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Pure Math

Ring Theory

Introduction

Ring theory is a branch of mathematics that is closely interlinked with group theory. To create a ring, we take a group with its “addition” operation, and add “multiplication” to the mix! We say that this group is a ring if we can apply the distribution law like standard addition and multiplication. If the nonzero elements of the ring form a group under multiplication, then we call the ring a field. It is important to include this “nonzero” part, because 0, or the identity of the group, does not have an inverse under multiplication. Instead, the identity of the multiplicative group is another element, commonly referred to as 1.

Examples

The integers, rationals, and reals are all examples of rings, using standard addition and multiplication. Another example is the set of n by n matrices under matrix addition and multiplication. Note that the rationals and reals are also fields, as we can take the multiplicative inverse of every nonzero element.

Extensions

Given some field F, we can actually construct another field by considering the polynomials in F, denoted as F(X). An element of this field has the form

    \[F(X) = f_0+f_1X+f_2X^2+\dots+f_nX^n\]

where f_0,f_1,f_2,\dots,f_n are elements of F.

Exercise: Check that F(X) is indeed a field.

If F(X) is a field, then we can substitute X for any number, and get a field as well. These fields are called field extensions of F.

Applications

Fields and their extensions are widely used in many fields relating to algebra and number theory, including abstract algebra and Galois theory. They are one of the most fundamental mathematical objects, and are a way to describe the overall structure of sets that F_p (integers modulo p), the rationals, and others share.

Prerequisites/What you need to know

Rings do not take a lot of background in mathematics to understand. However, ring theory can be thought of as an expansion to group theory that adds on an additional binary operation to the set.

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Comprehensive look at ring theory

Concise overview of theorems and definitions