# Complex Analysis/Complex numbers

## The field of the complex numbersEdit

Historically, it was observed that the equation has no solution for a real (since for ). Since mathematicians wanted to solve this equation, they just defined a number , called the imaginary unit, such that . Of course, there exists no such number. But if we write a two-tuple with as and calculate with these two-tuples using the calculation rule , that is,

- and

(where we already wrote a two-tuple as , which we will continue to do throughout this book), then the set of all two-tuples with this addition and multiplication forms a field. Indeed, the required axioms for a commutative ring are easy to check, and an inverse of , and not both zero, is given by

- ,

as can be checked by a direct computation.

**Definition 1.1**:

A **complex number** is a two-tuple of real numbers , written , where is called the **real part** of the number and is called the **imaginary part**. The **field of the complex numbers**, denoted by , is the set of all such numbers, together with addition

and multiplication

- .

## Absolute value, conjugationEdit

To each complex number, we can assign an absolute value as follows: A complex number ( ) is actually a two-tuple , which is as such an element of . Now in , we have the Euclidean absolute value, namely

- ,

and thus we just define:

**Definition 1.2**:

The absolute value of a complex number is defined as

- .

Note that the absolute value of the absolute value of a complex number is always a real number, since the root function maps everything in to (in fact to ).

To each complex number ( ), we also assign a different quantity, which is obtained by reflecting along the first axis:

**Definition 1.3**:

Let a complex number be given. Then the **conjugate** of , denoted , is defined as

- .

That is, the second component changed sign; if, in precise terms, , then .

We observe:

**Theorem 1.4**:

Let two complex number be given. Then

- .

**Proof**:

and

- .

With this notation, we can write the absolute value of a complex only in terms of without referring to or :

**Theorem 1.5**:

Let a complex number be given. Then

- .

Here juxtaposition denotes multiplication, as usual (albeit complex multiplication in this case).

**Proof**:

- .

From this follows that the absolute value has the following crucial property:

**Corollary 1.6**:

Let be complex numbers. Then

- .

**Proof**:

by theorems 1.4 and 1.5. Note that the argument of the square roots was always real.

## The complex planeEdit

Since each complex number is in fact a two-tuple , , the set of all complex numbers can be visualized as the plane, where is the first coordinate and the second coordinate. The situation is indicated in the following picture:

The horizontal axis (or -axis) indicates the real part and the vertical (or -) axis indicates the imaginary part.

## ExercisesEdit

- Compute the absolute value of the following complex numbers: , , .
- Assume that and are natural numbers which can be written as the sum of two squares of natural numbers: and for some . Prove that the product can also be written as the sum of two squares.
*Hint:*Plug in that (and similarly for ) and use the rules of computation for complex numbers. - Prove the following relation connecting complex multiplication and the standard scalar product of : .
- This exercise introduces central concepts in algebra. Make yourself familiar with the concept of a
*field*in algebra. If is a field, a*subfield*is defined to be a subset of which is closed under the addition, multiplication, subtraction and division inherited from and contains the elements and (ie. the neutral elements of addition and multiplication) of . Prove:- Let be a family of subfields of a field . Prove that the intersection is also a subfield of .
- Make yourself familiar with the concept of a partially ordered set, and prove that the set of subfields of a given field is partially ordered by inclusion (ie. ). Prove that with regard to that order, any family of subfields has a greatest lower bound.
- Prove that a field has a smallest subfield, called the
*prime field*, and identify the prime field of .