Euler's Equation

$Euler's equation$

Euler's equation is one of most remarkable and mysterious discoveries in Mathematics. Euler's equation (formula) shows a deep relationship between the trigonometric function and complex exponential function.

Discovery of Euler's Equation

First, take a look the Taylor series representation of exponential function, $e^x$ and trigonometric functions, sine, $sin(x)$ and cosine, $cos(x)$ .
$Taylor series of e^x, sin(x), cos(x)$ Les't compare $cos(x) + sin(x)$ with $e^x$ . Notice $cos(x) + sin(x)$ is almost identical to Taylor series of $e^x$ ; all terms in the series are exactly same except signs. As you know, the exponential function, $e^x$ increases exponentially as input x grows. But, what does this exponential function have to do with periodic (oscillating) functions, $cos(x)$ and $sin(x)$ ?
Mathematicians had tried to figure out this weird relationship between the exponential function and the sum of 2 oscillating functions. Finally, Leonhard Euler completed this relation by bringing the imaginary number, i into the above Taylor series; $e^{ix}$ instead of $e^x$ and $cos(x) + isin(x)$ instead of $cos(x) + sin(x)$ .
$Taylor series of e^{ix}, cos(x)+isin(x)$ Now, we find out $e^{ix}$ equals to $cos(x) + isin(x)$ , which is known as Euler's Equation.

Graph of Euler's Equation

We know that the exponential function, $e^x$ is increasing exponentially as x grows. But, what does the function $e^{ix}$ look like?
The following images show the graph of the complex exponential function, $complex exponential function, e^{ix}$ , by plotting the Taylor series of $e^{ix}$ in the 3D complex space (x - real - imaginary axis). Surprisingly, it is a spiral spring (coil) shape, rotating around a unit circle. And, when it is projected to the real number (top view) and imaginary number axis (side view), it becomes a trigonometric function, respectively cosine and sine.

$Graph of complex exponential function, e^{ix}$
Graph of $complex exponential function, e^{ix}$

$Projection of e^{ix} in real number axis$
Real number part of $Real part of complex exponential function, e^{ix}$

$Projection of e^{ix} in imaginary number axis$
Imaginary number part of $Imaginary part of complex exponential function, e^{ix}$

This graphical representation of the complex exponential function, $complex exponential function, e^{ix}$ clearly shows the relation to the trigonometric function; the real number part of $complex exponential function, e^{ix}$ is cosine and the imaginary part of $complex exponential function, e^{ix}$ is sine function with the period of $2pi$ in radians.

$Graph of e^{ix}$
Example of Complex Exponential Function, $e^{ix}$

This is an interactive OpenGL application to draw the complex exponential function, $complex exponential function, e^{ix}$ in 3D.

Use the mouse cursor to change the view and use 'd' key to change drawing modes.

Meaning of Euler's Equation

$Projection onto complex plane of complex exponential function, e^{ix}$
Graph of $complex exponential function, e^{ix}$ on the complex plane

When the graph of $complex exponential function, e^{ix}$ is projected to the complex plane, the function $complex exponential function, e^{ix}$ is tracing on the unit circle. It is a periodic function with the period $2pi$ .
It means that raising $mathematical constant, e$ to an imaginary power $ix$ produces the complex number with the angle x in radians. This polar form of $complex exponential function, e^{ix}$ is very convenient to represent rotating objects or periodic signals because it can represent a point in the complex plane with only single term instead of two terms, $a + ib$ . Plus, it simplifies the mathematics when used in multiplication, for example,

.
The complex exponential forms are frequently used in electrical engineering and physics. For example, A periodic signal can be represented the sum of sine and cosine functions in Fourier analysis, and the movement of a mass attached to a string is also sinusodial. These sinusodial functions can be replaced with the complex exponential forms for simpler computation.

$Polar form of a complex number$
Polar form of a complex number

We can extend this polar form representation for any complex number.
$Polar form of a complex number$ The magnitude (norm), |z| of the complex number is a scalar value and can be rewrtten by using the laws of exponent and logarithm:
$Polar form of a complex number$

2D Rotation with Euler's Equation

The multiplication of two complex numbers implies a rotation in 2D space.

Multiplication implies rotation

Take a look the following figure showing 2 complex numbers on the unit circle of the complex plane.
When we compare these two complex numbers, we notice that the sum of angles in the imaginary exponential form equals to the product of two complex numbers,

. It tells us multiplying

performs rotating

with angle Φ.
We can extend it to any arbitary complex number. In order to rotate a complex number z = x + iy with a certain angle Φ, we simply multiply

to the number. Then, the rotated result z' becomes

2D rotation of a complex number

If we re-write it as a matrix form, it becomes a 2x2 rotation matrix that we are familiar with.

Euler Identity

If we substitute the value

into Euler's equation, then we get:
$Euler Identity$ This equation is called Euler Identity showing the link between 5 fundamental mathematical constants; 0, 1, $pi$ , $e$ , and $i$ .
Logarithmic function is only defined for the domain x > 0. But, Euler Identity allows to define the logarithm of negative x by converting exponent to logarithm form: