# Dictionary:Complex number

Return to Complex Analysis.

Here we follow standard texts, such as Spiegel (1964) ^{[1]} or Levinson and Redheffer (1970). ^{[2]}

A number with both real and imaginary parts, such as

where , is called the *real part*, and is called the *imaginary part* of is called a **complex number**. [The symbol *j* is also used to indicate and
is the notation favored
by electrical engineers, as the symbol * is often reserved for representing electric current]. The ***modulus** or magnitude of the above complex number is and the angle indicating its direction with respect to the positive real axis is

A graph of a complex function or quantity (such as a frequency spectrum) is shown in Figure C-10.

## Contents

## Understanding complex numbers and functions of a complex variable.

In the computation of the roots of polynomial equations, additive quantities which are scaled by often appear.

### Example: a quadratic equation

For example, if we consider a general quadratic equation

where are real numbers. The roots of this equation may be obtained via the quadratic formula

Here, we see that when the quantity (known as the *discriminant* of the quadratic) the result will be

where we use the mathematician's convention of using

The point that is demonstrated here is that the relation between real and imaginary parts is additive.

### The fundamental theorem of algebra

The generalization of the quadratic result yields the **fundamental theorem of algebra** which states that
every polynomial of degree with complex coefficients has complex roots. Because real numbers are subsumed into the complex numbers (being considered as having imaginary part 0), this
describes all polynomials.

### The polar representation and the Argand plane

In general we consider a complex number where is the *real part* and
is called the *imaginary part* (called so, reflecting a time when mathematicians were uncomfortable with
quantities involving )

An amateur mathematician named Jean-Robert Argand is credited with being the first person to publish a representation of
complex numbers geometrically as defining a plane with the horizontal axis as the *real axis* and
as the *imaginary axis.*

If we consider the angle as the angle turned in a counter-clockwise direction from the positive real axis, then the natural polar representation results

Here
and is called the *modulus* of

The modulus may be written as where is called the **complex conjugate** of

Another polar representation is the following identities

and

implying that

We begin by formally writing the Taylor series representations of
and and sum the resulting series

where We may write the Taylor series form of

where . In both of the previous cases, free use has been made of the identity

The sum of these two series yields the series representation of the exponential function

where .

You will likely not see this argument in a textbook, as it is a *plausibility argument* rather than a proof, because it
depends on having the machinery of convergence of series in place.

### Functions of a complex variable

If a function is a mapping from the field of complex numbers to the field of complex numbers, then must, itself be the sum of a purely real valued function and a purely imaginary valued function thus,

A certain class of complex valued function, known as an *Analytic function* is of particular
importance in the application of complex-valued functions to problems in the physical sciences.

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