Complex Numbers

A complex number is an extension of a real number. Complex numbers appear in many engineering and math equations such as the Fundamental Theorem of Algebra, Euler’s Formula and the Mandelbrot Set. A complex number is denoted in the form shown below, where and are real numbers and .

Complex Number

Assumptions

Understanding what a complex number is and why it is useful requires a leap of faith. There are two assumptions, shown below, that form the definition of a complex number. Hopefully, by illustrating the properties that “fall out” of these two assumptions, you will be convinced that a complex number is very much real and useful.

  1. Assume there is some number so that .
  2. Give a home in the complex plane. This is shown below.

Definition of

The first assumption is that there exists some number that satisfies the equation . This by itself can be used in combination with the rules of algebra to solve previously unsolvable problems. However, a lot of the usefulness of a complex number comes from its application to two dimensional geometry in the coordinate plane. So first, we will give the notion of a complex number a home in the complex plane.

Definition of the Complex Plane

The second assumption is that a complex number has a “home” in the complex plane and can be drawn as an arrow from the origin to some point in the plane. The vertical axis marks the complex part of the number and the horizontal axis marks the real part of the number. For example, the complex number is drawn as an arrow starting from the origin and going units to the right and units up as shown in the figure below.

This figure illustrates the complex plane. The horizontal axis represents the real part of a complex number and the vertical axis represents the complex part of a complex number.
Figure 1: Complex Plane

Now that we have established the foundation of a complex number, we will look at some of the basic operations that can be perfomed from both a algebraic and geometric perspective.

Addition of Complex Numbers

The properties of addition for two complex numbers follow the rules of algebra. The real parts of the number are added together and the complex parts of the number are added together. Visually, this is represented by drawing the two arrows corresponding to each of the complex numbers “tail-to-tail” as shown below. The resulting number is the arrow drawn from the origin to the tip of the second arrow.

Complex Coordinate Plane Addition

Multiplication of Complex Numbers

The properties of complex number multiplication demonstrate why complex numbers are able to elegantly express an area of mathematics. Conceptually, multiplying by a complex number corresponds to stretching and rotating a notation of the complex plane.

Multiplication Property

Multiplying any complex number by results in . Visually, this correspond with stretching and rotating the number so that it lies on top of the complex number in the complex plane. Conceptually, you can imagine stretching and rotating all of the horizontal and vertical axis lines in the first plane.

Complex Coordinate Plane Multiplication

Multiplication Property

The second property of complex number mutliplication is multiplying a number by is equivalent to rotating by . For example, the complex number multiplied by is equal to the rotation shown below:

Complex Coordinate Plane Multiplication

This is where the usefulness of complex numbers starts to appear; the multiplication of two complex numbers can be geometrically related to rotation. Let’s verify the result we saw above using algebra and look at another example.

Multiplication Property

The last property of multiplication hopefully allows us to visualize what is happening when any two complex numbers are multiplied together. For example, to multiply the complex numbers and together, first we can visualize the stretch and rotation of the coordinate system from the first number , then we can draw the complex number arrow using this new stretched grid to find the product.

Complex Coordinate Plane Multiplication

We can verify this result using algebra.

Complex Plane Trigonometry

The fact that multiplying two complex numbers together results in a stretch and rotation in the complex plane suggestively hints that complex numbers may be useful when applied to trigonometry. Recall, a point on the unit circle which corresponds to the angle rotation along the perimeter of the circle.

Unit Circle Point

Euler’s Formula

Perhaps the most notorious formula in math and physics is Euler’s Formula as shown above.

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Trigonometric Identities

The trigonometric identities are a set of equations that are used to transform and manipulate math expressions. Assuming that you have had exposure to trigonometry and these identities, this is one of the areas where the usefulness of complex numbers shines. The complex plane represents an elegant way to think about and transform trigonometric expressions.