Complex numbers

Basic concepts

Effectively, the equation $$ x^{2} + 1 = 0 $$ Has solution, in fact it has two solutions wich are \( x = \pm \sqrt{-1} = \pm i \), the positive one is called as imaginary unit.

In general, numbers on the form $$ z=a+bi $$ Are called complex numbers, a is called real part of z and b is imaginary part of z. They are denoted respectively for \( Re\,z=a, Im\,z=b \).

The complex plane consists of all complex numbers, that is, pairs of real numbers (a, b), formally $$ \mathbb{C} = \{ z = a+bi : a, b \in \mathbb{R} \}$$ This notation, used for the first time by Euler in 1773, brought up a precious and prosperous branch of maths called Complex Variable analysis.

If we change the sign to the imaginary part we have the conjugate complex . This is, the number $$ \bar{z}=a-bi $$ It is called conjugate complex .

Complex plane definition

Complex plane is defined as follows $$ \mathbb{C} = \{ z = a+bi : a, b \in \mathbb{R} \}$$ and if \( z, w \in \mathbb{C} : z = a+bi, w = c + di \). There are addition and multiplication operations respectively are defined as: $$ z+w = a+c +(b+d)i $$ $$ zw = (ac - bc) +(bc+ad)i $$ The complex plane endowed with these two operations has a Field structure.

Note that an attentive student could claim that \( \mathbb{C} = \mathbb{R} ^2 \), which is true from a topological point of view but not from the algebraic one, because in the plane\( \mathbb{C} \) there are addition and multiplication operations which do not exist a at first in the usual plane of \( \mathbb{R} ^2 \).

Extended theory

Every complex number can be written in what we call polar form , like this

\(z = a+bi = re^{i\theta}\)

by polar form of a complex number we have

polar form
Figure 1: Polar form of complex number.

\(a=r \,cos\,\theta,\: b =r \,sin\,\theta\)

being r modulus of z. It is calculated as

\(|z| = \sqrt{x^{2}+y^{2}} \)

The argument of a complex number is well defined except multiples of \(2\pi\), this is $$ arg\, z = \theta + 2k\pi,\,\, k=1, 2, 3 ...$$ or $$ arg\,z = \theta ,\,\,\,\ mod\,2\pi$$ We will refer, hereafter \(Arg z = \theta \), then $$ arg\,z = Arg\,z + 2k\pi,\,\, k=1, 2, 3 ...$$ Suppose we want to calculate the square of z: , \(z^{2}\), then: $$z = re^{i \theta} \Rightarrow z^{2} = r^2 e^{2i \theta} \Rightarrow arg z^{2} = 2 Arg\,z + 4k\pi,\,\, k=1, 2, 3 ...$$ When we do the reverse operation, the square root, it happens that $$ arg \, z^{\frac{1}{2}} = \frac{arg \, z }{2} + \frac{2k\pi}{2} $$ So, now we can calculate square roots of negative numbers, for example, to calculate the square root of -1 $$ arg \, (-1)^{\frac{1}{2}} = \frac{\pi}{2} + k\pi $$. because of this i and -i are both square roots of -1.

Complex numbers operations

Given two complex numbers

\( z_{1} = r_{1} e^{i \theta_{1} }\)
\( z_{2} = r_{2} e^{i \theta_{2} }\)

It is easy

\(arg\, z_{1}z_{2} = arg\, z_{1} + arg\,z_{2}\)

\(arg\, \frac{z_{1}}{z_{2}} = arg\, z_{1} - arg\,z_{2}\)

this is because

\( z_{1}z_{2} = r_{1} e^{i \theta_{1} } . r_{2} e^{i \theta_{2} } = r_{1} r_{2} e^{i (\theta_{1}+ \theta_{2}) } \)

Lets use this link to see how to calculate power and other operations on complex numbers.

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