# Gaussian integers

You have already seen a few domain extensions of integers of the form , where is a square-free integer (i.e., ). The case of our interest in this post are Gaussian integers , that are obtained by setting .

## Theory:

We will now study prime elements of and we will see whether it has some interesting consequences. The following statement is very useful in this situation: Let be a prime number. If is NOT of the form for some , then is a prime element in . On the other hand, if , then and are prime elements. Moreover, up to multiplication by units, all prime elements of are of one of these types.

A very important consequence of this lemma is the following theorem: A prime number can be written as or mod . All prime elements in (up to multiplication by units) are

• ;
• prime numbers, such that mod ;
• , where mod .

## Properties:

• is a Euclidian domain for , but it is not Euclidian for .
• There is an operation of conjugation in that takes an element and sends it to the element .
• We have a Euclidian norm on : if , then • It is a general fact that is invertible. Therefore, units (invertible elements) of Gaussian integers are and ## Examples:

### Example 1:

Decide whether or are prime elements in .

### Solution:

We will use Lemma from above:

• , hence, is not a prime element , but and are prime.
• , since mod . Therefore, is a prime element.
• , so is not a prime element, but and are.

### Example 2:

Show that and are the only invertible elements in .

### Solution:

We use the general algebraic fact mentioned above: is invertible . So, we are trying to solve an equation , where Since the square of any integer is non-negative, the only possible solutions are & , or & , and we are done.

## Problems:

### Problem 1:

Prove that and are associated in .

### Problem 2:

Compute the norm of the following elements: , and .