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Complex Number

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{{wikibookspar|Algebra|Complex numbers}} In mathematics, a '''complex number''' is an expression of the form :$a\; +\; bi\; \backslash ,$ where ''a'' and ''b'' are real numbers, and ''i'' is a specific imaginary number, called the imaginary unit, with the property ''i'' ² = −1. The real number ''a'' is called the ''real part'' of the complex number, and the real number ''b'' is the ''imaginary part''. When the imaginary part ''b'' is 0, the complex number is just the real number ''a''. For example, 3 + 2''i'' is a ''complex number'', with real part 3 and imaginary part 2. Complex numbers can be added, subtracted, multiplied, and divided in a similar way to real numbers; but they have additional elegant properties. For example, every polynomial algebraic equation has a complex number as a solution, not just some, as in the real numbers. In some fields (in particular, electrical engineering, where ''i'' is a symbol for Electric current current), complex numbers are written as ''a'' + ''bj''.

Definitions


Notation and operations
The set of all complex numbers is usually denoted by '''C''', or in blackboard bold by $\backslash mathbb\{C\}$. The real numbers, '''R''', may be regarded as "lying in" '''C''' by considering every real number as a complex: $a\; =\; a\; +\; 0i$. Complex numbers are added, subtracted, and multiplied by formally applying the associative, commutative and distributive laws of algebra, together with the equation ''i'' ² = −1: :(''a'' + ''bi'') + (''c'' + ''di'') = (''a''+''c'') + (''b''+''d'')''i'' :(''a'' + ''bi'') − (''c'' + ''di'') = (''a''−''c'') + (''b''−''d'')''i'' :(''a'' + ''bi'')(''c'' + ''di'') = ''ac'' + ''bci'' + ''adi'' + ''bd i'' ² = (''ac''−''bd'') + (''bc''+''ad'')''i'' Division of complex numbers can also be defined (see below). Thus, the set of complex numbers forms a field (mathematics) field which, in contrast to the real numbers, is algebraically closed. In mathematics, the adjective "complex" means that the field of complex numbers is the underlying number field considered, for example complex analysis, matrix (mathematics) complex matrix, polynomial complex polynomial and Lie algebra complex Lie algebra.

The complex number field
Formally, the complex numbers can be defined as ordered pairs of real numbers (''a'', ''b'') together with the operations: * $(\; a\; ,\; b\; )\; +\; (\; c\; ,\; d\; )\; =\; (\; a\; +\; c\; ,\; b\; +\; d\; )\; \backslash ,$ * $(\; a\; ,\; b\; )\; \backslash cdot\; (\; c\; ,\; d\; )\; =\; (\; ac\; -\; bd\; ,\; bc\; +\; ad\; ).\; \backslash ,$ So defined, the complex numbers form a field (mathematics) field, the complex number field, denoted by '''C'''. Since a complex number ''a'' + ''bi'' is uniquely specified by an ordered pair (''a'', ''b'') of real numbers, the complex numbers are in one-to-one correspondence with points on a plane, called the complex plane. We identify the real number ''a'' with the complex number (''a'', 0), and in this way the field of real numbers '''R''' becomes a subfield of '''C'''. The imaginary unit ''i'' is the complex number (0, 1). In '''C''', we have: * additive identity ("zero"): (0, 0) * multiplicative identity ("one"): (1, 0) * additive inverse of (''a'',''b''): (−''a'', −''b'') * multiplicative inverse (reciprocal) of non-zero (''a'', ''b''): $\backslash left(\{a\backslash over\; a^2+b^2\},\{-b\backslash over\; a^2+b^2\}\backslash right).$ '''C''' can also be defined as the Closure (topology) topological closure of the algebraic numbers or as the algebraic closure of '''R''', both of which are described below.

The complex plane

image:complex.png

A complex number can be viewed as a point or a vector (spatial) position vector on a two-dimensional Cartesian coordinate system called the '''complex plane''' or '''Argand diagram''' (named after Jean-Robert Argand). The Cartesian coordinates of the complex number are the real part ''x'' and the imaginary part ''y'', while the Coordinates (mathematics)#Circular coordinates circular coordinates are ''r'' = |''z''|, called the ''Absolute value#Complex numbers absolute value or modulus'', and φ = arg(''z''), called the ''complex argument'' of ''z'' (mod-arg form). Together with Euler's formula we have :$z\; =\; x\; +\; iy\; =\; r\; (\backslash cos\; \backslash phi\; +\; i\backslash sin\; \backslash phi\; )\; =\; r\; e^\{i\; \backslash phi\}.\; \backslash ,$ Additionally the notation ''r'' cis φ is sometimes used. Note that the complex argument is unique modulo 2π, that is, if any two values of the complex argument exactly differ by an integer multiple of 2π, they are considered equivalent. By simple trigonometric identity trigonometric identities, we see that :$r\_1\; e^\{i\backslash phi\_1\}\; \backslash cdot\; r\_2\; e^\{i\backslash phi\_2\}\; =\; r\_1\; r\_2\; e^\{i(\backslash phi\_1\; +\; \backslash phi\_2)\}\; \backslash ,$ and that :$\backslash frac\{r\_1\; e^\{i\backslash phi\_1\}\}\; \{r\_2\; e^\{i\backslash phi\_2\}\}\; =\; \backslash frac\{r\_1\}\{r\_2\}\; e^\{i\; (\backslash phi\_1\; -\; \backslash phi\_2)\}.\; \backslash ,$ Now the addition of two complex numbers is just the vector space vector addition of two vectors, and the multiplication with a fixed complex number can be seen as a simultaneous rotation and stretching. Multiplication with i corresponds to a counter clockwise rotation by 90 degree (angle) degrees ($\backslash pi/2$ radians). The geometric content of the equation ''i''² = −1 is that a sequence of two 90 degree rotations results in a 180 degree ($\backslash pi$ radians) rotation. Even the fact (−1) · (−1) = +1 from arithmetic can be understood geometrically as the combination of two 180 degree turns.

Absolute value, conjugation and distance
The ''absolute value'' (or ''modulus'' or ''magnitude'') of a complex number ''z'' = ''r e''^''i''φ is defined as |''z''| = ''r''. Algebraically, if ''z'' = ''a'' + ''ib'', then $|\; z\; |\; =\; \backslash sqrt\{a^2+b^2\}.$ One can check readily that the absolute value has three important properties: :$|\; z\; |\; =\; 0\; \backslash ,$ iff $z\; =\; 0\; \backslash ,$ :$|\; z\; +\; w\; |\; \backslash leq\; |\; z\; |\; +\; |\; w\; |\; \backslash ,$ :$|\; z\; w\; |\; =\; |\; z\; |\; \backslash ;\; |\; w\; |\; \backslash ,$ for all complex numbers ''z'' and ''w''. It then follows, for example, that $|\; 1\; |\; =\; 1$ and $|z/w|=|z|/|w|$. By defining the distance function ''d''(''z'', ''w'') = |''z'' − ''w''| we turn the complex numbers into a metric space and we can therefore talk about limit (mathematics) limits and continuous function continuity. The addition, subtraction, multiplication and division of complex numbers are then continuous operations. Unless anything else is said, this is always the metric being used on the complex numbers. The complex conjugate of the complex number ''z'' = ''a'' + ''ib'' is defined to be ''a'' - ''ib'', written as $\backslash bar\{z\}$ or $z^*\backslash ,$. As seen in the figure, $\backslash bar\{z\}$ is the "reflection" of ''z'' about the real axis. The following can be checked: : $\backslash overline\{z+w\}\; =\; \backslash bar\{z\}\; +\; \backslash bar\{w\}$ : $\backslash overline\{zw\}\; =\; \backslash bar\{z\}\backslash bar\{w\}$ : $\backslash overline\{(z/w)\}\; =\; \backslash bar\{z\}/\backslash bar\{w\}$ : $\backslash bar\{\backslash bar\{z\}\}=z$ : $\backslash bar\{z\}=z$   if and only if ''z'' is real : $|z|=|\backslash bar\{z\}|$ : $|z|^2\; =\; z\backslash bar\{z\}$ : $z^\{-1\}\; =\; \backslash bar\{z\}|z|^\{-2\}$   if ''z'' is non-zero. The latter formula is the method of choice to compute the inverse of a complex number if it is given in rectangular coordinates. That conjugation commutes with all the algebraic operations (and many functions; ''e.g.'' $\backslash sin\backslash bar\; z=\backslash overline\{\backslash sin\; z\}$) is rooted in the ambiguity in choice of ''i'' (−1 has two square roots); note, however, that conjugation is not differentiable (see holomorphic).

Complex number division
Given a complex number (''a'' + ''bi'') which is to be divided by another complex number (''c'' + ''di'') whose magnitude is non-zero, there are two ways to do this; in either case it is the same as multiplying the first by the multiplicative inverse of the second. The first way has already been implied: to convert both complex numbers into exponential form, from which their quotient is easy to derive. The second way is to express the division as a fraction, then to multiply both numerator and denominator by the complex conjugate of the denominator. This causes the denominator to simplify into a real number: :$\{a\; +\; bi\; \backslash over\; c\; +\; di\}\; =\; \{(a\; +\; bi)\; (c\; -\; di)\; \backslash over\; (c\; +\; di)\; (c\; -\; di)\}\; =\; \{(ac\; +\; bd)\; +\; (bc\; -\; ad)\; i\; \backslash over\; c^2\; +\; d^2\}$ :::$=\; \backslash left(\{ac\; +\; bd\; \backslash over\; c^2\; +\; d^2\}\backslash right)\; +\; i\backslash left(\; \{bc\; -\; ad\; \backslash over\; c^2\; +\; d^2\}\; \backslash right).$

Matrix representation of complex numbers
While usually not useful, alternative representations of complex fields can give some insight into their nature. One particularly elegant representation interprets every complex number as 2×2 matrix (mathematics) matrix with real number real entries which stretches and rotates the points of the plane. Every such matrix has the form : with real numbers ''a'' and ''b''. The sum and product of two such matrices is again of this form. Every non-zero such matrix is invertible, and its inverse is again of this form. Therefore, the matrices of this form are a field. In fact, this is exactly the field of complex numbers. Every such matrix can be written as : which suggests that we should identify the real number 1 with the matrix : and the imaginary unit ''i'' with : a counter-clockwise rotation by 90 degrees. Note that the square of this latter matrix is indeed equal to −1. The absolute value of a complex number expressed as a matrix is equal to the square root of the determinant of that matrix. If the matrix is viewed as a transformation of a plane, then the transformation rotates points through an angle equal to the argument of the complex number and scales by a factor equal to the complex number's absolute value. The conjugate of the complex number ''z'' corresponds to the transformation which rotates through the same angle as ''z'' but in the opposite direction, and scales in the same manner as ''z''; this can be described by the transpose of the matrix corresponding to ''z''. If the matrix elements are themselves complex numbers, then the resulting algebra is that of the quaternions. In this way, the matrix representation can be seen as a way of expressing the Cayley-Dickson construction of algebras.

Geometric interpretation of the operations on complex numbers
Image:Complex numbers addition.png right|thumb|The point ''X'' is the sum of ''A'' and ''B''. Choose a point in the plane which will be the origin, $0$. Given two points ''A'' and ''B'' in the plane, their ''sum'' is the point ''X'' in the plane such that the triangles with vertices 0, ''A'', ''B'' and ''X'', ''B'', ''A'' are similarity (mathematics)#Similar triangles similar. Image:Complex numbers multiplication.png right|thumb|The point ''X'' is the product of ''A'' and ''B''. Choose in addition a point in the plane different from zero, which will be the unity, 1. Given two points ''A'' and ''B'' in the plane, their ''product'' is the point ''X'' in the plane such that the triangles with vertices 0, 1, ''A'', and ''0'', ''B'', ''X'' are similar. Image:Complex numbers conjugation.png right|thumb|The point ''X'' is the complex conjugate of ''A''. Given a point ''A'' in the plane, its ''complex conjugate'' is a point ''X'' in the plane such that the triangles with vertices 0, 1, ''A'' and 0, 1, ''X'' are mirror image of each other.

Some properties


Real vector space
'''C''' is a two-dimensional real vector space. Unlike the reals, complex numbers cannot be ordered in any way that is compatible with its arithmetic operations: '''C''' cannot be turned into an ordered field. Linear transformation#Definition and first consequences '''R'''-linear maps '''C''' → '''C''' have the general form :$f(z)=az+b\backslash overline\{z\}$ with complex coefficients ''a'' and ''b''. Only the first term is '''C'''-linear; also only the first term is Holomorphic function holomorphic; the second term is real-differentiable, but does not satisfy the Cauchy-Riemann equations. The function :$f(z)=az\backslash ,$ corresponds to rotations combined with scaling, while the function :$f(z)=b\backslash overline\{z\}$ corresponds to reflections combined with scaling.

Solutions of polynomial equations
A ''root'' of the polynomial ''p'' is a complex number ''z'' such that ''p''(''z'') = 0. A most striking result is that all polynomials of degree ''n'' with real or complex coefficients have exactly ''n'' complex roots (counting multiple roots of a polynomial multiple roots according to their multiplicity). This is known as the fundamental theorem of algebra, and shows that the complex numbers are an algebraically closed field. Indeed, the complex number field is the algebraically closed field algebraic closure of the real number field, and Cauchy constructed complex numbers in this way. It can be identified as the quotient ring of the polynomial ring (mathematics) ring '''R'''[''X''] by the Ideal (ring theory) ideal generated by the polynomial ''X''² + 1: :$\backslash mathbb\{C\}\; =\; \backslash mathbb\{R\}[\; X\; ]\; /\; (\; X^2\; +\; 1).\; \backslash ,$ This is indeed a field because ''X''² + 1 is irreducible polynomial irreducible, hence generating a maximal ideal, in '''R'''[''X'']. The image of ''X'' in this quotient ring becomes the imaginary unit ''i''.

Algebraic characterization
The field '''C''' is (up to field isomorphism) characterization (mathematics) characterized by the following three facts: * its characteristic (algebra) characteristic is 0 * its transcendence degree over the prime field is the cardinality of the continuum * it is algebraically closed Consequently, '''C''' contains many proper subfields which are isomorphic to '''C'''. Another consequence of this characterization is that the Galois group of '''C''' over the rational numbers is enormous, with cardinality equal to Beth two that of the power set of the continuum.

Characterization as a topological field
As noted above, the algebraic characterization of '''C''' fails to capture some of its most important properties. These properties, which underpin the foundations of complex analysis, arise from the topology of '''C'''. The following properties characterize '''C''' as a topological ring topological field: *'''C''' is a field. *'''C''' contains a subset ''P'' of nonzero elements satisfying: **''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'''. *'''C''' has a nontrivial involutive automorphism ''x->x*'', fixing ''P'' and such that ''xx*'' is in ''P'' for any nonzero ''x'' in '''C'''. Given these properties, one can then define a topology on '''C''' by taking the sets *$B(x,p)\; =\; \backslash \{y\; |\; p\; -\; (y-x)(y-x)^*\backslash in\; P\backslash \}$ as a base (topology) base, where ''x'' ranges over '''C''', and ''p'' ranges over ''P''. To see that these properties characterize '''C''' as a topological ring topological field, one notes that ''P'' ∪ {0} ∪ ''-P'' is an ordered Dedekind completion Dedekind-complete field and thus can be identified with the real numbers '''R''' by a unique field isomorphism. The last property is easily seen to imply that the Galois group over the real numbers is of order two, completing the characterization. Lev Semenovich Pontryagin Pontryagin has shown that the only connected space connected locally compact topological ring topological fields are '''R''' and '''C'''. This gives another characterization of '''C''' as a topological field, since '''C''' can be distinguished from '''R''' by noting the nonzero complex numbers are connected space connected whereas the nonzero real numbers are not.

Complex analysis
{{details|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 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.

Applications


Control theory
In control theory, systems are often transformed from the time domain to the frequency domain using the Laplace transform. The system's pole (complex analysis) poles and zero (complex analysis) zeros 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 especially important whether the pole (complex analysis) poles and zero (complex analysis) zeros are in the left or right half planes, i.e. have real part greater than or less than zero. If a system has poles that are *in the right half plane, it will be unstable, *all in the left half plane, it will be stability 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.

Signal analysis
Complex numbers are used in signal analysis and other fields as a convenient description for periodically varying signals. The absolute value .html">amplitude and the argument arg(''z'') as the phase_(waves) phase.html" title="Meaning of _is interpreted as the phase">phase of a sine wave of given frequency. 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 the real part of complex valued functions of the form :$f\; (\; t\; )\; =\; z\; e^\{i\backslash omega\; t\}\; \backslash ,$ where ω represents the angular frequency and the complex number ''z'' encodes the phase and amplitude as explained above. In electrical engineering, the Fourier transform is used to analyze varying voltages and current (electricity) 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. (Electrical engineers and some physicists use the letter ''j'' for the imaginary unit since ''i'' is typically reserved for varying currents and may come into conflict with ''i''.) 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, compression compress, restore, and otherwise process digital Sound audio signals, still images, and video signals.

Improper integrals
In applied fields, the use of complex analysis is 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.

Quantum mechanics
The complex number field is also of utmost importance in quantum mechanics since the underlying theory is built on (infinite dimensional) Hilbert spaces over '''C'''.

Relativity
In special relativity special and general relativity, some formulas for the metric on spacetime become simpler if one takes the time variable to be imaginary.

Applied mathematics
In differential equations, it is common to first find all complex roots ''r'' of the characteristic equation of a linear differential equation and then attempt to solve the system in terms of base functions of the form ''f''(''t'') = ''e''^''rt''.

Fluid dynamics
In fluid dynamics, complex functions are used to describe potential flow in 2d.

Fractals
Certain fractals are plotted in the complex plane e.g. Mandelbrot set and Julia set.

History
The earliest fleeting reference to square roots of negative numbers perhaps occurred in the work of the Hellenized Ancient Egypt Egyptian Hellenistic mathematics mathematician and inventor Hero of Alexandria Heron of Alexandria in the 1st century common era CE, when he considered the volume of an impossible frustum of a pyramid {{citation needed}}, though negative numbers were not conceived in the Hellenistic civilization Hellenistic world. In India, the Indian mathematics mathematician and Indian science astronomer Madhava of Sangamagrama in the 14th century gave approximations of transcendental numbers (non-algebraic complex numbers) by means of continued fractions. Complex numbers became more prominent in the 16th century, when closed formulas for the roots of Cube (arithmetic) cubic and quartic polynomials were discovered by Italian mathematicians (see Niccolo 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. For example, Tartaglia's cubic formula gives the following solution to the equation $x^3-x=0$: :$\backslash frac\{1\}\{\backslash sqrt\{3\}\}\backslash left(\backslash sqrt\{-1\}^\{1/3\}+\backslash frac\{1\}\{\backslash sqrt\{-1\}^\{1/3\}\}\backslash right).$ At first glance this looks like nonsense. However formal calculations with complex numbers show that the equation $z^3=i$ has solutions −''i'', $\backslash frac\{\backslash sqrt\{3\}\}\{2\}+\backslash frac\{1\}\{2\}i$ and $\backslash frac\{-\backslash sqrt\{3\}\}\{2\}+\backslash frac\{1\}\{2\}i$. Substituting these in turn for $\backslash sqrt\{-1\}^\{1/3\}$ into the cubic formula and simplifying, one gets 0, 1 and −1 as the solutions of $x^3-x=0.$ This was doubly unsettling since not even negative numbers were considered to be on firm ground at the time. The term "imaginary" for these quantities was coined by René Descartes in 1637 and was meant to be derogatory (see imaginary number for a discussion of the "reality" of complex numbers). A further source of confusion was that the equation $\backslash sqrt\{-1\}^2=\backslash sqrt\{-1\}\backslash sqrt\{-1\}=-1$ seemed to be capriciously inconsistent with the algebraic identity $\backslash sqrt\{a\}\backslash sqrt\{b\}=\backslash sqrt\{ab\}$, which is valid for positive 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 $\backslash frac\{1\}\{\backslash sqrt\{a\}\}=\backslash sqrt\{\backslash frac\{1\}\{a\}\}$) 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 $\backslash sqrt\{-1\}$ to guard against this mistake. The 18th century saw the labors of Abraham de Moivre and Leonhard Euler. To De Moivre is due (1730) the well-known formula which bears his name, de Moivre's formula: :$(\backslash cos\; \backslash theta\; +\; i\backslash sin\; \backslash theta)^\{n\}\; =\; \backslash cos\; n\; \backslash theta\; +\; i\backslash sin\; n\; \backslash theta\; \backslash ,$ and to Euler (1748) Euler's formula of complex analysis: :$\backslash cos\; \backslash theta\; +\; i\backslash sin\; \backslash theta\; =\; e\; ^\{i\backslash theta\; \}.\; \backslash ,$ The existence of complex numbers was not completely accepted until the geometrical interpretation (see below) had been described by Caspar Wessel in 1799; it was rediscovered several years later and popularized by Carl Friedrich Gauss, and as a result the theory of complex numbers received a notable expansion. The idea of the graphic representation of complex numbers had appeared, however, as early as 1685, in John Wallis Wallis's ''De Algebra tractatus''. Wessel's memoir appeared in the Proceedings of the Copenhagen Academy for 1799, and is exceedingly clear and complete, even in comparison with modern works. He also considers the sphere, and gives a quaternion theory from which he develops a complete spherical trigonometry. In 1804 the Abbé Buée independently came upon the same idea which Wallis had suggested, that $\backslash pm\backslash sqrt\{-1\}$ should represent a unit line, and its negative, perpendicular to the real axis. Buée's paper was not published until 1806, in which year Jean-Robert Argand also issued a pamphlet on the same subject. It is to Argand's essay that the scientific foundation for the graphic representation of complex numbers is now generally referred. Nevertheless, in 1831 Gauss found the theory quite unknown, and in 1832 published his chief memoir on the subject, thus bringing it prominently before the mathematical world. Mention should also be made of an excellent little treatise by Mourey (1828), in which the foundations for the theory of directional numbers are scientifically laid. The general acceptance of the theory is not a little due to the labors of Augustin Louis Cauchy and Niels Henrik Abel, and especially the latter, who was the first to boldly use complex numbers with a success that is well known. The common terms used in the theory are chiefly due to the founders. Argand called $\backslash cos\; \backslash phi\; +\; i\backslash sin\; \backslash phi$ the ''direction factor'', and $r\; =\; \backslash sqrt\{a^2+b^2\}$ the ''modulus''; Cauchy (1828) called $\backslash cos\; \backslash phi\; +\; i\backslash sin\; \backslash phi$ the ''reduced form'' (l'expression réduite); Gauss used ''i'' for $\backslash sqrt\{-1\}$, introduced the term ''complex number'' for $a+bi$, and called $a^2+b^2$ the ''norm''. The expression ''direction coefficient'', often used for $\backslash cos\; \backslash phi\; +\; i\; \backslash sin\; \backslash phi$, is due to Hankel (1867), and ''absolute value,'' for ''modulus,'' is due to Weierstrass. Following Cauchy and Gauss have come a number of contributors of high rank, of whom the following may be especially mentioned: Ernst Kummer Kummer (1844), Leopold Kronecker (1845), Scheffler (1845, 1851, 1880), Bellavitis (1835, 1852), Peacock (1845), and Augustus De Morgan De Morgan (1849). August Ferdinand Möbius Möbius must also be mentioned for his numerous memoirs on the geometric applications of complex numbers, and Johann Peter Gustav Lejeune Dirichlet Dirichlet for the expansion of the theory to include primes, congruences, reciprocity, etc., as in the case of real numbers. A complex ring (mathematics) ring or Field (mathematics) field is a set of complex numbers which is closed under addition, subtraction, and multiplication. Carl Friedrich Gauss Gauss studied complex numbers of the form $a\; +\; bi$, where ''a'' and ''b'' are integral, or rational (and ''i'' is one of the two roots of $x^2\; +\; 1\; =\; 0$). His student, Ferdinand Eisenstein, studied the type $a\; +\; b\backslash omega$, where $\backslash omega$ is a complex root of $x^3\; -\; 1\; =\; 0$. Other such classes (called cyclotomic fields) of complex numbers are derived from the roots of unity $x^k\; -\; 1\; =\; 0$ for higher values of $k$. This generalization is largely due to Kummer, who also invented ideal numbers, which were expressed as geometrical entities by Felix Klein in 1893. The general theory of fields was created by Évariste Galois, who studied the fields generated by the roots of any polynomial equation :$\backslash \; F(x)\; =\; 0.$ The late writers (from 1884) on the general theory include Karl Weierstrass Weierstrass, Hermann Schwarz Schwarz, Richard Dedekind, Otto Hölder, Berloty, Henri Poincaré, Eduard Study, and Alexander MacFarlane. The formally correct definition using pairs of real numbers was given in the 19th century.

See also
* Circular motion#Using complex numbers * Complex geometry * De Moivre's formula * Euler's identity * Hypercomplex number * Leonhard Euler * Local field * Mandelbrot Set * Phasor (physics) * Phasor (electronics) * Quaternion * Riemann sphere (extended complex plane) * Split-complex number

Further reading
* ''An Imaginary Tale: The Story of $\backslash sqrt\{-1\}$'', by Paul J. Nahin; Princeton University Press; ISBN 0691027951 (hardcover, 1998). A gentle introduction to the history of complex numbers and the beginnings of complex analysis. * ''Numbers'', by H.-D. Ebbinghaus, H. Hermes, F. Hirzebruch, M. Koecher, K. Mainzer, J. Neukirch, A. Prestel, R. Remmert; Springer; ISBN 0-387-97497 (hardcover, 1991). An advanced perspective on the historical development of the concept of number. * ''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.

External links

- John and Betty's Journey Through Complex Numbers
- Complex Number from MathWorld
- SOS Math - Complex Variables
- Windows calculator that supports complex numbers
- Algebraic Structure of Complex Numbers from cut-the-knot Category:Complex numbers Category:Elementary mathematics bg:КомплекÑ?но чиÑ?ло bn:জটিল সংখà§?যা ca:Nombre complex cs:Komplexní Ä?íslo da:Komplekse tal de:Komplexe Zahl el:Μιγαδικός αÏ?ιθμός et:Kompleksarv es:Número complejo eo:Kompleksa Nombro fa:اعداد مختلط fr:Nombre complexe ko:복소수 is:Tvinntölur it:Numero complesso he:מספר מרוכב lt:Kompleksiniai skaiÄ?iai nl:Complex getal ja:複素数 nb:Komplekst tall pl:Liczby zespolone pt:Número complexo ro:Număr complex ru:КомплекÑ?ное чиÑ?ло scn:Nummuru complessu simple:complex number sl:Kompleksno Å¡tevilo sr:КомплекÑ?ан број fi:Kompleksiluku sv:Komplexa tal th:จำนวนเชิงซ้อน tr:Karmaşık sayılar zh:複數 (數學) hu:Komplex számok ta:கலபà¯?பெணà¯? see Complex number

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