Complex Numbers

A Complex Number

Let's talk some more about imaginary numbers ... 

The "unit" imaginary number (like 1 for Real Numbers) is i, which is the square root of −1

When we square i we get −1

i² = −1

Examples of Imaginary Numbers:

3i

1.04i

−2.8i

3i/4

(√2)i

1998i

And we keep that little "i" there to remind us we still need to multiply by √−1

Complex Numbers

When we combine a Real Number and an Imaginary Number we get a Complex Number:

Examples:

Either Part Can Be Zero

So, a Complex Number has a real part and an imaginary part.

But either part can be 0, so all Real Numbers and Imaginary Numbers are also Complex Numbers.

Complicated?

A Visual Explanation

You know how the number line goes left-right?

Well let's have the imaginary numbers go up-down:

And we get the Complex Plane

A complex number can now be shown as a point:

The complex number 3 + 4i

Properties

The letter z is often used for a complex number:

z = a + bi

• z is a Complex Number
• a and b are Real Numbers
• i is the unit imaginary number = √−1

we refer to the real part and imaginary part using Re and Im like this:

Re(z) = a, Im(z) = b

The conjugate (it changes the sign in the middle) of z is shown with a star:

z^* = a − bi

We can also use angle and distance like this (called polar form):

So the complex number 3 + 4i can also be shown as distance 5 and angle 0.927 radians. To convert from one form to the other use Cartesian to Polar conversion.

The magnitude of z is:

|z| = √(a² + b²)

And the angle of z, also called is:

Arg(z) = tan^-1(b/a)   (for a>0)

Adding

To add two complex numbers we add each part separately:

(a+bi) + (c+di) = (a+c) + (b+d)i

Let's try another:

Multiplying

To multiply complex numbers:

Each part of the first complex number gets multiplied by
each part of the second complex number

Just use "FOIL", which stands for "Firsts, Outers, Inners, Lasts" (see Binomial Multiplication for more details):

	Firsts: a × c Outers: a × di Inners: bi × c Lasts: bi × di
(a+bi)(c+di) = ac + adi + bci + bdi2

Like this:

And this:

But There is a Quicker Way!

Use this rule:

(a+bi)(c+di) = (ac−bd) + (ad+bc)i

Why Does That Rule Work?

It is just the "FOIL" method after a little work:

And there we have the (ac − bd) + (ad + bc)i  pattern.

This rule is certainly faster, but if you forget it, just remember the FOIL method.

Let us try i²

Just for fun, let's use the method to calculate i²

And that agrees nicely with the definition that i² = −1

So it all works wonderfully!

Learn more at Complex Number Multiplication.

Conjugates

We will need to use conjugates in a minute!

A conjugate is where we change the sign in the middle like this:

A conjugate can be shown with a little star, or with a bar over it:

Dividing

The conjugate is used to help complex division.

The trick is to multiply both top and bottom by the conjugate of the bottom.

Yes, there is a bit of calculation to do. But it can be done.

Multiplying By the Conjugate

There is a faster way though.

In the previous example, what happened on the bottom was interesting:

(4 − 5i)(4 + 5i) = 16 + 20i − 20i − 25i²

The middle terms (20i − 20i) cancel out:

(4 − 5i)(4 + 5i) = 16 − 25i²

Also i² = −1 :

(4 − 5i)(4 + 5i) = 16 + 25

And 16 and 25 are (magically) squares of the 4 and 5:

(4 − 5i)(4 + 5i) = 4² + 5²

Which is really quite a simple result. The general rule is:

That can save us time when we do division, like this:

Notation

We often use z for a complex number. And Re() for the real part and Im() for the imaginary part, like this:

Which looks like this on the complex plane:

The Mandelbrot Set

	The beautiful Mandelbrot Set (pictured here) is based on Complex Numbers. It is a plot of what happens when we take the simple equation z2+c (both complex numbers) and feed the result back into z time and time again. The color shows how fast z2+c grows, and black means it stays within a certain range.
Here is an image made by zooming into the Mandelbrot set
And here is the center of the previous one zoomed in even further: