# Rings and Algebras

Paris, Chennai

The ring is defined for a complex number . is algebraic if it can be expressed as the roots of a polynomial with integer coefficients; otherwise, it is transcendental.

Elements with multiplicative inverses are terms units of a ring. In , the only units are 1 and -1. In , they are nonzero constant polynomials.

The kernel of a homomorphism is defined as:

Closure under multiplication is formalized by a subset , the ideal of , such that:

1. is a subgroup of
2. and implies

Any ring with has characteristic . Otherwise characteristic is where . , , and have characteristic , while has characteristic .

## Homomorphisms

Let be a homomorphism, such that are the set of cosets. Then .

For , . Moreover, there is a bijective correspondence between ideals of , and those of .

Notation: = ring obtained by adjoining to the ring .

Let denote the polynomial ring generated by and . By adjoining the solution of to the ring , we get the isomorphism .

can be viewed as a polynomial in with coefficients in : is an isomorphism. For example:

## Fields and integral domains

Integral domains are fields without zerodivisors, and fields have the additional property that every element has a multiplicative inverse.

No zerodivisors: satisfies the cancellation law that if , then .

Fields are characterized by having exactly two ideals: and . In integral domains, on the other hand, every ideal is a principal ideal (these ideals generate the whole ring): for instance, in , every prime number is a principal ideal.

## Maximal ideals

is termed as a maximal ideal if there are no ideals between and the whole ring : in other words, if is the maximal ideal, then there is no such that . Every maximal ideal has the property that is a field, having exactly two ideals.

## Nullstellansatz

Given three functions in two variables:

We can write , a linear combination with polynomial coefficients.

## Algebraic Geometry

The set of solutions of is a variety, as is a set of points.

Two polynomials of degree and have finitely many points of intersection, and this is known as Bezout bound , as long as they have no common factor.

## Unique Factorization Domain and Principal Ideal Domain

is not a UFD because there exist two different factorizations of the element .

However, a UFD allows for associate factorizations: since the units of are , and are associates: .

Irreducible elements in a UFD are prime, but irreducible and prime are not equivalent in the general case: in 𝟝, is irreducible because it has no proper factors, but it is not prime because, although it divides , it does not divide either factor.

An integral domain in which all ideals are principal (generated by a single element) is termed a principal ideal domain.

## Gauss Primes

The ring of Gauss Integers given by has units : is an example of a Gauss prime, while is not.

1. The prime number is either a Gauss prime, or the product of complex conjugate Gaussian primes.
2. Let be a Guass prime. Then, is either a prime integer or the square of a prime integer.

## Algebraic numbers

A number is algebraic if and only if it is the root of a polynomial with integer coefficients. Moreover, it's an algebraic integer if and only if the polynomial is monic. The cube root of unity is an example of an algebraic number since it is the root of the monic .

Algebraic integers in the quadratic field are of two types:

1. If , then
2. If , then

is an integer of the second type.