## Complex numbers

**-**

## Complex Numbers

# Complex Numbers

Complex numbers are those that can be put into the form a←+ bi, where a and b are real numbers and

Typical complex numbers are 3– i, 1/2+ 7i, and 6– 2i. If one writes the real number 17 as 17+ 0i and the imaginary number–2.5i as 0– 2.5i, they too can be considered complex numbers.

Complex numbers are so called because they are made up of two parts that cannot be combined. Even though the parts are joined by a plus sign, the addition cannot be performed. The expression must be left as an indicated sum.

Complex numbers are occasionally represented with ordered pairs, (z, b). Doing so shows the two-component nature of complex numbers but renders the operations with them somewhat obscure and hides the kind of numbers they are.

Inklings of the need for complex numbers were felt as early as the sixteenth century. In about 1545, Renaissance mathematician Jerome Cardan recognized that his method of solving cubic equations often led to solutions that contained the square root of negative numbers. It was not until the seventeenth and early eighteenth centuries that de Moivre, the Bernoullis, Euler, and others legitimized imaginary and complex numbers.

## Arithmetic

Complex numbers extend the set of real numbers to include imaginary numbers. The numbers must obey the mathematical laws already in place with two exceptions: the “sum” a+ bi must be left uncombined, and i^{2}=–1, which runs counter to the rule that the product of two numbers of like sign is positive.

Arithmetic with complex numbers is much like that of binomials such as 5x+ 7 with an important exception. When a binomial is squared, the term 25x^{2}appears, and it does not go away. When a+ bi is squared, the i^{2} in the term b^{2}i^{2} does go away. It becomes–b^{2}. These are the rules:

Equality: To be equal, two complex numbers must have equal real parts and equal imaginary parts. That is a+ bi= c+ di if and only if a= c and b= d.

Addition: To add two complex numbers, add the real parts and the imaginary parts separately. The sum of a+ bi and c+ di is (a+ c)+ (b+ d)i. The sum (3+ 5i)+ (8– 7i) is 11– 2i.

Subtraction: To subtract a complex number, subtract the real part from the real part, and the imaginary part from the imaginary part. The difference (a+ bi)– (c+ di) is (a– c)+ (b– d)i; (6+ 4i)– (3– 2i) is 3+ 6i.

Zero: To equal zero, a complex number must have both its real and its imaginary parts equal to zero: a+ bi= 0 if and only if a= 0 and b= 0.

Opposites: To form the opposite of a complex number, take the opposite of each part:–(a+ bi)= a+ (–b)i. The opposite of 6– 2i is–6+ 2i.

Multiplication: To form the product of two complex numbers multiply each part of one number by each part of the other: (a+ bi)(c+ di)= ac+ adi+ bci+ bdi^{2}, or (ac– bd)+ (ad+ bc)i. The product (5– 2i)(4– 3i) is 14– 23i.

Conjugates: Two numbers whose imaginary parts are opposites are called complex conjugates: a+ bi and a– bi are conjugates. Pairs of complex conjugates have many applications, because the product of two complex conjugates is real: (6– 12i)(6+ 12i)= 36– 144i^{2}, or 180.

Division: Except for division by zero, the set of complex numbers is closed with respect to division: If a+ bi is not zero, then (c+ di)/(a+ bi) is a complex number. To divide c+ di by a+ bi, multiply them both by the conjugate a– bi, which eliminates the need to divide by a complex number. Forexample

While the foregoing rules suffice for ordinary complex-number arithmetic, they must often be coupled with ingenuity for nonroutine problems. An example of this can be seen in the problem of computing asquare root of 3– 4i.

One starts by assuming that the square root is a complex number a+ bi. Then 3– 4i is the square of a+ bi, or a^{2}– b^{2}+ 2abi.

For two complex numbers to be equal, their real and imaginary parts must be equal.

Solving these equations for a yields four roots, namely 2, –2, i, and–i. Discarding the imaginary roots and solving for b gives 2– i or–2+ i as the square roots of 3– 4i. These roots seem a little strange, but their squares are in fact 3– 4i.

## Graphical representation

The mathematicians Wessel, Argand, and Gauss separately devised a graphical method of representing complex numbers. Real numbers have only a real component and can be represented as points on a one-dimensional number line. Complex numbers, on the other hand, with their real and imaginary components, require a two-dimensional complex-number plane (Figure 1).

The complex number plane can also by represented with polar coordinates. The relation between these coordinates and rectangular coordinates are given by the equations

Thus the complex number x+ iy can also be written in polar form r cos θ+ ir sin θ or r (cos θ+ i sin θ), abbreviated r cis θ. When written in polar form, r is the “modulus” or “absolute value” of the number. When the point is plotted on a Gauss-Argand diagram, r represents the distance from the point to the origin (Figure 2).

The angle θ is called the “argument” or the amplitude of the complex number, and represents the angle shown in Figure 2. Because adding or subtracting 360°to θ will not change the position of the ray drawn to the point, r cis θ, r cis (θ+ 360°), r cis (θ– 360°), and others all represent the same complex number. When θ is measured in radians, adding or subtracting a multiple of 2π will change the representation of the number, but not the number itself.

In polar form the five points shown in Figure 1 are A: (cos 45°+ I sin 45°) or cis 45°. B: 153.4°. C:2 cis 225°. D: 5 cis 306.9°. E: cis cis 333.4°.Except for A and C, the polar forms seem more awkward. Often, however, it is the other way about. 1 cis 72°, which represents the fifth root of 1 is simple in its polar form, but considerably less so in its rectangular form: 0.3090+.9511 i—and even this is only an approximation.

The polar form of a complex number has two remarkable features that the rectangular form lacks. For one, multiplication is very easy. The product of

r 1 cis θ 1 and r 2 cis θ 2 is simply r 1r 2 cis (θ 1+ θ 2). For example, (3 cis 30°) (6 cis 60°) is 18 cis 90°. The other feature is known as de Moivre’s theorem: (r cis θ)^{n}= r^{n}cis nθ, where n is any real number (actually any complex number). This is a powerful theorem. For example, if one wants to compute (1+ i)^{n}, multiplying it out can take a lot of time. If one converts it to polar form,= not equal 2 cis 45°, however, (= not equal 2 cis 45°)^{5}is= not equal 32 cis 225° or–4– 4i.

One can use de Moivre’s theorem to compute roots. Since the nth root of a real or complex number zis z^{1/n}, the nth root of r cis θ is r^{1/n} cis θ/n.

It is interesting to apply this to the cube root of 1. Writing 1 as a complex number in polar form one has 1

### KEY TERMS

**Complex number—** A number composed of two separate parts, a realpart and an imaginary part, joined by a+ sign.

**Imaginary number—** A number whose square is a negative number.

cis 0°. Its cube root is 1^{1/3} cis 0/3°, or simply 1. But 1 cis 0° is the same number as 1 cis 360° and 1 cis 720°. Applying de Moivre’s theorem to these alternate forms yields 1 cis 120° and 1 cis 240°, which are not simply 1. In fact they are and i in rectangular form.

## Uses of complex numbers

Complex numbers are needed for the fundamental theorem of algebra: Every polynomial equation, P(x)= 0, with complex coefficients has a complex root. For example, the polynomial equation x^{2}+ 1= 0 has no real roots. If one allows complex numbers, however, it has two: 0+ i and– i.

Complex numbers are also used in the branch of mathematics known as functions of complex variables. Such functions can often describe physical situations, as in electrical engineering and fluid dynamics, which real-valued functions cannot.

## Resources

### Books

Ball, W.W. Rouse. *A Short Account of the History of Mathematics.* London: Sterling Publications, 2002.

Bittinger, Marvin L., and David Ellenbogen. *Intermediate Algebra: Concepts and Applications.* 6th ed. Reading, MA: Addison-Wesley Publishing, 2001.

Stein, Sherman K. *Mathematics, the Man-Made Universe.* San Francisco: W. H. Freeman and Co., 1969.

### Periodicals

Jourdain, Philip E.B. “The Nature of Mathematics.” In *The World of Mathematics,* edited by James Newman. New York: Simon and Schuster, 1956.

### OTHER

*Clark University, Department of Mathematics and Computer Science.* “Dave’s Short Course on Complex Numbers” <http://www.clarku.edu/~djoyce/complex> (accessed October 7, 2006).

J. Paul Moulton

## Complex Numbers

# Complex numbers

Complex numbers are numbers that consist of two parts, one real and one imaginary. An imaginary number is the square root of a real number, such as √−4;. The expression √−4 is said to be imaginary because no real number can satisfy the condition stated. That is, there is no number that can be squared to give the value −4, which is what √−4 means. The imaginary number √−1 has a special designation in mathematics. It is represented by the letter i.

Complex numbers can be represented as a binomial (a mathematical expression consisting of one term added to or subtracted from another) of the form a + bi. In this binomial, a and b represent real numbers and i = √−1. Some examples of complex numbers are 3 − i, ½ + 7i, and −6 − 2i.

The two parts of a complex number cannot be combined. Even though the parts are joined by a plus sign, the addition cannot be performed. The expression must be left as an indicated sum.

## History

One of the first mathematicians to realize the need for complex numbers was Italian mathematician Girolamo Cardano (1501–1576). Around 1545, Cardano recognized that his method of solving cubic equations often led to solutions containing the square root of negative numbers. Imaginary numbers did not fully become a part of mathematics, however, until they were studied at length by French-English mathematician Abraham De Moivre (1667–1754), a Swiss family of mathematicians named the Bernoullis, Swiss mathematician Leonhard Euler (1707–1783), and others in the eighteenth century.

## Arithmetic

In many ways, operations with complex numbers follow the same rules as those for real numbers. Two exceptions to those rules arise because of the nature of complex numbers. First, what appears to be an addition operation, a + bi, must be left uncombined. Second, the general expression

for any imaginary number, such as i^{2} = −1, violates the rule that the product of two numbers of a like sign is positive.

The general rules for working with complex numbers are as follows:

1. Equality: To be equal, two complex numbers must have equal real parts and equal imaginary parts. That is, assume that we know that the expressions (a + bi) and (c + di) are equal. That condition can be true if and only if a = c and b = d.

2. Addition: To add two complex numbers, the real parts and the imaginary parts are added separately. The following examples illustrate this rule:

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

(3 + 5i) + (8 − 7i) = 11 − 2i

3. Subtraction: To subtract a complex number, subtract the real part from the real part and the imaginary part from the imaginary part. For example:

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

(6 + 4i) − (3 − 2i) = 3 + 6i

4. Zero: To equal zero, a complex number must have both its real part and its imaginary part equal to zero. That is, a + bi = 0 if and only if a = 0 *and* b = 0.

5. Opposites: To form the opposite of a complex number, take the opposite of each part. The opposite of a + bi is −(a + bi), or −a + (−b)i. The opposite of 6 − 2i is −6 + 2i.

## Words to Know

**Complex number:** A number composed of two separate parts, a real part and an imaginary part, joined by a + sign.

**Imaginary number:** A number whose square (the number multiplied by itself) is a negative number.

6. Multiplication: To form the product of two complex numbers, multiply each part of one number by each part of the other. The product of (a + bi) × (c + di) is ac + adi + bci + bdi^{2}. Since bdi^{2} = −bd, the final product is ac + adi + bci − bd. This expression can be expressed as a complex number as (ac − bd) + (ad + bc)i. Similarly, the product (5 − 2i) × (4 − 3i) is 14 − 23i.

7. Conjugates: Two numbers whose imaginary parts are opposites are called complex conjugates. The complex numbers a + bi and a − bi are complex conjugates because the terms bi have opposite signs. Pairs of complex conjugates have many applications because the product of two complex conjugates is real. For example, (6 − 12i) × (6 + 12i) = 36 − 144i^{2}, or 36 + 144 = 180.

8. Division: Division of complex numbers is restricted by the fact that an imaginary number cannot be divided by itself. Division can be carried out, however, if the divisor is first converted to a real number. To make this conversion, the divisor can be multiplied by its complex conjugate.

## Graphical representation

After complex numbers were discovered in the eighteenth century, mathematicians searched for ways of representing these combinations of real and imaginary numbers. One suggestion was to represent the numbers graphically, as shown in Figure 1. In graphical systems, the real part of a complex number is plotted along the horizontal axis and the imaginary part is plotted on the vertical axis. Thus, in Figure 1, point A stands for the complex number 2 + 2i and point B stands for the complex number −2 + i.

## Uses of complex numbers

For all the "imaginary" component they contain, complex numbers occur frequently in scientific and engineering calculations. Whenever the solution to an equation yields the square root of a negative number (such as √−9), complex numbers are involved. One of the problems faced by a scientist or engineer, then, is to figure out what the imaginary and complex numbers represent in the real world.

## Numbers, Complex

# Numbers, Complex

The set of complex numbers includes all the numbers we commonly work with in school mathematics (whole numbers, fractions, decimals, square roots, etc.), plus many more numbers that are generally not encountered until the study of higher mathematics. Complex numbers were invented centuries ago in order to provide solutions to certain equations that previously had seemed impossible to solve.

Imagine trying to find a solution to *x* + 6 = 4 but being able to look for a solution only in the set of whole numbers. This is impossible. However, if we expand our domain to all integers, − 2 provides a solution. Similarly, it is impossible to find a solution to 2*x* = 7 using only integers, but we can expand our domain to the set of rational numbers, and or 3.5 provides a solution. Now suppose you wanted to find a solution to *x* ^{2} = 2 using only rational numbers. This, too, is impossible. However, the set of rational numbers can be expanded to create still another new set of numbers—the real numbers. Clearly, is one solution to the equation *x* ^{2} = 2 because by definition the square root of any number multiplied by itself equals the number: . Another solution is because it is also true that the negative square root of a number multiplied by itself equals the number: .

In about 50 c.e. another seemingly impossible problem emerged when Heron of Alexandria, a Greek mathematician, was trying to evaluate the square root of a negative number. Consequently, the square root of a negative number cannot be evaluated using only real numbers. To find a solution, another new number system needed to be invented. In the mid-sixteenth century an Italian mathematician named Girolamo Cardano began to do just that. He is recognized as the discoverer of the imaginary numbers that play an essential role in understanding the complex number system.

## Imaginary Numbers

Contrary to their name, imaginary numbers are not imaginary at all. Imaginary numbers were invented in an effort to evaluate negative square roots, a mathematical operation that before their existence was impossible. Thus, the argument to justify the existence of imaginary numbers is similar to the argument for the existence of integers, rational numbers, and real numbers.

Leonhard Euler, an eighteenth-century Swiss mathematician known for his prolific writing in mathematics and his standardization of modern mathematics notation, chose the symbol *i* to stand for the square root of −1. Since that time, the imaginary number set has included *i* and any real number times *i.* So, for example, and (which we write as .

It is interesting and important to observe the behavior of *i* when it is multiplied by itself. To begin, *i* multiplied by itself is *i* ^{2} (also written as × , which is − 1). So *i* ^{2} = − 1. To continue this line of reasoning, *i* ^{3}* = − i* (because *i* ^{3} can be written as *i* ^{2}* × i* and thus − 1 × *i* or *−i* ). Similarly, *i* ^{4} = 1 (because it can be thought of as *i* ^{2} *× i* ^{2} or − 1 ×− 1). Continuing, we find that *i* ^{5} *= i,* and this is where the pattern begins again (*i,* − 1, *− i,* 1).

Incredibly, by using imaginary numbers it is possible to solve many equations that were deemed impossible for centuries. Consider *x* ^{2} + 4 = 0. Using algebraic manipulation (subtracting 4 from both sides of the equals sign), the equation becomes *x* ^{2} = − 4, and *x* can be either or . So *x* = 2*i* or − 2*i*. To check your answer, substitute the 2*i* or − 2*i* for *x*. So, for example, the equation becomes (2*i* )^{2}+ 4 = 0, which is (2^{2}* × i* ^{2}) + 4 = 0. Because *i* ^{2}= −1 and 2^{2} = 4 then (4 × −1) + 4 = 0, or − 4 + 4 = 0.

## Complex Number System

Carl Friedrich Gauss, a nineteenth-century German mathematician, is credited with inventing and naming the complex number system. Complex numbers are generally expressed in the form *a + bi,* where *a* and *b* are real numbers and *i* is the imaginary number described above (that is, ). The *a* part is considered the real part of the complex number and the *bi* part is the imaginary part of the complex number. Upon further inspection, we can see that the set of complex numbers includes all the pure real numbers, together with all the pure imaginary numbers, together with many more numbers that are sums of these. In other words, whenever a complex number has *b* = 0, it is actually a pure real number too because it is equal to *a* + 0*i*, which is just *a* (a real number). Whenever a complex number has *a* = 0, it is actually a pure imaginary number because it is equal to 0 *+ bi*, which is just *bi*, an imaginary number.

The complex number system consists of all complex numbers, *a + bi* (where *a* and *b* are real numbers), together with the rules that define the four basic operations on this set of numbers (addition, subtraction, multiplication, and division). Indeed, in order to define any number system, there are certain rules that must be obeyed. First, addition and multiplication must be well defined (that is, it must be clear how to add and how to multiply any two numbers in the set). Addition of complex numbers occurs by adding their real parts and their imaginary parts separately. For example, (4 + 2*i* ) + (6 + 3*i* ) = (4 + 6) + (2*i* + 3*i* ) = 10 + 5*i*. In general, (*a + bi* ) + (*c + di* ) is equal to (*a + c* ) + (*b + d* )*i*. Subtraction is performed similarly.

To multiply any two complex numbers, use the distributive property (in a method sometimes referred to in elementary algebra classes as the "foil" method) and then combine real terms and imaginary terms separately. Thus, (*a + bi* ) × (*c + di* ) *= ac* + (*ad* )*i* + (*bc* )*i* + (*bi* )(*di* ) = (*ac + bd* ) + (*ad + bc* )*i*. For example, (4 + 5*i* )(6 + 3*i* ) = 24 + 12*i* + 30*i* + 15*i* ^{2} = 24 + 42*i* - 15 = 9 + 42*i*. Remember from previous discussions that *i* ^{2}= - 1. Multiplying a complex number by a constant is a simpler case of this process in which the distributive property is similarly invoked. For example, 4 (4 + 2*i* ) = 16 + 8*i*. In general, *c* (*a + bi* ) = *ac* + (*bc* )*i*.

Every number system also must have both an additive identity and a multiplicative identity—that is, numbers that when added to (or multiplied by) any number in the set produce the same number started with. For the complex numbers—just as for the whole numbers, the integers, the rational numbers, and the real numbers—the additive identity is 0 and the multiplicative identity is 1.

For any number system, there must also be both additive and multiplicative inverses—that is, numbers that when added to (or multiplied by) a number in the set produce the additive (or multiplicative) identity, respectively. For the real numbers, the opposite of any number is its additive inverse. That is, *−a* is the additive inverse of *a* (since, for example, −4 + 4 = 0, and 0 is the additive identity). For the complex numbers, we must take the opposite of both the real and imaginary parts of a number to find its inverse. Thus, (*−a −bi* ) is the additive inverse of (*a + bi* ) (because adding these together gives 0 + 0*i*, or simply 0, the additive identity).

Finally, as for the real numbers, multiplication and addition of complex numbers must be commutative, associative, and distributive. This is indeed the case because the operations in complex numbers are based on the operations in real numbers.

Dividing any complex number *a + bi* by a real number (say, *r* ) is done by dividing each part of the number by *r*. Thus, (*a + bi* ) ÷ *r = a* /*r* + (*b* /*r* )*i*. To divide a complex number by a complex number is somewhat more complicated. However, the process is similar, in some ways, to a process you probably learned for dividing by a decimal.

Consider the example, 135.5 ÷ 0.25. To perform this division, you were probably taught to multiply both dividend and divisor by 100 before proceeding with the division. Thus, the problem is transformed to 13550 ÷ 25. The purpose of performing this transformation is to create a new (easier) problem that will have the same answer as the original problem.

The process of dividing by a complex number proceeds in a similar fashion. To divide by a complex number, we choose first to multiply both dividend and divisor by something to make the divisor non-complex. In the case of division by a complex number, we choose to multiply by a number known as the "complex conjugate" of the divisor. The complex conjugate of any complex number (*a + bi* ) is simply the complex number (*a - bi* ). Multiplying these two numbers together produces a product that is not complex (that is, the product has imaginary part equal to 0).

For example, the complex conjugate of 2 + 3*i* is 2 − 3*i*. When you multiply these two numbers you obtain 13 because (2 + 3*i* )(2 − 3*i* ) = 4 + 6*i* − 6*i* − 9*i* ^{2}= 4 − 9*i* ^{2} = 4 − 9 (−1) = 4 + 9 = 13. The product (13) has imaginary part equal to 0 because the middle (imaginary) terms cancel out.

Thus, if you want to perform (3 + 4*i* ) ÷ (2 + 3*i* ), you must first multiply both of these numbers by (2 − 3*i* ) to produce an equivalent division problem: (3 + 4*i* )(2 − 3*i* ) ÷ (2 + 3*i* )(2 − 3*i* ). This works out to (18 − *i* ) ÷ 13. Now the division is easily performed by dividing both real and imaginary parts by 13, producing as the answer.

Complex conjugates also arise when finding the roots of **polynomials** . When a polynomial is factored, the total number of roots is always equal to the degree of the polynomial, as proven by the Fundamental Theorem of Algebra. For example, *x* ^{2} + 5*x* + 6 = 0 is an equation of degree 2 (since the highest power of *x* is 2). By factoring this equation into (*x* + 3)(*x* + 2)= 0, we can see that it has two solutions, −3 and −2. Thus, the polynomial *x* ^{2} + 5*x* + 6 has exactly two roots.

It is quite possible, however, that some of the roots of a polynomial will not be real because (as seen above) some equations can be solved only by appealing to the set of complex numbers. For instance, a polynomial of degree 4 might have two real roots and two complex roots.

Consider, for example, the equation *x* ^{4} − 16 = 0. This can be factored into (*x* ^{2}− 4)(*x* ^{2} + 4) = 0. Thus, we see that either *x* ^{2} − 4 = 0 or *x* ^{2} + 4 = 0. The former is true only when *x* = 2 or − 2. The latter is true only when *x* = 2*i* or −2*i*. Thus, *x* ^{4} 16 is a polynomial of degree 4 with two real roots and two imaginary (or complex) roots. In fact, complex solutions always come in pairs of complex conjugates. That is, whenever *a + bi* is a root of a polynomial, then *a − bi* will also be a root.

## Geometric Representations of Complex Numbers

In the early nineteenth century, Jean Robert Argand, an amateur French mathematician; Caspar Wessell, a Norwegian cartographer; and Carl Friedrich Gauss, a German mathematician, all worked on developing geometric representations of complex numbers in a plane. Because complex numbers have both a real part and an imaginary part, a two-dimensional plane (rather than a one-dimensional line) is needed to plot them.

The real part and the imaginary part of a complex number can be written as a coordinate pair. For example, 3 + 2*i* can be written as (3,2), where the first coordinate represents the real part of the complex number and the second coordinate represents the imaginary part. Then, we can use this ordered pair to produce a geometric representation of the complex number. In fact, mathematicians use two different, but related geometric representations of complex numbers.

First, the number *a + bi* or (*a, b* ) can be thought of as the point *P* (*a, b* ) in the complex plane (or Argand diagram) by starting at the origin and plotting a point over *a* units and up *b* units. Or, second, (*a, b* ) can be thought of as a vector from (0,0), the origin, to *P* in the complex plane.

Notice that construction of the complex plane is similar to that of the Cartesian plane. However, for the purpose of plotting complex numbers, the horizontal axis is considered to be the real axis and the vertical axis to be the imaginary axis. Thus, ordered pairs for complex numbers are plotted in the same manner as plotting ordered pairs of real numbers on a Cartesian plane.

Many relationships can be defined using a geometric model of complex numbers. For example, consider the notion of absolute value. Recall that for real numbers, the absolute value of any number *a* is just the distance of *a* from 0 (or, geometrically, the length of the segment from 0 to the point *a* on the number line). For example, the absolute value of −3 is 3, since −3 is 3 units from 0 on the number line. For the same reason, the absolute value of ± 13 is 13.

In the complex number system, the absolute value of a number *a + bi* or (*a,b* ) is defined in an analogous way as the distance from the origin (0,0) to the point *P* (*a,b* ) or length of the vector that (*a,b* ) represents. Thus, the absolute value of *a + bi* is determined by computing the length of the vector from the origin to *P*. This distance can be found by dropping a perpendicular line segment from *P* (*a,b* ) to the *x* -axis.

Doing so forms a right triangle whose hypotenuse is the length of the vector and whose horizontal leg has length *a* and whose vertical leg has length *b*. The length of the vector, *c*, can be computed using the Pythagorean Theorem (*c* ^{2}*= a* ^{2}*+ b* ^{2}). In other words, the absolute value of *a + bi* is equal to the square root of (*a* ^{2}*+ b* ^{2}). For example, the absolute value of the complex number 3 + 4*i* is the square root of 3^{2}+ 4^{2}, which is , or 5.

## Uses of Complex Numbers

Complex numbers are used both in the study of pure mathematics and in a variety of technical, real-world applications. When we study the real number system, many of its properties are easier to illustrate by looking at the real numbers within the more inclusive set of complex numbers. In other words, complex numbers are important in the study of number theory. Another example of the utility of complex numbers in mathematical study is that some functions, which generate fractals, contain complex numbers.

Complex numbers also have practical applications in technical fields. For instance, complex numbers are used in the study of electromagnetic fields. An electromagnetic field has both an electric and magnetic component, so two measures are required: one for the intensity of the electric field and one for the intensity of the magnetic field. Complex numbers help to describe the field's strength.

Electrical engineers use complex numbers to measure electrical current and to explain how electric circuits behave. Mechanical engineers use complex numbers to analyze the stresses of beams in buildings and bridges. Complex numbers appear when the engineers look for the **eigenvalues** and **eigenvectors** of the matrix that the engineers configure to explain numerically the stresses of the beams.

see also Euler, Leonhard; Integers; Number System, Real; Numbers, Rational; Numbers, Real; Vectors.

*Rose Kathleen Lynch*

## Bibliography

Bolton, William. *Complex Numbers.* New York: Addison Wesley Longman, Inc., 1996.

Devlin, Keith J. *Mathematics: The New Golden Age.* New York: Columbia University Press, 1999.

Frucht, William, ed. *Imaginary Numbers: An Anthology of Marvelous Mathematical Stories, Diversions and Musings.* New York: John Wiley & Sons, Inc., 1999.

Nahin, Paul J. *An Imaginary Tale: The Story of the Square Root of Minus One.* Princeton, NJ: Princeton University Press, 1998.

National Council of Teachers of Mathematics. *Historical Topics for the Mathematics Classroom.* Reston, VA: National Council of Teachers of Mathematics, 1989.

### Internet Resources

The Math Forum. *Ask Dr. Math.* Swarthmore College. <http://forum.swarthmore.edu/dr.math/>.

## Complex Numbers

# Complex numbers

Complex numbers are numbers which can be put into the form a + bi, where a and b are real numbers and i2 = -1.

Typical complex numbers are 3 - i, 1/2 + 7i, and -6 - 2i. If one writes the real number 17 as 17 + 0i and the **imaginary number** -2.5i as 0 - 2.5i, they too can be considered complex numbers.

Complex numbers are so called because they are made up of two parts which cannot be combined. Even though the parts are joined by a plus sign, the **addition** cannot be performed. The expression must be left as an indicated sum.

Complex numbers are occasionally represented with ordered pairs, (z,b). Doing so shows the two-component nature of complex numbers but renders the operations with them somewhat obscure and hides the kind of numbers they are.

Inklings of the need for complex numbers were felt as early as the sixteenth century. Cardan, in about 1545, recognized that his method of solving **cubic equations** often led to solutions with the **square root** of **negative** numbers in them. It was not until the seventeenth and early eighteenth centuries that de Moivre, the Bernoullis, Euler, and others gave formal recognition to imaginary and complex numbers as legitimate numbers.

## Arithmetic

Complex numbers can be thought of as an extension of the set of real numbers to include the imaginary numbers. These numbers must obey the laws, such as the distributive law, which are already in place. This they do with two exceptions, the fact that the "sum" a + bi must be left uncombined, and the law i2 = -1, which runs counter to the rule that the product of two numbers of like sign is positive.

**Arithmetic** with complex numbers is much like the "arithmetic" of binomials such as 5x + 7 with an important exception. When such a binomial is squared, the **term** 25x2 appears, and it doesn't go away. When a + bi is squared, the i2 in the term b2i2 does go away. It becomes -b2. These are the rules:

- Equality: To be equal two complex numbers must have equal real parts and equal imaginary parts. That is a + bi = c + di if and only if a = c and b = d.
- Addition: To add two complex numbers, add the real parts and the imaginary parts separately. The sum of a + bi and c + di is (a + c) + (b + d)i. The sum (3 + 5i) + (8 - 7i) is 11 - 2i.
**Subtraction**: To subtract a complex number, subtract the real part from the real part and the imaginary part from the imaginary part. The difference (a + bi) - (c + di) is (a - c) + (b - d)i; (6 + 4i) - (3 - 2i) is 3 + 6i.**Zero**: To equal zero, a complex number must have both its real part and its imaginary part equal to zero: a + bi = 0 if and only if a = 0 and b = 0.- Opposites: To form the opposite of a complex number, take the opposite of each part: -(a + bi) = -a + (-b)i. The opposite of 6 - 2i is -6 + 2i.
**Multiplication**: To form the product of two complex numbers multiply each part of one number by each part of the other: (a + bi)(c + di) = ac + adi + bci + bdi2, or (ac - bd) + (ad + bc)i. The product (5 - 2i)(4 - 3i) is 14 - 23i.- Conjugates: Two numbers whose imaginary parts are opposites are called "complex conjugates." These complex numbers a + bi and a - bi are conjugates. Pairs of complex conjugates have many applications because the product of two complex conjugates is real: (6 - 12i)(6 + 12i) = 36 - 144i2, or 180.
**Division**: Division of complex numbers is an example. Except for division by zero, the set of complex numbers is closed with respect to division: If a + bi is not zero, then (c + di)/(a + bi) is a complex number. To divide c + di by a + bi, multiply them both by the conjugate a - bi, which eliminates the need to divide by a complex number. For example

While the foregoing rules suffice for ordinary complex-number arithmetic, they must often be coupled with ingenuity for non-routine problems. An example of this can be seen in the problem of computing a square root of 3 - 4i.

One starts by assuming that the square root is a complex number a + bi. Then 3 - 4i is the square of a + bi, or a2 - b2 + 2abi.

For two complex numbers to be equal, their real and imaginary parts must be equal

Solving these equations for a yields four roots, namely 2, -2, i, and -i. Discarding the imaginary roots and solving for b gives 2 - i or -2 + i as the square roots of 3 - 4i. These roots seem strange, but their squares are in fact 3 - 4i.

## Graphical representation

The mathematicians Wessel, Argand, and Gauss, separately devised a graphical method of representing
complex numbers. Real numbers have only a real component and can be represented as points on a one-dimensional number line. Complex numbers, on the other hand, with their real and imaginary components, require a two-dimensional complex-number **plane** .

The complex number plane can also by represented with **polar coordinates** . The **relation** between these coordinates and rectangular coordinates are given by the equations

Thus the complex number x + iy can also be written in polar form r cos θ + ir sin θ or r(cos θ + i sin θ), abbreviated r cis θ. When written in polar form, r is called the "modulus" or "absolute value" of the number. When the **point** is plotted on a Gauss-Argand diagram, r represents the **distance** from the point to the origin.

The **angle** θ is called the "argument" or the "amplitude" of the complex number, and represents the angle shown in Figure 2. Because adding or subtracting 360° to θ will not change the position of the ray drawn to the point, r cis θ, r cis ( θ + 360°), r cis ( θ - 360°), and others all represent the same complex number. When θ is measured in radians, adding or subtracting a multiple of 2π will change the representation of the number, but not the number itself.

In polar form the five points shown in Figure 1 are A: √θ (cos 45° + I sin 45°) or √θ cis 45°. B: √+5 cis 153.4°. C: 2 cis 225°. D: 5 cis 306.9°. E: √+5 cis 333.4°. Except for A and C, the polar forms seem more awkward. Often, however, it is the other way about. 1 cis 72°, which represents the fifth root of 1 is simple in its polar form, but considerably less so in its rectangular form: .3090 +.9511 i, and even this is only an **approximation** .

The polar form of a complex number has two remarkable features which the rectangular form lacks. For one, multiplication is very easy. The product of r1 cis θ1 and r2 cis θ2 is simply r1 r2 cis ( θ1 + θ2). For example, (3 cis 30°) (6 cis 60°) is 18 cis 90°. The other feature is known as de Moivre's **theorem** : (r cis θ)n = rn cis n θ, where n is any real number (actually any complex number). This is a powerful theorem. For example, if one wants to compute (1 + i)n, multiplying it out can take a lot of time. If one converts it to polar form, ≠l2 cis 45°, however, (≠2 cis 45°)5 ≠32 cis 225° or -4 -4i.

One can use de Moivre's theorem to compute roots. Since the nth root of a real or complex number z is z1/n, the nth root of r cis θ is r1/n cis θ/n.

It is interesting to apply this to the cube root of 1. Writing 1 as a complex number in polar form one has 1 cis 0°. Its cube root is 11/3 cis 0/3°, or simply 1. But 1 cis 0° is the same number as 1 cis 360° and 1 cis 720°. Applying de Moivre's theorem to these alternate forms yields 1 cis 120° and 1 cis 240°, which are not simply 1. In fact they are -1/2 + √+2 /2 i and -1/2 - √+3/2i in rectangular form.

## Uses of complex numbers

Complex numbers are needed for the fundamental theorem of **algebra** : Every polynomial equation, P(x) = 0, with complex coefficients has a complex root. For example, the polynomial equation x2 + 1 = 0 has no real roots. If one allows complex numbers, however, it has two: 0 + i and - i.

Complex numbers are also used in the branch of **mathematics** known as "functions of complex variables." Such functions can often describe physical situations, as in electrical **engineering** and **fluid dynamics** , which real-valued functions cannot.

## Resources

### books

Ball, W.W. Rouse. *A Short Account of the History of Mathematics.* London: Sterling Publications, 2002.

Bittinger, Marvin L., and Davic Ellenbogen. *Intermediate Algebra: Concepts and Applications.* 6th ed. Reading, MA: Addison-Wesley Publishing, 2001.

Jourdain, Philip E. B. "The Nature of Mathematics." *The World of Mathematics.* Newman, James, ed. New York: Simon and Schuster, 1956.

Stein, Sherman K. *Mathematics, the Man-made Universe.* San Francisco: W. H. Freeman and Co., 1969.

J. Paul Moulton

## KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .**Complex number**—A number composed of two separate parts, a real part and an imaginary part, joined by a "+" sign.

**Imaginary number**—A number whose square is a negative number.

## complex number

**complex number** A number that, for mathematical convenience, is regarded as being composed of two scalars (called the *real* and *imaginary parts* of the complex number), and is subject to a standard set of operations according to the rules of complex algebra.

In engineering, and especially control engineering and electrical and electronic engineering, complex numbers and complex operations are an essential tool without which calculations would be much more difficult to express and understand. Consequently, support for complex numbers and operations is an important consideration in the design of programming languages, packages, and libraries for use in engineering applications.

## complex number

**complex number** Number of the form *a* + *b*i, where i = √−1, and *a* and *b* are real numbers. In order to obtain a solution to the equation *x*^{2} + 1 = 0, we need to introduce a new number i such that i^{2} = −1. The solutions to similar equations then give rise to a set of numbers of the general form *a* + *b*i. These are known as the complex numbers. Since *b* can be equal to zero, the set of complex numbers includes the real numbers.