# Complex number

This standard basis makes the complex numbers a Cartesian plane, called the complex plane. This allows a geometric interpretation of the complex numbers and their operations, and conversely expressing in terms of complex numbers some geometric properties and constructions. For example, the real numbers form the real line which is identified to the horizontal axis of the complex plane. The complex numbers of absolute value one form the unit circle. The addition of a complex number is a translation in the complex plane, and the multiplication by a complex number is a similarity centered at the origin. The complex conjugation is the reflection symmetry with respect to the real axis. The complex absolute value is a Euclidean norm.

In summary, the complex numbers form a rich structure that is simultaneously an algebraically closed field, a commutative algebra over the reals, and a Euclidean vector space of dimension two.

A complex number is a number of the form *a* + *bi*, where a and b are real numbers, and *i* is an indeterminate satisfying *i*^{2} = −1. For example, 2 + 3*i* is a complex number.^{[6]}^{[3]}

This way, a complex number is defined as a polynomial with real coefficients in the single indeterminate *i*, for which the relation *i*^{2} + 1 = 0 is imposed. Based on this definition, complex numbers can be added and multiplied, using the addition and multiplication for polynomials. The relation *i*^{2} + 1 = 0 induces the equalities *i*^{4k} = 1, *i*^{4k+1} = *i*, *i*^{4k+2} = −1, and *i*^{4k+3} = −*i*, which hold for all integers k; these allow the reduction of any polynomial that results from the addition and multiplication of complex numbers to a linear polynomial in i, again of the form *a* + *bi* with real coefficients a, b.

The real number a is called the *real part* of the complex number *a* + *bi*; the real number b is called its *imaginary part*. To emphasize, the imaginary part does not include a factor i; that is, the imaginary part is b, not *bi*.^{[7]}^{[8]}^{[3]}

Formally, the complex numbers are defined as the quotient ring of the polynomial ring in the indeterminate *i*, by the ideal generated by the polynomial *i*^{2} + 1 (see below).^{[9]}

A real number a can be regarded as a complex number *a* + 0*i*, whose imaginary part is 0. A purely imaginary number *bi* is a complex number 0 + *bi*, whose real part is zero. As with polynomials, it is common to write a for *a* + 0*i* and *bi* for 0 + *bi*. Moreover, when the imaginary part is negative, that is, *b* = −*|b|* < 0, it is common to write *a* − *|b|i* instead of *a* + (−*|b|*)*i*; for example, for *b* = −4, 3 − 4*i* can be written instead of 3 + (−4)*i*.

Since the multiplication of the indeterminate *i* and a real is commutative in polynomials with real coefficients, the polynomial *a* + *bi* may be written as *a* + *ib*. This is often expedient for imaginary parts denoted by expressions, for example, when b is a radical.^{[10]}

In some disciplines, particularly in electromagnetism and electrical engineering, j is used instead of i as i is frequently used to represent electric current.^{[11]} In these cases, complex numbers are written as *a* + *bj*, or *a* + *jb*.

The definition of the complex numbers involving two arbitrary real values immediately suggests the use of Cartesian coordinates in the complex plane. The horizontal (*real*) axis is generally used to display the real part, with increasing values to the right, and the imaginary part marks the vertical (*imaginary*) axis, with increasing values upwards.

A charted number may be viewed either as the coordinatized point or as a position vector from the origin to this point. The coordinate values of a complex number z can hence be expressed in its *Cartesian*, *rectangular*, or *algebraic* form.

Notably, the operations of addition and multiplication take on a very natural geometric character, when complex numbers are viewed as position vectors: addition corresponds to vector addition, while multiplication (see below) corresponds to multiplying their magnitudes and adding the angles they make with the real axis. Viewed in this way, the multiplication of a complex number by *i* corresponds to rotating the position vector counterclockwise by a quarter turn (90°) about the origin—a fact which can be expressed algebraically as follows:

An alternative option for coordinates in the complex plane is the polar coordinate system that uses the distance of the point z from the origin (O), and the angle subtended between the positive real axis and the line segment Oz in a counterclockwise sense. This leads to the polar form of complex numbers.

The *absolute value* (or *modulus* or *magnitude*) of a complex number *z* = *x* + *yi* is^{[14]}

If z is a real number (that is, if *y* = 0), then *r* = |*x*|. That is, the absolute value of a real number equals its absolute value as a complex number.

By Pythagoras' theorem, the absolute value of a complex number is the distance to the origin of the point representing the complex number in the complex plane.

The *argument* of z (in many applications referred to as the "phase" φ)^{[13]} is the angle of the radius Oz with the positive real axis, and is written as arg *z*. As with the modulus, the argument can be found from the rectangular form x + yi^{[15]}—by applying the inverse tangent to the quotient of imaginary-by-real parts. By using a half-angle identity, a single branch of the arctan suffices to cover the range of the arg-function, (−*π*, *π*], and avoids a more subtle case-by-case analysis

Normally, as given above, the principal value in the interval (−π, π] is chosen. If the arg value is negative, values in the range (−π, π] or [0, 2π) can be obtained by adding 2*π*. The value of φ is expressed in radians in this article. It can increase by any integer multiple of 2*π* and still give the same angle, viewed as subtended by the rays of the positive real axis and from the origin through z. Hence, the arg function is sometimes considered as multivalued. The polar angle for the complex number 0 is indeterminate, but arbitrary choice of the polar angle 0 is common.

Together, r and φ give another way of representing complex numbers, the *polar form*, as the combination of modulus and argument fully specify the position of a point on the plane. Recovering the original rectangular co-ordinates from the polar form is done by the formula called *trigonometric form*

In angle notation, often used in electronics to represent a phasor with amplitude r and phase φ, it is written as^{[16]}

When visualizing complex functions, both a complex input and output are needed. Because each complex number is represented in two dimensions, visually graphing a complex function would require the perception of a four dimensional space, which is possible only in projections. Because of this, other ways of visualizing complex functions have been designed.

In domain coloring the output dimensions are represented by color and brightness, respectively. Each point in the complex plane as domain is *ornated*, typically with *color* representing the argument of the complex number, and *brightness* representing the magnitude. Dark spots mark moduli near zero, brighter spots are farther away from the origin, the gradation may be discontinuous, but is assumed as monotonous. The colors often vary in steps of π/3 for 0 to 2π from red, yellow, green, cyan, blue, to magenta. These plots are called color wheel graphs. This provides a simple way to visualize the functions without losing information. The picture shows zeros for ±1, (2 + *i*) and poles at ±√−2 −2 *i* .

Riemann surfaces are another way to visualize complex functions.^{[further explanation needed]} Riemann surfaces can be thought of as deformations of the complex plane; while the horizontal axes represent the real and imaginary inputs, the single vertical axis only represents either the real or imaginary output. However, Riemann surfaces are built in such a way that rotating them 180 degrees shows the imaginary output, and vice versa. Unlike domain coloring, Riemann surfaces can represent multivalued functions like √*z*.

The solution in radicals (without trigonometric functions) of a general cubic equation contains the square roots of negative numbers when all three roots are real numbers, a situation that cannot be rectified by factoring aided by the rational root test if the cubic is irreducible (the so-called *casus irreducibilis*). This conundrum led Italian mathematician Gerolamo Cardano to conceive of complex numbers in around 1545,^{[17]} though his understanding was rudimentary.

Work on the problem of general polynomials ultimately led to the fundamental theorem of algebra, which shows that with complex numbers, a solution exists to every polynomial equation of degree one or higher. Complex numbers thus form an algebraically closed field, where any polynomial equation has a root.

Many mathematicians contributed to the development of complex numbers. The rules for addition, subtraction, multiplication, and root extraction of complex numbers were developed by the Italian mathematician Rafael Bombelli.^{[18]} A more abstract formalism for the complex numbers was further developed by the Irish mathematician William Rowan Hamilton, who extended this abstraction to the theory of quaternions.^{[19]}

The earliest fleeting reference to square roots of negative numbers can perhaps be said to occur in the work of the Greek mathematician Hero of Alexandria in the 1st century AD, where in his *Stereometrica* he considers, apparently in error, the volume of an impossible frustum of a pyramid to arrive at the term √81 − 144 = 3*i*√7 in his calculations, although negative quantities were not conceived of in Hellenistic mathematics and Hero merely replaced it by its positive (√144 − 81 = 3√7).^{[20]}

The impetus to study complex numbers as a topic in itself first arose in the 16th century when algebraic solutions for the roots of cubic and quartic polynomials were discovered by Italian mathematicians (see Niccolò Fontana Tartaglia, Gerolamo Cardano). It was soon realized (but proved much later)^{[21]} that these formulas, even if one was interested only in real solutions, sometimes required the manipulation of square roots of negative numbers. As an example, Tartaglia's formula for a cubic equation of the form *x*^{3} = *px* + *q*^{[c]} gives the solution to the equation *x*^{3} = *x* as

At first glance this looks like nonsense. However, formal calculations with complex numbers show that the equation *z*^{3} = *i* has solutions −*i*,
√3+*i*/2 and
−√3+*i*/2. Substituting these in turn for √−1^{1/3} in Tartaglia's cubic formula and simplifying, one gets 0, 1 and −1 as the solutions of *x*^{3} − *x* = 0. Of course this particular equation can be solved at sight but it does illustrate that when general formulas are used to solve cubic equations with real roots then, as later mathematicians showed rigorously,^{[d]} the use of complex numbers is unavoidable. Rafael Bombelli was the first to address explicitly these seemingly paradoxical solutions of cubic equations and developed the rules for complex arithmetic trying to resolve these issues.

The term "imaginary" for these quantities was coined by René Descartes in 1637, who was at pains to stress their unreal nature^{[22]}

... sometimes only imaginary, that is one can imagine as many as I said in each equation, but sometimes there exists no quantity that matches that which we imagine.

[]

*... quelquefois seulement imaginaires c'est-à-dire que l'on peut toujours en imaginer autant que j'ai dit en chaque équation, mais qu'il n'y a quelquefois aucune quantité qui corresponde à celle qu'on imagine.*

A further source of confusion was that the equation √−1^{2} = √−1√−1 = −1 seemed to be capriciously inconsistent with the algebraic identity √*a*√*b* = √*ab*, which is valid for non-negative real numbers a and b, and which was also used in complex number calculations with one of a, b positive and the other negative. The incorrect use of this identity (and the related identity
1/√*a* = √
1/*a*) in the case when both a and b are negative even bedeviled Euler. This difficulty eventually led to the convention of using the special symbol *i* in place of √−1 to guard against this mistake.^{[citation needed]} Even so, Euler considered it natural to introduce students to complex numbers much earlier than we do today. In his elementary algebra text book, Elements of Algebra, he introduces these numbers almost at once and then uses them in a natural way throughout.

In the 18th century complex numbers gained wider use, as it was noticed that formal manipulation of complex expressions could be used to simplify calculations involving trigonometric functions. For instance, in 1730 Abraham de Moivre noted that the complicated identities relating trigonometric functions of an integer multiple of an angle to powers of trigonometric functions of that angle could be simply re-expressed by the following well-known formula which bears his name, de Moivre's formula:

In 1748 Leonhard Euler went further and obtained Euler's formula of complex analysis:^{[23]}

by formally manipulating complex power series and observed that this formula could be used to reduce any trigonometric identity to much simpler exponential identities.

The idea of a complex number as a point in the complex plane (above) was first described by Danish–Norwegian mathematician Caspar Wessel in 1799,^{[24]} although it had been anticipated as early as 1685 in Wallis's *A Treatise of Algebra*.^{[25]}

Wessel's memoir appeared in the Proceedings of the Copenhagen Academy but went largely unnoticed. In 1806 Jean-Robert Argand independently issued a pamphlet on complex numbers and provided a rigorous proof of the fundamental theorem of algebra.^{[26]} Carl Friedrich Gauss had earlier published an essentially topological proof of the theorem in 1797 but expressed his doubts at the time about "the true metaphysics of the square root of −1".^{[27]} It was not until 1831 that he overcame these doubts and published his treatise on complex numbers as points in the plane,^{[28]}^{[29]}^{: 638 } largely establishing modern notation and terminology.

If one formerly contemplated this subject from a false point of view and therefore found a mysterious darkness, this is in large part attributable to clumsy terminology. Had one not called +1, −1, √−1 positive, negative, or imaginary (or even impossible) units, but instead, say, direct, inverse, or lateral units, then there could scarcely have been talk of such darkness. — Gauss (1831)^{[29]}^{: 638 }^{[28]}

In the beginning of the 19th century, other mathematicians discovered independently the geometrical representation of the complex numbers: Buée,^{[30]}^{[31]} Mourey,^{[32]} Warren,^{[33]} Français and his brother, Bellavitis.^{[34]}^{[35]}

The English mathematician G.H. Hardy remarked that Gauss was the first mathematician to use complex numbers in 'a really confident and scientific way' although mathematicians such as Norwegian Niels Henrik Abel and Carl Gustav Jacob Jacobi were necessarily using them routinely before Gauss published his 1831 treatise.^{[36]}

Augustin Louis Cauchy and Bernhard Riemann together brought the fundamental ideas of complex analysis to a high state of completion, commencing around 1825 in Cauchy's case.

The common terms used in the theory are chiefly due to the founders. Argand called cos *φ* + *i* sin *φ* the *direction factor*, and *r* = √*a*^{2} + *b*^{2} the *modulus*;^{[e]}^{[38]} Cauchy (1821) called cos *φ* + *i* sin *φ* the *reduced form* (l'expression réduite)^{[39]} and apparently introduced the term *argument*; Gauss used *i* for √−1,^{[f]} introduced the term *complex number* for *a* + *bi*,^{[g]} and called *a*^{2} + *b*^{2} the *norm*.^{[h]} The expression *direction coefficient*, often used for cos *φ* + *i* sin *φ*, is due to Hankel (1867),^{[40]} and *absolute value,* for *modulus,* is due to Weierstrass.

Later classical writers on the general theory include Richard Dedekind, Otto Hölder, Felix Klein, Henri Poincaré, Hermann Schwarz, Karl Weierstrass and many others. Important work (including a systematization) in complex multivariate calculus has been started at beginning of the 20th century. Important results have been achieved by Wilhelm Wirtinger in 1927.

Complex numbers have a similar definition of equality to real numbers; two complex numbers *a*_{1} + *b*_{1}*i* and *a*_{2} + *b*_{2}*i* are equal if and only if both their real and imaginary parts are equal, that is, if *a*_{1} = *a*_{2} and *b*_{1} = *b*_{2}. Nonzero complex numbers written in polar form are equal if and only if they have the same magnitude and their arguments differ by an integer multiple of 2*π*.

Unlike the real numbers, there is no natural ordering of the complex numbers. In particular, there is no linear ordering on the complex numbers that is compatible with addition and multiplication – the complex numbers cannot have the structure of an ordered field. This is e.g. because every non-trivial sum of squares in an ordered field is ≠ 0, and *i*^{2} + 1^{2} = 0 is a non-trivial sum of squares.
Thus, complex numbers are naturally thought of as existing on a two-dimensional plane.

The *complex conjugate* of the complex number *z* = *x* + *yi* is given by *x* − *yi*. It is denoted by either z or *z**.^{[41]} This unary operation on complex numbers cannot be expressed by applying only their basic operations addition, subtraction, multiplication and division.

Geometrically, z is the "reflection" of z about the real axis. Conjugating twice gives the original complex number

which makes this operation an involution. The reflection leaves both the real part and the magnitude of z unchanged, that is

The imaginary part and the argument of a complex number z change their sign under conjugation

The product of a complex number *z* = *x* + *yi* and its conjugate is known as the *absolute square*. It is always a non-negative real number and equals the square of the magnitude of each:

This property can be used to convert a fraction with a complex denominator to an equivalent fraction with a real denominator by expanding both numerator and denominator of the fraction by the conjugate of the given denominator. This process is sometimes called "rationalization" of the denominator (although the denominator in the final expression might be an irrational real number), because it resembles the method to remove roots from simple expressions in a denominator.

The real and imaginary parts of a complex number z can be extracted using the conjugation:

Moreover, a complex number is real if and only if it equals its own conjugate.

Conjugation is also employed in inversive geometry, a branch of geometry studying reflections more general than ones about a line. In the network analysis of electrical circuits, the complex conjugate is used in finding the equivalent impedance when the maximum power transfer theorem is looked for.

Two complex numbers a and b are most easily added by separately adding their real and imaginary parts of the summands. That is to say:

Using the visualization of complex numbers in the complex plane, the addition has the following geometric interpretation: the sum of two complex numbers a and b, interpreted as points in the complex plane, is the point obtained by building a parallelogram from the three vertices O, and the points of the arrows labeled a and b (provided that they are not on a line). Equivalently, calling these points A, B, respectively and the fourth point of the parallelogram X the triangles OAB and XBA are congruent. A visualization of the subtraction can be achieved by considering addition of the negative subtrahend.

The rules of the distributive property, the commutative properties (of addition and multiplication), and the defining property *i*^{2} = −1 apply to complex numbers. It follows that

Using the conjugation, the reciprocal of a nonzero complex number *z* = *x* + *yi* can always be broken down to

This can be used to express a division of an arbitrary complex number *w* = *u* + *vi* by a non-zero complex number z as

Formulas for multiplication, division and exponentiation are simpler in polar form than the corresponding formulas in Cartesian coordinates. Given two complex numbers *z*_{1} = *r*_{1}(cos *φ*_{1} + *i* sin *φ*_{1}) and *z*_{2} = *r*_{2}(cos *φ*_{2} + *i* sin *φ*_{2}), because of the trigonometric identities

In other words, the absolute values are multiplied and the arguments are added to yield the polar form of the product. For example, multiplying by *i* corresponds to a quarter-turn counter-clockwise, which gives back *i*^{2} = −1. The picture at the right illustrates the multiplication of

Since the real and imaginary part of 5 + 5*i* are equal, the argument of that number is 45 degrees, or *π*/4 (in radian). On the other hand, it is also the sum of the angles at the origin of the red and blue triangles are arctan(1/3) and arctan(1/2), respectively. Thus, the formula

holds. As the arctan function can be approximated highly efficiently, formulas like this – known as Machin-like formulas – are used for high-precision approximations of π.

The functional equation implies thus that, if x and y are real, one has

which is the decomposition of the exponential function into its real and imaginary parts.

However, because cosine and sine are periodic functions, the addition of an integer multiple of 2*π* to φ does not change z. For example, *e*^{iπ} = *e*^{3iπ} = −1 , so both iπ and 3*iπ* are possible values for the natural logarithm of −1.

Therefore, if the complex logarithm is not to be defined as a multivalued function

one has to use a branch cut and to restrict the codomain, resulting in the bijective function

It seems natural to extend this formula to complex values of x, but there are some difficulties resulting from the fact that the complex logarithm is not really a function, but a multivalued function.

It follows that if z is as above, and if t is another complex number, then the *exponentiation* is the multivalued function

If, in the preceding formula, t is an integer, then the sine and the cosine are independent of k. Thus, if the exponent n is an integer, then *z*^{n} is well defined, and the exponentiation formula simplifies to de Moivre's formula:

While the nth root of a positive real number r is chosen to be the *positive* real number c satisfying *c*^{n} = *r*, there is no natural way of distinguishing one particular complex nth root of a complex number. Therefore, the nth root is a n-valued function of z. This implies that, contrary to the case of positive real numbers, one has

since the left-hand side consists of n values, and the right-hand side is a single value.

These two laws and the other requirements on a field can be proven by the formulas given above, using the fact that the real numbers themselves form a field.

When the underlying field for a mathematical topic or construct is the field of complex numbers, the topic's name is usually modified to reflect that fact. For example: complex analysis, complex matrix, complex polynomial, and complex Lie algebra.

Given any complex numbers (called coefficients) *a*_{0}, ..., *a*_{n}, the equation

There are various proofs of this theorem, by either analytic methods such as Liouville's theorem, or topological ones such as the winding number, or a proof combining Galois theory and the fact that any real polynomial of *odd* degree has at least one real root.

Complex numbers *a* + *bi* can also be represented by 2 × 2 matrices that have the form:

Here the entries a and b are real numbers. As the sum and product of two such matrices is again of this form, these matrices form a subring of the ring 2 × 2 matrices.

is a ring isomorphism from the field of complex numbers to the ring of these matrices. This isomorphism associates the square of the absolute value of a complex number with the determinant of the corresponding matrix, and the conjugate of a complex number with the transpose of the matrix.

The geometric description of the multiplication of complex numbers can also be expressed in terms of rotation matrices by using this correspondence between complex numbers and such matrices. The action of the matrix on a vector (*x*, *y*) corresponds to the multiplication of *x* + *iy* by *a* + *ib*. In particular, if the determinant is 1, there is a real number t such that the matrix has the form:

The study of functions of a complex variable is known as complex analysis and has enormous practical use in applied mathematics as well as in other branches of mathematics. Often, the most natural proofs for statements in real analysis or even number theory employ techniques from complex analysis (see prime number theorem for an example). Unlike real functions, which are commonly represented as two-dimensional graphs, complex functions have four-dimensional graphs and may usefully be illustrated by color-coding a three-dimensional graph to suggest four dimensions, or by animating the complex function's dynamic transformation of the complex plane.

The notions of convergent series and continuous functions in (real) analysis have natural analogs in complex analysis. A sequence of complex numbers is said to converge if and only if its real and imaginary parts do. This is equivalent to the (ε, δ)-definition of limits, where the absolute value of real numbers is replaced by the one of complex numbers. From a more abstract point of view, ℂ, endowed with the metric

is a complete metric space, which notably includes the triangle inequality

Like in real analysis, this notion of convergence is used to construct a number of elementary functions: the *exponential function* exp *z*, also written *e*^{z}, is defined as the infinite series

The series defining the real trigonometric functions sine and cosine, as well as the hyperbolic functions sinh and cosh, also carry over to complex arguments without change. For the other trigonometric and hyperbolic functions, such as tangent, things are slightly more complicated, as the defining series do not converge for all complex values. Therefore, one must define them either in terms of sine, cosine and exponential, or, equivalently, by using the method of analytic continuation.

Unlike in the situation of real numbers, there is an infinitude of complex solutions z of the equation

for any complex number *w* ≠ 0. It can be shown that any such solution z – called complex logarithm of w – satisfies

where arg is the argument defined above, and ln the (real) natural logarithm. As arg is a multivalued function, unique only up to a multiple of 2*π*, log is also multivalued. The principal value of log is often taken by restricting the imaginary part to the interval (−*π*, *π*].

and is multi-valued, except when ω is an integer. For *ω* = 1 / *n*, for some natural number n, this recovers the non-uniqueness of nth roots mentioned above.

Complex numbers, unlike real numbers, do not in general satisfy the unmodified power and logarithm identities, particularly when naïvely treated as single-valued functions; see failure of power and logarithm identities. For example, they do not satisfy

Both sides of the equation are multivalued by the definition of complex exponentiation given here, and the values on the left are a subset of those on the right.

A function *f* : ℂ → ℂ is called holomorphic if it satisfies the Cauchy–Riemann equations. For example, any ℝ-linear map ℂ → ℂ can be written in the form

Complex analysis shows some features not apparent in real analysis. For example, any two holomorphic functions f and g that agree on an arbitrarily small open subset of ℂ necessarily agree everywhere. Meromorphic functions, functions that can locally be written as *f*(*z*)/(*z* − *z*_{0})^{n} with a holomorphic function f, still share some of the features of holomorphic functions. Other functions have essential singularities, such as sin(1/*z*) at *z* = 0.

Complex numbers have applications in many scientific areas, including signal processing, control theory, electromagnetism, fluid dynamics, quantum mechanics, cartography, and vibration analysis. Some of these applications are described below.

As mentioned above, any nonconstant polynomial equation (in complex coefficients) has a solution in ℂ. *A fortiori*, the same is true if the equation has rational coefficients. The roots of such equations are called algebraic numbers – they are a principal object of study in algebraic number theory. Compared to ℚ, the algebraic closure of ℚ, which also contains all algebraic numbers, ℂ has the advantage of being easily understandable in geometric terms. In this way, algebraic methods can be used to study geometric questions and vice versa. With algebraic methods, more specifically applying the machinery of field theory to the number field containing roots of unity, it can be shown that it is not possible to construct a regular nonagon using only compass and straightedge – a purely geometric problem.

Another example are Gaussian integers, that is, numbers of the form *x* + *iy*, where x and y are integers, which can be used to classify sums of squares.

Analytic number theory studies numbers, often integers or rationals, by taking advantage of the fact that they can be regarded as complex numbers, in which analytic methods can be used. This is done by encoding number-theoretic information in complex-valued functions. For example, the Riemann zeta function ζ(*s*) is related to the distribution of prime numbers.

In applied fields, complex numbers are often used to compute certain real-valued improper integrals, by means of complex-valued functions. Several methods exist to do this; see methods of contour integration.

In differential equations, it is common to first find all complex roots r of the characteristic equation of a linear differential equation or equation system and then attempt to solve the system in terms of base functions of the form *f*(*t*) = *e*^{rt}. Likewise, in difference equations, the complex roots r of the characteristic equation of the difference equation system are used, to attempt to solve the system in terms of base functions of the form *f*(*t*) = *r*^{t}.

In control theory, systems are often transformed from the time domain to the frequency domain using the Laplace transform. The system's zeros and poles are then analyzed in the *complex plane*. The root locus, Nyquist plot, and Nichols plot techniques all make use of the complex plane.

In the root locus method, it is important whether zeros and poles are in the left or right half planes, that is, have real part greater than or less than zero. If a linear, time-invariant (LTI) system has poles that are

If a system has zeros in the right half plane, it is a nonminimum phase system.

Complex numbers are used in signal analysis and other fields for a convenient description for periodically varying signals. For given real functions representing actual physical quantities, often in terms of sines and cosines, corresponding complex functions are considered of which the real parts are the original quantities. For a sine wave of a given frequency, the absolute value |*z*| of the corresponding z is the amplitude and the argument arg *z* is the phase.

If Fourier analysis is employed to write a given real-valued signal as a sum of periodic functions, these periodic functions are often written as complex-valued functions of the form

where ω represents the angular frequency and the complex number *A* encodes the phase and amplitude as explained above.

This use is also extended into digital signal processing and digital image processing, which utilize digital versions of Fourier analysis (and wavelet analysis) to transmit, compress, restore, and otherwise process digital audio signals, still images, and video signals.

Another example, relevant to the two side bands of amplitude modulation of AM radio, is:

In electrical engineering, the Fourier transform is used to analyze varying voltages and currents. The treatment of resistors, capacitors, and inductors can then be unified by introducing imaginary, frequency-dependent resistances for the latter two and combining all three in a single complex number called the impedance. This approach is called phasor calculus.

In electrical engineering, the imaginary unit is denoted by j, to avoid confusion with I, which is generally in use to denote electric current, or, more particularly, i, which is generally in use to denote instantaneous electric current.

Since the voltage in an AC circuit is oscillating, it can be represented as

The complex-valued signal *V*(*t*) is called the analytic representation of the real-valued, measurable signal *v*(*t*).
^{[55]}

In fluid dynamics, complex functions are used to describe potential flow in two dimensions.

The complex number field is intrinsic to the mathematical formulations of quantum mechanics, where complex Hilbert spaces provide the context for one such formulation that is convenient and perhaps most standard. The original foundation formulas of quantum mechanics – the Schrödinger equation and Heisenberg's matrix mechanics – make use of complex numbers.

In special and general relativity, some formulas for the metric on spacetime become simpler if one takes the time component of the spacetime continuum to be imaginary. (This approach is no longer standard in classical relativity, but is used in an essential way in quantum field theory.) Complex numbers are essential to spinors, which are a generalization of the tensors used in relativity.

Just as by applying the construction to reals the property of ordering is lost, properties familiar from real and complex numbers vanish with each extension. The quaternions lose commutativity, that is, *x*·*y* ≠ *y*·*x* for some quaternions *x*, *y*, and the multiplication of octonions, additionally to not being commutative, fails to be associative: (*x*·*y*)·*z* ≠ *x*·(*y*·*z*) for some octonions *x*, *y*, *z*.

for some fixed complex number w can be represented by a 2 × 2 matrix (once a basis has been chosen). With respect to the basis (1, *i*), this matrix is

has the property that its square is the negative of the identity matrix: *J*^{2} = −*I*. Then