumura.
A Primer of Commutative Algebra, J.S. Milne.

## Problem Sets

Weekly problem sets will be posted on line. Solutions are to be prepared in typeset form (typically via latex) and submitted electronically as a pdf file by noon on the Monday due date (the first problem set will be due September 17). Late problem sets will not be accepted -- your lowest score is dropped, so you can afford to skip one problem set without penalty. Collaboration is permitted/encouraged, but you must write up your own solutions and explicity identify your collaborators, as well as any resources you consulted that are not listed above. If there are none, please indicate this by writing Sources consulted: none at the top of your submission.