Complex number
A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers, and i is a solution of the equation x^{2} = −1. Because no real number satisfies this equation, i is called an imaginary number. For the complex number a + bi, a is called the real part, and b is called the imaginary part. Despite the historical nomenclature "imaginary", complex numbers are regarded in the mathematical sciences as just as "real" as the real numbers, and are fundamental in many aspects of the scientific description of the natural world.^{[1]}^{[2]}
The complex number system can be defined as the algebraic extension of the ordinary real numbers by an imaginary number i.^{[3]} This means that complex numbers can be added, subtracted, and multiplied, as polynomials in the variable i, with the rule i^{2} = −1 imposed. Furthermore, complex numbers can also be divided by nonzero complex numbers. Overall, the complex number system is a field.
Most importantly the complex numbers give rise to the fundamental theorem of algebra: every nonconstant polynomial equation with complex coefficients has a complex solution. This property is true of the complex numbers, but not the reals. The 16th century Italian mathematician Gerolamo Cardano is credited with introducing complex numbers in his attempts to find solutions to cubic equations.^{[4]}
Geometrically, complex numbers extend the concept of the onedimensional number line to the twodimensional complex plane by using the horizontal axis for the real part and the vertical axis for the imaginary part. The complex number a + bi can be identified with the point (a, b) in the complex plane. A complex number whose real part is zero is said to be purely imaginary; the points for these numbers lie on the vertical axis of the complex plane. A complex number whose imaginary part is zero can be viewed as a real number; its point lies on the horizontal axis of the complex plane. Complex numbers can also be represented in polar form, which associates each complex number with its distance from the origin (its magnitude) and with a particular angle known as the argument of this complex number.
Overview
Complex numbers allow solutions to certain equations that have no solutions in real numbers. For example, the equation
has no real solution, since the square of a real number cannot be negative. Complex numbers provide a solution to this problem. The idea is to extend the real numbers with an indeterminate i (sometimes called the imaginary unit) that is taken to satisfy the relation i^{2} = −1, so that solutions to equations like the preceding one can be found. In this case the solutions are −1 + 3i and −1 − 3i, as can be verified using the fact that i^{2} = −1:
According to the fundamental theorem of algebra, all polynomial equations with real or complex coefficients in a single variable have a solution in complex numbers.
Definition
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 + 3i is a complex number.^{[5]}
A complex number may therefore be defined as a polynomial in the single indeterminate i, with the relation i^{2} + 1 = 0 imposed. From this definition, complex numbers can be added or multiplied, using the addition and multiplication for polynomials. Formally, the set of complex numbers is the quotient ring of the polynomial ring in the indeterminate i, by the ideal generated by the polynomial i^{2} + 1 (see below).^{[6]} The set of all complex numbers is denoted by (upright bold) or (blackboard bold).
The real number a is called the real part of the complex number a + bi; the real number b is called the imaginary part of a + bi. By this convention, the imaginary part does not include a factor of i: hence b, not bi, is the imaginary part.^{[7]}^{[8]} The real part of a complex number z is denoted by Re(z) or ℜ(z); the imaginary part of a complex number z is denoted by Im(z) or ℑ(z). For example,
A real number a can be regarded as a complex number a + 0i whose imaginary part is 0. A purely imaginary number bi is a complex number 0 + bi whose real part is zero. It is common to write a for a + 0i and bi for 0 + bi. Moreover, when the imaginary part is negative, it is common to write a − bi with b > 0 instead of a + (−b)i, for example 3 − 4i instead of 3 + (−4)i.
Cartesian form and definition via ordered pairs
A complex number can thus be identified with an ordered pair (Re(z),Im(z)) in the Cartesian plane, an identification sometimes known as the Cartesian form of z. In fact, a complex number can be defined as an ordered pair (a,b), but then rules for addition and multiplication must also be included as part of the definition (see below).^{[9]} William Rowan Hamilton introduced this approach to define the complex number system.^{[10]}
Complex plane
A complex number can be viewed as a point or position vector in a twodimensional Cartesian coordinate system called the complex plane or Argand diagram (see Pedoe 1988 and Solomentsev 2001), named after JeanRobert Argand. The numbers are conventionally plotted using the real part as the horizontal component, and imaginary part as vertical (see Figure 1). These two values used to identify a given complex number are therefore called its Cartesian, rectangular, or algebraic form.
A position vector may also be defined in terms of its magnitude and direction relative to the origin. These are emphasized in a complex number's polar form. Using the polar form of the complex number in calculations may lead to a more intuitive interpretation of mathematical results. 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 corresponds to multiplying their magnitudes and adding their arguments (i.e. the angles they make with the x 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+bi)i = ai+bi^{2} = b+ai.
History in brief
 Main section: History
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 socalled casus irreducibilis). This conundrum led Italian mathematician Gerolamo Cardano to conceive of complex numbers in around 1545,^{[11]} 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 full development of complex numbers. The rules for addition, subtraction, multiplication, and division of complex numbers were developed by the Italian mathematician Rafael Bombelli.^{[12]} 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.
Notation
Because it is a polynomial in the indeterminate i, a + ib may be written instead of a + bi, which is often expedient when b is a radical.^{[13]} In some disciplines, in particular electromagnetism and electrical engineering, j is used instead of i,^{[14]} since i is frequently used for electric current. In these cases complex numbers are written as a + bj or a + jb.
Equality and order relations
Two complex numbers are equal if and only if both their real and imaginary parts are equal. That is, complex numbers and are equal if and only if and . If the complex numbers are written in polar form, they are equal if and only if they have the same argument and the same magnitude.
Because complex numbers are naturally thought of as existing on a twodimensional plane, there is no natural linear ordering on the set of complex numbers. Furthermore, 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 because any square in an ordered field is at least 0, but i^{2} = −1.
Elementary operations
Conjugate
The complex conjugate of the complex number z = x + yi is defined to be x − yi. It is denoted by either or z*.^{[15]}
Geometrically, is the "reflection" of z about the real axis. Conjugating twice gives the original complex number:
The real and imaginary parts of a complex number z can be extracted using the conjugate:
Moreover, a complex number is real if and only if it equals its own conjugate.
Conjugation distributes over the standard arithmetic operations:
Addition and subtraction
Complex numbers are added by separately adding the real and imaginary parts of the summands. That is to say:
Similarly, subtraction is defined by
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 of the complex plane, is the point X obtained by building a parallelogram, three of whose vertices are O, A and B. Equivalently, X is the point such that the triangles with vertices O, A, B, and X, B, A, are congruent.
Multiplication and division
The multiplication of two complex numbers is defined by the following formula:
In particular, the square of i is −1:
The preceding definition of multiplication of general complex numbers follows naturally from this fundamental property of i. Indeed, if i is treated as a number so that di means d times i, the above multiplication rule is identical to the usual rule for multiplying two sums of two terms.

(distributive property)
 (commutative property of addition—the order of the summands can be changed)
 (commutative and distributive properties)
 (fundamental property of i).
The division of two complex numbers is defined in terms of complex multiplication, which is described above, and real division. When at least one of c and d is nonzero, we have
Division can be defined in this way because of the following observation:
As shown earlier, c − di is the complex conjugate of the denominator c + di. At least one of the real part c and the imaginary part d of the denominator must be nonzero for division to be defined. This is called "rationalization" of the denominator (although the denominator in the final expression might be an irrational real number).
Reciprocal
The reciprocal of a nonzero complex number z = x + yi is given by
This formula can be used to compute the multiplicative inverse of a complex number if it is given in rectangular coordinates. Inversive geometry, a branch of geometry studying reflections more general than ones about a line, can also be expressed in terms of complex numbers. In the network analysis of electrical circuits, the complex conjugate is used in finding the equivalent impedance when the maximum power transfer theorem is used.
Square root
The square roots of a + bi (with b ≠ 0) are , where
and
where sgn is the signum function. This can be seen by squaring to obtain a + bi.^{[16]}^{[17]} Here is called the modulus of a + bi, and the square root sign indicates the square root with nonnegative real part, called the principal square root; also where ^{[18]}
Polar form
Absolute value and argument
An alternative way of defining a point P in the complex plane, other than using the x and ycoordinates, is to use the distance of the point from O, the point whose coordinates are (0, 0) (the origin), together with the angle subtended between the positive real axis and the line segment OP in a counterclockwise direction. This idea leads to the polar form of complex numbers.
The absolute value (or modulus or magnitude) of a complex number z = x + yi is^{[19]}
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 complex number is the distance to the origin of the point representing the complex number in the complex plane.
The square of the absolute value is
where is the complex conjugate of
The argument of z (in many applications referred to as the "phase") is the angle of the radius OP with the positive real axis, and is written as . As with the modulus, the argument can be found from the rectangular form :^{[20]}
Normally, as given above, the principal value in the interval (−π,π] is chosen. Values in the range [0,2π) are obtained by adding 2π if the value is negative. The value of φ is expressed in radians in this article. It can increase by any integer multiple of 2π and still give the same angle. Hence, the arg function is sometimes considered as multivalued. The polar angle for the complex number 0 is indeterminate, but arbitrary choice of the angle 0 is common.
The value of φ equals the result of atan2:
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 coordinates from the polar form is done by the formula called trigonometric form
Using Euler's formula this can be written as
Using the cis function, this is sometimes abbreviated to
In angle notation, often used in electronics to represent a phasor with amplitude r and phase φ, it is written as^{[21]}
Multiplication and division in polar form
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 wellknown trigonometric identities
we may derive
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 quarterturn counterclockwise, which gives back i^{2} = −1. The picture at the right illustrates the multiplication of
Since the real and imaginary part of 5 + 5i 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 Machinlike formulas—are used for highprecision approximations of π.
Similarly, division is given by
Exponentiation
Euler's formula
Euler's formula states that, for any real number x,
 ,
where e is the base of the natural logarithm. This can be proved through induction by observing that
and so on, and by considering the Taylor series expansions of e^{ix}, cos x and sin x:
The rearrangement of terms is justified because each series is absolutely convergent.
Natural logarithm
It follows from Euler's formula that, for any complex number z written in polar form,
where r is a nonnegative real number, one possible value for the complex logarithm of z is
Because cosine and sine are periodic functions, other possible values may be obtained. For example, , so both and are two possible values for the natural logarithm of .
To deal with the existence of more than one possible value for a given input, the complex logarithm may be considered a multivalued function, with
Alternatively, a branch cut can be used to define a singlevalued "branch" of the complex logarithm.
Integer and fractional exponents
We may use the identity
to define complex exponentiation, which is likewise multivalued:
When n is an integer, this simplifies to de Moivre's formula:
The nth roots of z are given by
for any integer k satisfying 0 ≤ k ≤ n − 1. Here ^{n}√r is the usual (positive) nth root of the positive real number r. 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 of z is considered as a multivalued function (in z), as opposed to a usual function f, for which f(z) is a uniquely defined number. Formulas such as
(which holds for positive real numbers), do in general not hold for complex numbers.
Properties
Field structure
The set C of complex numbers is a field.^{[22]} Briefly, this means that the following facts hold: first, any two complex numbers can be added and multiplied to yield another complex number. Second, for any complex number z, its additive inverse −z is also a complex number; and third, every nonzero complex number has a reciprocal complex number. Moreover, these operations satisfy a number of laws, for example the law of commutativity of addition and multiplication for any two complex numbers z_{1} and z_{2}:
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.
Unlike the reals, C is not an ordered field, that is to say, it is not possible to define a relation z_{1} < z_{2} that is compatible with the addition and multiplication. In fact, in any ordered field, the square of any element is necessarily positive, so i^{2} = −1 precludes the existence of an ordering on C.^{[23]}
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.
Solutions of polynomial equations
Given any complex numbers (called coefficients) a_{0}, …, a_{n}, the equation
has at least one complex solution z, provided that at least one of the higher coefficients a_{1}, …, a_{n} is nonzero.^{[24]} This is the statement of the fundamental theorem of algebra, of Carl Friedrich Gauss and Jean le Rond d'Alembert. Because of this fact, C is called an algebraically closed field. This property does not hold for the field of rational numbers Q (the polynomial x^{2} − 2 does not have a rational root, since √2 is not a rational number) nor the real numbers R (the polynomial x^{2} + a does not have a real root for a > 0, since the square of x is positive for any real number x).
There are various proofs of this theorem, either by 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.
Because of this fact, theorems that hold for any algebraically closed field, apply to C. For example, any nonempty complex square matrix has at least one (complex) eigenvalue.
Algebraic characterization
The field C has the following three properties: first, it has characteristic 0. This means that 1 + 1 + ⋯ + 1 ≠ 0 for any number of summands (all of which equal one). Second, its transcendence degree over Q, the prime field of C, is the cardinality of the continuum. Third, it is algebraically closed (see above). It can be shown that any field having these properties is isomorphic (as a field) to C. For example, the algebraic closure of Q_{p} also satisfies these three properties, so these two fields are isomorphic (as fields, but not as topological fields).^{[25]} Also, C is isomorphic to the field of complex Puiseux series. However, specifying an isomorphism requires the axiom of choice. Another consequence of this algebraic characterization is that C contains many proper subfields that are isomorphic to C.
Characterization as a topological field
The preceding characterization of C describes only the algebraic aspects of C. That is to say, the properties of nearness and continuity, which matter in areas such as analysis and topology, are not dealt with. The following description of C as a topological field (that is, a field that is equipped with a topology, which allows the notion of convergence) does take into account the topological properties. C contains a subset P (namely the set of positive real numbers) of nonzero elements satisfying the following three conditions:
 P is closed under addition, multiplication and taking inverses.
 If x and y are distinct elements of P, then either x − y or y − x is in P.
 If S is any nonempty subset of P, then S + P = x + P for some x in C.
Moreover, C has a nontrivial involutive automorphism x ↦ x* (namely the complex conjugation), such that x x* is in P for any nonzero x in C.
Any field F with these properties can be endowed with a topology by taking the sets B(x, p) = { y  p − (y − x)(y − x)* ∈ P } as a base, where x ranges over the field and p ranges over P. With this topology F is isomorphic as a topological field to C.
The only connected locally compact topological fields are R and C. This gives another characterization of C as a topological field, since C can be distinguished from R because the nonzero complex numbers are connected, while the nonzero real numbers are not.^{[26]}
Formal construction
Construction as ordered pairs
The set C of complex numbers can be defined as the set R^{2} of ordered pairs (a, b) of real numbers, in which the following rules for addition and multiplication are imposed:^{[27]}
It is then just a matter of notation to express (a, b) as a + bi.
Construction as a quotient field
Though this lowlevel construction does accurately describe the structure of the complex numbers, the following equivalent definition reveals the algebraic nature of C more immediately. This characterization relies on the notion of fields and polynomials. A field is a set endowed with addition, subtraction, multiplication and division operations that behave as is familiar from, say, rational numbers. For example, the distributive law
must hold for any three elements x, y and z of a field. The set R of real numbers does form a field. A polynomial p(X) with real coefficients is an expression of the form
 ,
where the a_{0}, ..., a_{n} are real numbers. The usual addition and multiplication of polynomials endows the set R[X] of all such polynomials with a ring structure. This ring is called the polynomial ring over the real numbers.
The set of complex numbers is defined as the quotient ring R[X]/(X ^{2} + 1).^{[28]} This extension field contains two square roots of −1, namely (the cosets of) X and −X, respectively. (The cosets of) 1 and X form a basis of R[X]/(X ^{2} + 1) as a real vector space, which means that each element of the extension field can be uniquely written as a linear combination in these two elements. Equivalently, elements of the extension field can be written as ordered pairs (a, b) of real numbers. The quotient ring is a field, because X^{2} + 1 is irreducible over R, so the ideal it generates is maximal.
The formulas for addition and multiplication in the ring R[X], modulo the relation X^{2} = 1, correspond to the formulas for addition and multiplication of complex numbers defined as ordered pairs. So the two definitions of the field C are isomorphic (as fields).
Accepting that C is algebraically closed, since it is an algebraic extension of R in this approach, C is therefore the algebraic closure of R.
Matrix representation of complex numbers
Complex numbers a + bi can also be represented by 2 × 2 matrices that have the following form:
Here the entries a and b are real numbers. The sum and product of two such matrices is again of this form, and the sum and product of complex numbers corresponds to the sum and product of such matrices, the product being:
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. Moreover, the square of the absolute value of a complex number expressed as a matrix is equal to the determinant of that matrix:
The conjugate corresponds to the transpose of the matrix.
Though this representation of complex numbers with matrices is the most common, many other representations arise from matrices other than that square to the negative of the identity matrix. See the article on 2 × 2 real matrices for other representations of complex numbers.
Complex analysis
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 twodimensional graphs, complex functions have fourdimensional graphs and may usefully be illustrated by colorcoding a threedimensional graph to suggest four dimensions, or by animating the complex function's dynamic transformation of the complex plane.
Complex exponential and related functions
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
for any two complex numbers z_{1} and z_{2}.
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.
Euler's formula states:
for any real number φ, in particular
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 2π, log is also multivalued. The principal value of log is often taken by restricting the imaginary part to the interval (−π,π].
Complex exponentiation z^{ω} is defined as
and is multivalued, except when is an integer. For ω = 1 / n, for some natural number n, this recovers the nonuniqueness 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 singlevalued 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.
Holomorphic functions
A function f : C → C is called holomorphic if it satisfies the Cauchy–Riemann equations. For example, any Rlinear map C → C can be written in the form
with complex coefficients a and b. This map is holomorphic if and only if b = 0. The second summand is realdifferentiable, but does not satisfy the Cauchy–Riemann equations.
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.
Applications
Complex numbers have essential concrete applications in a variety of scientific and related areas such as signal processing, control theory, electromagnetism, fluid dynamics, quantum mechanics, cartography, and vibration analysis. Some applications of complex numbers are:
Control theory
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, i.e. have real part greater than or less than zero. If a linear, timeinvariant (LTI) system has poles that are
 in the right half plane, it will be unstable,
 all in the left half plane, it will be stable,
 on the imaginary axis, it will have marginal stability.
If a system has zeros in the right half plane, it is a nonminimum phase system.
Improper integrals
In applied fields, complex numbers are often used to compute certain realvalued improper integrals, by means of complexvalued functions. Several methods exist to do this; see methods of contour integration.
Fluid dynamics
In fluid dynamics, complex functions are used to describe potential flow in two dimensions.
Dynamic equations
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}.
Electromagnetism and electrical engineering
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, frequencydependent 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
To obtain the measurable quantity, the real part is taken:
The complexvalued signal is called the analytic representation of the realvalued, measurable signal . ^{[29]}
Signal analysis
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 realvalued signal as a sum of periodic functions, these periodic functions are often written as complex valued functions of the form
and
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:
Quantum mechanics
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.
Relativity
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.
Geometry
Shapes
Three noncollinear points in the plane determine the shape of the triangle . Locating the points in the complex plane, this shape of a triangle may be expressed by complex arithmetic as
The shape of a triangle will remain the same, when the complex plane is transformed by translation or dilation (by an affine transformation), corresponding to the intuitive notion of shape, and describing similarity. Thus each triangle is in a similarity class of triangles with the same shape.^{[30]}
Fractals
The Mandelbrot set is an example of a fractal formed on the complex plane by plotting every location where iterating the sequence does not diverge when iterated infinitely. Similarly, Julia sets have the same rules, except where remains constant.
Triangles
Every triangle has a unique Steiner inellipse—an ellipse inside the triangle and tangent to the midpoints of the three sides of the triangle. The foci of a triangle's Steiner inellipse can be found as follows, according to Marden's theorem:^{[31]}^{[32]} Denote the triangle's vertices in the complex plane as a = x_{A} + y_{A}i, b = x_{B} + y_{B}i, and c = x_{C} + y_{C}i. Write the cubic equation , take its derivative, and equate the (quadratic) derivative to zero. Marden's Theorem says that the solutions of this equation are the complex numbers denoting the locations of the two foci of the Steiner inellipse.
Algebraic number theory
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
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 numbertheoretic information in complexvalued functions. For example, the Riemann zeta function ζ(s) is related to the distribution of prime numbers.
History
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 in his calculations, although negative quantities were not conceived of in Hellenistic mathematics and Heron merely replaced it by its positive ( ).^{[33]}
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 that these formulas, even if one was only interested 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 ^{[34]} 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, and . Substituting these in turn for 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, the use of complex numbers is unavoidable. Rafael Bombelli was the first to explicitly address 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, although he was at pains to stress their imaginary nature^{[35]}
[...] 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’estàdire 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 seemed to be capriciously inconsistent with the algebraic identity , which is valid for nonnegative 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 ) 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. 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 reexpressed by the following wellknown formula which bears his name, de Moivre's formula:
In 1748 Leonhard Euler went further and obtained Euler's formula of complex analysis:
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 Caspar Wessel in 1799, although it had been anticipated as early as 1685 in Wallis's De Algebra tractatus.
Wessel's memoir appeared in the Proceedings of the Copenhagen Academy but went largely unnoticed. In 1806 JeanRobert Argand independently issued a pamphlet on complex numbers and provided a rigorous proof of the fundamental theorem of algebra. 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". It was not until 1831 that he overcame these doubts and published his treatise on complex numbers as points in the plane, largely establishing modern notation and terminology. In the beginning of the 19th century, other mathematicians discovered independently the geometrical representation of the complex numbers: Buée, Mourey, Warren, Français and his brother, Bellavitis.^{[36]}
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 Niels Henrik Abel and Carl Gustav Jacob Jacobi were necessarily using them routinely before Gauss published his 1831 treatise.^{[37]} 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 the direction factor, and the modulus; Cauchy (1828) called the reduced form (l'expression réduite) and apparently introduced the term argument; Gauss used i for , introduced the term complex number for a + bi, and called a^{2} + b^{2} the norm. The expression direction coefficient, often used for , is due to Hankel (1867), 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.
Generalizations and related notions
The process of extending the field R of reals to C is known as the Cayley–Dickson construction. It can be carried further to higher dimensions, yielding the quaternions H and octonions O which (as a real vector space) are of dimension 4 and 8, respectively. In this context the complex numbers have been called the binarions.^{[38]}
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, i.e.: 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.
Reals, complex numbers, quaternions and octonions are all normed division algebras over R. By Hurwitz's theorem they are the only ones; the sedenions, the next step in the Cayley–Dickson construction, fail to have this structure.
The Cayley–Dickson construction is closely related to the regular representation of C, thought of as an Ralgebra (an Rvector space with a multiplication), with respect to the basis (1, i). This means the following: the Rlinear map
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
i.e., the one mentioned in the section on matrix representation of complex numbers above. While this is a linear representation of C in the 2 × 2 real matrices, it is not the only one. Any matrix
has the property that its square is the negative of the identity matrix: J^{2} = −I. Then
is also isomorphic to the field C, and gives an alternative complex structure on R^{2}. This is generalized by the notion of a linear complex structure.
Hypercomplex numbers also generalize R, C, H, and O. For example, this notion contains the splitcomplex numbers, which are elements of the ring R[x]/(x^{2} − 1) (as opposed to R[x]/(x^{2} + 1)). In this ring, the equation a^{2} = 1 has four solutions.
The field R is the completion of Q, the field of rational numbers, with respect to the usual absolute value metric. Other choices of metrics on Q lead to the fields Q_{p} of padic numbers (for any prime number p), which are thereby analogous to R. There are no other nontrivial ways of completing Q than R and Q_{p}, by Ostrowski's theorem. The algebraic closures of Q_{p} still carry a norm, but (unlike C) are not complete with respect to it. The completion of turns out to be algebraically closed. This field is called padic complex numbers by analogy.
The fields R and Q_{p} and their finite field extensions, including C, are local fields.
See also
Wikimedia Commons has media related to Complex numbers. 
 Algebraic surface
 Circular motion using complex numbers
 Complexbase system
 Complex geometry
 Complex square root
 Eisenstein integer
 Euler's identity
 Gaussian integer
 Riemann sphere (extended complex plane)
 Root of unity
 Unit complex number
Notes
 ↑ An extensive account of the history, from initial skepticism to ultimate acceptance, can be found in Nicolas Bourbaki, "1. Foundations of mathematics; logic; set theory", Elements of the history of mathematics, Springer, pp. 18&ndash, 24 .
 ↑ Penrose, Roger (2016). The Road to Reality: A Complete Guide to the Laws of the Universe (reprinted ed.). Random House. pp. 72–73. ISBN 9781446418208. Extract of page 73: "complex numbers, as much as reals, and perhaps even more, find a unity with nature that is truly remarkable. It is as though Nature herself is as impressed by the scope and consistency of the complexnumber system as we are ourselves, and has entrusted to these numbers the precise operations of her world at its minutest scales."
 ↑ Nicolas Bourbaki. "VIII.1". General topology. SpringerVerlag.
 ↑ Burton (1995, p. 294)
 ↑ Sheldon Axler (2010). College algebra. Wiley. p. 262.
 ↑ Nicolas Bourbaki. "VIII.1". General topology. SpringerVerlag.
 ↑ Complex Variables (2nd Edition), M.R. Spiegel, S. Lipschutz, J.J. Schiller, D. Spellman, Schaum's Outline Series, Mc Graw Hill (USA), ISBN 9780071615693
 ↑ Aufmann, Richard N.; Barker, Vernon C.; Nation, Richard D. (2007), "Chapter P", College Algebra and Trigonometry (6 ed.), Cengage Learning, p. 66, ISBN 0618825150
 ↑ Tom Apostol (1981). Mathematical analysis. AddisonWesley. pp. 15&ndash, 16.
 ↑ Leo Corry (2015). A Brief History of Numbers. Oxford University Press. pp. 215&ndash, 216.
 ↑ Morris Kline. A history of mathematical thought, volume 1. p. 253.
 ↑ Katz (2004, §9.1.4)
 ↑ For example Ahlfors (1979).
 ↑ Brown, James Ward; Churchill, Ruel V. (1996), Complex variables and applications (6th ed.), New York: McGrawHill, p. 2, ISBN 0079121470,
In electrical engineering, the letter j is used instead of i.
 ↑ For the former notation, see for instance Tom Apostol (1981). Mathematical analysis. AddisonWesley. pp. 15&ndash, 16. .
 ↑ Abramowitz, Milton; Stegun, Irene A. (1964), Handbook of mathematical functions with formulas, graphs, and mathematical tables, Courier Dover Publications, p. 17, ISBN 0486612724 , Section 3.7.26, p. 17
 ↑ Cooke, Roger (2008), Classical algebra: its nature, origins, and uses, John Wiley and Sons, p. 59, ISBN 0470259523 , Extract: page 59
 ↑ Ahlfors (1979, p. 3)
 ↑ Tom Apostol (1981). Mathematical analysis. AddisonWesley. p. 18. .
 ↑ Kasana, H.S. (2005), "Chapter 1", Complex Variables: Theory And Applications (2nd ed.), PHI Learning Pvt. Ltd, p. 14, ISBN 8120326415
 ↑ Nilsson, James William; Riedel, Susan A. (2008), "Chapter 9", Electric circuits (8th ed.), Prentice Hall, p. 338, ISBN 0131989251
 ↑ Tom Apostol (1981). Mathematical analysis. AddisonWesley. pp. 15&ndash, 16.
 ↑ Tom Apostol (1981). Mathematical analysis. AddisonWesley. p. 25.
 ↑ Nicolas Bourbaki. "VIII.1". General topology. SpringerVerlag.
 ↑ Marker, David (1996), "Introduction to the Model Theory of Fields", in Marker, D.; Messmer, M.; Pillay, A., Model theory of fields, Lecture Notes in Logic, 5, Berlin: SpringerVerlag, pp. 1–37, ISBN 3540607412, MR 1477154
 ↑ Nicolas Bourbaki. "VIII.4". General topology. SpringerVerlag.
 ↑ Tom Apostol (1981). Mathematical analysis. AddisonWesley. pp. 15&ndash, 16.
 ↑ Nicolas Bourbaki. "VIII.1". General topology. SpringerVerlag.
 ↑ Electromagnetism (2nd edition), I.S. Grant, W.R. Phillips, Manchester Physics Series, 2008 ISBN 0471927120
 ↑ J.A. Lester (1996) "Triangles I: Shapes", Aequationes Mathematicae 52:30–54
 ↑ Kalman, Dan (2008a), "An Elementary Proof of Marden's Theorem", The American Mathematical Monthly, 115: 330–38, ISSN 00029890
 ↑ Kalman, Dan (2008b), "The Most Marvelous Theorem in Mathematics", Journal of Online Mathematics and its Applications External link in
journal=
(help)  ↑ Nahin, Paul J. (2007), An Imaginary Tale: The Story of √−1, Princeton University Press, ISBN 9780691127989, retrieved 20 April 2011
 ↑ In modern notation, Tartaglia's solution is based on expanding the cube of the sum of two cube roots: With , , , u and v can be expressed in terms of p and q as and , respectively. Therefore, . When is negative (casus irreducibilis), the second cube root should be regarded as the complex conjugate of the first one.
 ↑ Descartes, René (1954) [1637], La Géométrie  The Geometry of René Descartes with a facsimile of the first edition, Dover Publications, ISBN 0486600688, retrieved 20 April 2011
 ↑ Caparrini, Sandro (2000), "On the Common Origin of Some of the Works on the Geometrical Interpretation of Complex Numbers", in Kim Williams (ed.), Two Cultures, Birkhäuser, p. 139, ISBN 3764371862 Extract of page 139
 ↑ Hardy, G. H.; Wright, E. M. (2000) [1938], An Introduction to the Theory of Numbers, OUP Oxford, p. 189 (fourth edition), ISBN 0199219869
 ↑ Kevin McCrimmon (2004) A Taste of Jordan Algebras, pp 64, Universitext, Springer ISBN 0387954473 MR 2014924
References
Mathematical references
 Ahlfors, Lars (1979), Complex analysis (3rd ed.), McGrawHill, ISBN 9780070006577
 Conway, John B. (1986), Functions of One Complex Variable I, Springer, ISBN 0387903283
 Joshi, Kapil D. (1989), Foundations of Discrete Mathematics, New York: John Wiley & Sons, ISBN 9780470211526
 Pedoe, Dan (1988), Geometry: A comprehensive course, Dover, ISBN 0486658120
 Press, WH; Teukolsky, SA; Vetterling, WT; Flannery, BP (2007), "Section 5.5 Complex Arithmetic", Numerical Recipes: The Art of Scientific Computing (3rd ed.), New York: Cambridge University Press, ISBN 9780521880688
 Solomentsev, E.D. (2001) [1994], "Complex number", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 9781556080104
Historical references
 Burton, David M. (1995), The History of Mathematics (3rd ed.), New York: McGrawHill, ISBN 9780070094659
 Katz, Victor J. (2004), A History of Mathematics, Brief Version, AddisonWesley, ISBN 9780321161932
 Nahin, Paul J. (1998), An Imaginary Tale: The Story of
, Princeton University Press, ISBN 0691027951
 A gentle introduction to the history of complex numbers and the beginnings of complex analysis.
 H. D. Ebbinghaus; H. Hermes; F. Hirzebruch; M. Koecher; K. Mainzer; J. Neukirch; A. Prestel; R. Remmert (1991), Numbers (hardcover ed.), Springer, ISBN 0387974970
 An advanced perspective on the historical development of the concept of number.
Further reading
 The Road to Reality: A Complete Guide to the Laws of the Universe, by Roger Penrose; Alfred A. Knopf, 2005; ISBN 0679454438. Chapters 4–7 in particular deal extensively (and enthusiastically) with complex numbers.
 Unknown Quantity: A Real and Imaginary History of Algebra, by John Derbyshire; Joseph Henry Press; ISBN 030909657X (hardcover 2006). A very readable history with emphasis on solving polynomial equations and the structures of modern algebra.
 Visual Complex Analysis, by Tristan Needham; Clarendon Press; ISBN 0198534477 (hardcover, 1997). History of complex numbers and complex analysis with compelling and useful visual interpretations.
 Conway, John B., Functions of One Complex Variable I (Graduate Texts in Mathematics), Springer; 2 edition (12 September 2005). ISBN 0387903283.
External links
Wikiversity has learning resources about Complex Numbers 
Wikibooks has a book on the topic of: Calculus/Complex numbers 
Wikisource has the text of the 1911 Encyclopædia Britannica article Number/Complex Numbers. 
 Hazewinkel, Michiel, ed. (2001) [1994], "Complex number", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 9781556080104
 Introduction to Complex Numbers from Khan Academy
 Imaginary Numbers on In Our Time at the BBC
 Euler's Investigations on the Roots of Equations at Convergence. MAA Mathematical Sciences Digital Library.
 John and Betty's Journey Through Complex Numbers
 Dimensions: a math film. Chapter 5 presents an introduction to complex arithmetic and stereographic projection. Chapter 6 discusses transformations of the complex plane, Julia sets, and the Mandelbrot set.