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In
mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, the complex plane is the
plane Plane(s) most often refers to: * Aero- or airplane, a powered, fixed-wing aircraft * Plane (geometry), a flat, 2-dimensional surface Plane or planes may also refer to: Biology * Plane (tree) or ''Platanus'', wetland native plant * ''Planes' ...
formed by the
complex number In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the form ...
s, with a Cartesian coordinate system such that the -axis, called the real axis, is formed by the
real number In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every real ...
s, and the -axis, called the imaginary axis, is formed by the
imaginary number An imaginary number is a real number multiplied by the imaginary unit , is usually used in engineering contexts where has other meanings (such as electrical current) which is defined by its property . The square of an imaginary number is . Fo ...
s. The complex plane allows a geometric interpretation of complex numbers. Under addition, they add like
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
s. The multiplication of two complex numbers can be expressed more easily in
polar coordinates In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...
—the magnitude or ''modulus'' of the product is the product of the two
absolute value In mathematics, the absolute value or modulus of a real number x, is the non-negative value without regard to its sign. Namely, , x, =x if is a positive number, and , x, =-x if x is negative (in which case negating x makes -x positive), an ...
s, or moduli, and the
angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''Side (plane geometry), sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), vertex'' of the angle. Angles formed by two ...
or ''argument'' of the product is the sum of the two angles, or arguments. In particular, multiplication by a complex number of modulus 1 acts as a rotation. The complex plane is sometimes known as the Argand plane or Gauss plane.


Notational conventions


Complex numbers

In
complex analysis Complex analysis, traditionally known as the theory of functions of a complex variable, is the branch of mathematical analysis that investigates Function (mathematics), functions of complex numbers. It is helpful in many branches of mathemati ...
, the complex numbers are customarily represented by the symbol ''z'', which can be separated into its real (''x'') and imaginary (''y'') parts: z = x + iy for example: ''z'' = 4 + 5''i'', where ''x'' and ''y'' are real numbers, and ''i'' is the
imaginary unit The imaginary unit or unit imaginary number () is a solution to the quadratic equation x^2+1=0. Although there is no real number with this property, can be used to extend the real numbers to what are called complex numbers, using addition an ...
. In this customary notation the complex number ''z'' corresponds to the point (''x'', ''y'') in the
Cartesian plane A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in ...
. In the Cartesian plane the point (''x'', ''y'') can also be represented in
polar coordinates In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to th ...
as (x, y) = (r\cos\theta, r\sin\theta)\qquad(r, \theta) = \left(\sqrt, \quad \arctan\frac\right). In the Cartesian plane it may be assumed that the arctangent takes values from −''π''/2 to ''π''/2 (in
radian The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before tha ...
s), and some care must be taken to define the more complete arctangent function for points (''x'', ''y'') when ''x'' ≤ 0. In the complex plane these polar coordinates take the form z = x + iy = , z, \left(\cos\theta + i\sin\theta\right) = , z, e^ where , z, = \sqrt; \quad \theta = \arg(z) = \frac\ln\frac = -i\ln\frac. Here , ''z'', is the ''absolute value'' or ''modulus'' of the complex number ''z''; ''θ'', the ''argument'' of ''z'', is usually taken on the interval ; and the last equality (to , ''z'', ''e''''iθ'') is taken from
Euler's formula Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the fundamental relationship between the trigonometric functions and the complex exponential function. Euler's formula states that fo ...
. Without the constraint on the range of ''θ'', the argument of ''z'' is multi-valued, because the complex exponential function is periodic, with period 2''π i''. Thus, if ''θ'' is one value of arg(''z''), the other values are given by , where ''n'' is any non-zero integer. While seldom used explicitly, the geometric view of the complex numbers is implicitly based on its structure of a Euclidean vector space of dimension 2, where the
inner product In mathematics, an inner product space (or, rarely, a Hausdorff pre-Hilbert space) is a real vector space or a complex vector space with an operation called an inner product. The inner product of two vectors in the space is a scalar, often ...
of complex numbers and is given by \Re(w\overline); then for a complex number its absolute value coincides with its Euclidean norm, and its argument with the angle turning from 1 to . The theory of
contour integration In the mathematical field of complex analysis, contour integration is a method of evaluating certain integrals along paths in the complex plane. Contour integration is closely related to the calculus of residues, a method of complex analysis. ...
comprises a major part of complex analysis. In this context, the direction of travel around a closed curve is important – reversing the direction in which the curve is traversed multiplies the value of the integral by −1. By convention the ''positive'' direction is counterclockwise. For example, the
unit circle In mathematics, a unit circle is a circle of unit radius—that is, a radius of 1. Frequently, especially in trigonometry, the unit circle is the circle of radius 1 centered at the origin (0, 0) in the Cartesian coordinate system in the Eucli ...
is traversed in the positive direction when we start at the point ''z'' = 1, then travel up and to the left through the point ''z'' = ''i'', then down and to the left through −1, then down and to the right through −''i'', and finally up and to the right to ''z'' = 1, where we started. Almost all of complex analysis is concerned with
complex functions Complex analysis, traditionally known as the theory of functions of a complex variable, is the branch of mathematical analysis that investigates functions of complex numbers. It is helpful in many branches of mathematics, including algebrai ...
 – that is, with functions that map some subset of the complex plane into some other (possibly overlapping, or even identical) subset of the complex plane. Here it is customary to speak of the
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined **Domain of definition of a partial function **Natural domain of a partial function **Domain of holomorphy of a function * Do ...
of ''f''(''z'') as lying in the ''z''-plane, while referring to the
range Range may refer to: Geography * Range (geographic), a chain of hills or mountains; a somewhat linear, complex mountainous or hilly area (cordillera, sierra) ** Mountain range, a group of mountains bordered by lowlands * Range, a term used to i ...
of ''f''(''z'') as a set of points in the ''w''-plane. In symbols we write z = x + iy; \qquad f(z) = w = u + iv and often think of the function ''f'' as a transformation from the ''z''-plane (with coordinates (''x'', ''y'')) into the ''w''-plane (with coordinates (''u'', ''v'')).


Complex plane notation

Complex plane is denoted as \mathbb.


Argand diagram

Argand diagram refers to a geometric
plot Plot or Plotting may refer to: Art, media and entertainment * Plot (narrative), the story of a piece of fiction Music * ''The Plot'' (album), a 1976 album by jazz trumpeter Enrico Rava * The Plot (band), a band formed in 2003 Other * ''Plot' ...
of complex numbers as points using the ''x''-axis as the real axis and the ''y''-axis as the imaginary axis. Such plots are named after
Jean-Robert Argand Jean-Robert Argand (, , ; July 18, 1768 – August 13, 1822) was an amateur mathematician. In 1806, while managing a bookstore in Paris, he published the idea of geometrical interpretation of complex numbers known as the Argand diagram and is know ...
(1768–1822), although they were first described by Norwegian–Danish land surveyor and mathematician
Caspar Wessel Caspar Wessel (8 June 1745, Vestby – 25 March 1818, Copenhagen) was a Danish– Norwegian mathematician and cartographer. In 1799, Wessel was the first person to describe the geometrical interpretation of complex numbers as points in the comp ...
(1745–1818). Argand diagrams are frequently used to plot the positions of the
zeros and poles In complex analysis (a branch of mathematics), a pole is a certain type of singularity of a complex-valued function of a complex variable. In some sense, it is the simplest type of singularity. Technically, a point is a pole of a function if ...
of a function in the complex plane.


Stereographic projections

It can be useful to think of the complex plane as if it occupied the surface of a sphere. Given a
sphere A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
of unit radius, place its center at the origin of the complex plane, oriented so that the equator on the sphere coincides with the unit circle in the plane, and the north pole is "above" the plane. We can establish a
one-to-one correspondence In mathematics, a bijection, also known as a bijective function, one-to-one correspondence, or invertible function, is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other ...
between the points on the surface of the sphere minus the north pole and the points in the complex plane as follows. Given a point in the plane, draw a straight line connecting it with the north pole on the sphere. That line will intersect the surface of the sphere in exactly one other point. The point will be projected onto the south pole of the sphere. Since the interior of the unit circle lies inside the sphere, that entire region () will be mapped onto the southern hemisphere. The unit circle itself () will be mapped onto the equator, and the exterior of the unit circle () will be mapped onto the northern hemisphere, minus the north pole. Clearly this procedure is reversible – given any point on the surface of the sphere that is not the north pole, we can draw a straight line connecting that point to the north pole and intersecting the flat plane in exactly one point. Under this stereographic projection the north pole itself is not associated with any point in the complex plane. We perfect the one-to-one correspondence by adding one more point to the complex plane – the so-called ''
point at infinity In geometry, a point at infinity or ideal point is an idealized limiting point at the "end" of each line. In the case of an affine plane (including the Euclidean plane), there is one ideal point for each pencil of parallel lines of the plane. Ad ...
'' – and identifying it with the north pole on the sphere. This topological space, the complex plane plus the point at infinity, is known as the
extended complex plane In mathematics, the Riemann sphere, named after Bernhard Riemann, is a model of the extended complex plane: the complex plane plus one point at infinity. This extended plane represents the extended complex numbers, that is, the complex numbers ...
. We speak of a single "point at infinity" when discussing complex analysis. There are two points at infinity (positive, and negative) on the
real number line In elementary mathematics, a number line is a picture of a graduated straight line that serves as visual representation of the real numbers. Every point of a number line is assumed to correspond to a real number, and every real number to a poin ...
, but there is only one point at infinity (the north pole) in the extended complex plane. Imagine for a moment what will happen to the lines of latitude and longitude when they are projected from the sphere onto the flat plane. The lines of latitude are all parallel to the equator, so they will become perfect circles centered on the origin . And the lines of longitude will become straight lines passing through the origin (and also through the "point at infinity", since they pass through both the north and south poles on the sphere). This is not the only possible yet plausible stereographic situation of the projection of a sphere onto a plane consisting of two or more values. For instance, the north pole of the sphere might be placed on top of the origin in a plane that is tangent to the circle. The details don't really matter. Any stereographic projection of a sphere onto a plane will produce one "point at infinity", and it will map the lines of latitude and longitude on the sphere into circles and straight lines, respectively, in the plane.


Cutting the plane

When discussing functions of a complex variable it is often convenient to think of a cut in the complex plane. This idea arises naturally in several different contexts.


Multi-valued relationships and branch points

Consider the simple two-valued relationship w = f(z) = \pm\sqrt = z^. Before we can treat this relationship as a single-valued
function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...
, the range of the resulting value must be restricted somehow. When dealing with the square roots of non-negative real numbers this is easily done. For instance, we can just define y = g(x) = \sqrt = x^ to be the non-negative real number ''y'' such that . This idea doesn't work so well in the two-dimensional complex plane. To see why, let's think about the way the value of ''f''(''z'') varies as the point ''z'' moves around the unit circle. We can write z = re^\quad\text\quad w=z^ = \sqrt\,e^\qquad(0\leq\theta\leq 2\pi). Evidently, as ''z'' moves all the way around the circle, ''w'' only traces out one-half of the circle. So one continuous motion in the complex plane has transformed the positive square root ''e''0 = 1 into the negative square root . This problem arises because the point ''z'' = 0 has just one square root, while every other complex number ''z'' ≠ 0 has exactly two square roots. On the real number line we could circumvent this problem by erecting a "barrier" at the single point ''x'' = 0. A bigger barrier is needed in the complex plane, to prevent any closed contour from completely encircling the
branch point In the mathematical field of complex analysis, a branch point of a multi-valued function (usually referred to as a "multifunction" in the context of complex analysis) is a point such that if the function is n-valued (has n values) at that point, ...
''z'' = 0. This is commonly done by introducing a branch cut; in this case the "cut" might extend from the point ''z'' = 0 along the positive real axis to the point at infinity, so that the argument of the variable ''z'' in the cut plane is restricted to the range 0 ≤ arg(''z'') < 2''π''. We can now give a complete description of . To do so we need two copies of the ''z''-plane, each of them cut along the real axis. On one copy we define the square root of 1 to be , and on the other we define the square root of 1 to be ''e''''iπ'' = −1. We call these two copies of the complete cut plane ''sheets''. By making a continuity argument we see that the (now single-valued) function maps the first sheet into the upper half of the ''w''-plane, where , while mapping the second sheet into the lower half of the ''w''-plane (where ).See , pp. 113–119. The branch cut in this example doesn't have to lie along the real axis. It doesn't even have to be a straight line. Any continuous curve connecting the origin ''z'' = 0 with the point at infinity would work. In some cases the branch cut doesn't even have to pass through the point at infinity. For example, consider the relationship w = g(z) = \left(z^2 - 1\right)^. Here the polynomial ''z''2 − 1 vanishes when , so ''g'' evidently has two branch points. We can "cut" the plane along the real axis, from −1 to 1, and obtain a sheet on which ''g''(''z'') is a single-valued function. Alternatively, the cut can run from ''z'' = 1 along the positive real axis through the point at infinity, then continue "up" the negative real axis to the other branch point, ''z'' = −1. This situation is most easily visualized by using the stereographic projection described above. On the sphere one of these cuts runs longitudinally through the southern hemisphere, connecting a point on the equator (''z'' = −1) with another point on the equator (''z'' = 1), and passing through the south pole (the origin, ''z'' = 0) on the way. The second version of the cut runs longitudinally through the northern hemisphere and connects the same two equatorial points by passing through the north pole (that is, the point at infinity).


Restricting the domain of meromorphic functions

A
meromorphic function In the mathematical field of complex analysis, a meromorphic function on an open subset ''D'' of the complex plane is a function that is holomorphic on all of ''D'' ''except'' for a set of isolated points, which are poles of the function. The ...
is a complex function that is
holomorphic In mathematics, a holomorphic function is a complex-valued function of one or more complex variables that is complex differentiable in a neighbourhood of each point in a domain in complex coordinate space . The existence of a complex derivati ...
and therefore analytic everywhere in its domain except at a finite, or countably infinite, number of points. The points at which such a function cannot be defined are called the
pole Pole may refer to: Astronomy *Celestial pole, the projection of the planet Earth's axis of rotation onto the celestial sphere; also applies to the axis of rotation of other planets *Pole star, a visible star that is approximately aligned with the ...
s of the meromorphic function. Sometimes all of these poles lie in a straight line. In that case mathematicians may say that the function is "holomorphic on the cut plane". Here's a simple example. The
gamma function In mathematics, the gamma function (represented by , the capital letter gamma from the Greek alphabet) is one commonly used extension of the factorial function to complex numbers. The gamma function is defined for all complex numbers except ...
, defined by \Gamma (z) = \frac \prod_^\infty \left left(1+\frac\right)^e^\right/math> where ''γ'' is the
Euler–Mascheroni constant Euler's constant (sometimes also called the Euler–Mascheroni constant) is a mathematical constant usually denoted by the lowercase Greek letter gamma (). It is defined as the limiting difference between the harmonic series and the natural l ...
, and has simple poles at 0, −1, −2, −3, ... because exactly one denominator in the
infinite product In mathematics, for a sequence of complex numbers ''a''1, ''a''2, ''a''3, ... the infinite product : \prod_^ a_n = a_1 a_2 a_3 \cdots is defined to be the limit of the partial products ''a''1''a''2...''a'n'' as ''n'' increases without bound. ...
vanishes when ''z'' is zero, or a negative integer. Since all its poles lie on the negative real axis, from ''z'' = 0 to the point at infinity, this function might be described as "holomorphic on the cut plane, the cut extending along the negative real axis, from 0 (inclusive) to the point at infinity." Alternatively, Γ(''z'') might be described as "holomorphic in the cut plane with −''π'' < arg(''z'') < ''π'' and excluding the point ''z'' = 0." This cut is slightly different from the branch cut we've already encountered, because it actually ''excludes'' the negative real axis from the cut plane. The branch cut left the real axis connected with the cut plane on one side (0 ≤ ''θ''), but severed it from the cut plane along the other side (''θ'' < 2''π''). Of course, it's not actually necessary to exclude the entire line segment from ''z'' = 0 to −∞ to construct a domain in which Γ(''z'') is holomorphic. All we really have to do is puncture the plane at a countably infinite set of points . But a closed contour in the punctured plane might encircle one or more of the poles of Γ(''z''), giving a
contour integral In the mathematical field of complex analysis, contour integration is a method of evaluating certain integrals along paths in the complex plane. Contour integration is closely related to the calculus of residues, a method of complex analysis. ...
that is not necessarily zero, by the
residue theorem In complex analysis, the residue theorem, sometimes called Cauchy's residue theorem, is a powerful tool to evaluate line integrals of analytic functions over closed curves; it can often be used to compute real integrals and infinite series as wel ...
. By cutting the complex plane we ensure not only that Γ(''z'') is holomorphic in this restricted domain – we also ensure that the contour integral of Γ over any closed curve lying in the cut plane is identically equal to zero.


Specifying convergence regions

Many complex functions are defined by
infinite series In mathematics, a series is, roughly speaking, a description of the operation of adding infinitely many quantities, one after the other, to a given starting quantity. The study of series is a major part of calculus and its generalization, math ...
, or by
continued fractions In mathematics, a continued fraction is an expression obtained through an iterative process of representing a number as the sum of its integer part and the reciprocal of another number, then writing this other number as the sum of its integer ...
. A fundamental consideration in the analysis of these infinitely long expressions is identifying the portion of the complex plane in which they converge to a finite value. A cut in the plane may facilitate this process, as the following examples show. Consider the function defined by the infinite series f(z) = \sum_^\infty \left(z^2 + n\right)^. Since ''z''2 = (−''z'')2 for every complex number ''z'', it's clear that ''f''(''z'') is an even function of ''z'', so the analysis can be restricted to one half of the complex plane. And since the series is undefined when z^2 + n = 0 \quad \iff \quad z = \pm i\sqrt, it makes sense to cut the plane along the entire imaginary axis and establish the convergence of this series where the real part of ''z'' is not zero before undertaking the more arduous task of examining ''f''(''z'') when ''z'' is a pure imaginary number. In this example the cut is a mere convenience, because the points at which the infinite sum is undefined are isolated, and the ''cut'' plane can be replaced with a suitably ''punctured'' plane. In some contexts the cut is necessary, and not just convenient. Consider the infinite periodic continued fraction f(z) = 1 + \cfrac. It can be shown that ''f''(''z'') converges to a finite value if and only if ''z'' is not a negative real number such that . In other words, the convergence region for this continued fraction is the cut plane, where the cut runs along the negative real axis, from − to the point at infinity.See , p. 39.


Gluing the cut plane back together

We have already seen how the relationship w = f(z) = \pm\sqrt = z^ can be made into a single-valued function by splitting the domain of ''f'' into two disconnected sheets. It is also possible to "glue" those two sheets back together to form a single Riemann surface on which can be defined as a holomorphic function whose image is the entire ''w''-plane (except for the point ). Here's how that works. Imagine two copies of the cut complex plane, the cuts extending along the positive real axis from to the point at infinity. On one sheet define , so that , by definition. On the second sheet define , so that , again by definition. Now flip the second sheet upside down, so the imaginary axis points in the opposite direction of the imaginary axis on the first sheet, with both real axes pointing in the same direction, and "glue" the two sheets together (so that the edge on the first sheet labeled "" is connected to the edge labeled "" on the second sheet, and the edge on the second sheet labeled "" is connected to the edge labeled "" on the first sheet). The result is the Riemann surface domain on which is single-valued and holomorphic (except when ). To understand why ''f'' is single-valued in this domain, imagine a circuit around the unit circle, starting with on the first sheet. When we are still on the first sheet. When we have crossed over onto the second sheet, and are obliged to make a second complete circuit around the branch point before returning to our starting point, where is equivalent to , because of the way we glued the two sheets together. In other words, as the variable ''z'' makes two complete turns around the branch point, the image of ''z'' in the ''w''-plane traces out just one complete circle. Formal differentiation shows that f(z) = z^ \quad\Rightarrow\quad f' (z) = \tfrac z^ from which we can conclude that the derivative of ''f'' exists and is finite everywhere on the Riemann surface, except when (that is, ''f'' is holomorphic, except when ). How can the Riemann surface for the function w = g(z) = \left(z^2 - 1\right)^, also discussed above, be constructed? Once again we begin with two copies of the ''z''-plane, but this time each one is cut along the real line segment extending from to – these are the two branch points of ''g''(''z''). We flip one of these upside down, so the two imaginary axes point in opposite directions, and glue the corresponding edges of the two cut sheets together. We can verify that ''g'' is a single-valued function on this surface by tracing a circuit around a circle of unit radius centered at . Commencing at the point on the first sheet we turn halfway around the circle before encountering the cut at . The cut forces us onto the second sheet, so that when ''z'' has traced out one full turn around the branch point , ''w'' has taken just one-half of a full turn, the sign of ''w'' has been reversed (since ), and our path has taken us to the point on the second sheet of the surface. Continuing on through another half turn we encounter the other side of the cut, where , and finally reach our starting point ( on the first sheet) after making two full turns around the branch point. The natural way to label in this example is to set on the first sheet, with on the second. The imaginary axes on the two sheets point in opposite directions so that the counterclockwise sense of positive rotation is preserved as a closed contour moves from one sheet to the other (remember, the second sheet is ''upside down''). Imagine this surface embedded in a three-dimensional space, with both sheets parallel to the ''xy''-plane. Then there appears to be a vertical hole in the surface, where the two cuts are joined together. What if the cut is made from down the real axis to the point at infinity, and from , up the real axis until the cut meets itself? Again a Riemann surface can be constructed, but this time the "hole" is horizontal. Topologically speaking, both versions of this Riemann surface are equivalent – they are
orientable In mathematics, orientability is a property of some topological spaces such as real vector spaces, Euclidean spaces, surfaces, and more generally manifolds that allows a consistent definition of "clockwise" and "counterclockwise". A space i ...
two-dimensional surfaces of
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
one.


Use in control theory

In
control theory Control theory is a field of mathematics that deals with the control of dynamical systems in engineered processes and machines. The objective is to develop a model or algorithm governing the application of system inputs to drive the system to a ...
, one use of the complex plane is known as the ''
s-plane In mathematics, the Laplace transform, named after its discoverer Pierre-Simon Laplace (), is an integral transform that converts a function of a real variable (usually t, in the ''time domain'') to a function of a complex variable s (in the com ...
''. It is used to visualise the roots of the equation describing a system's behaviour (the characteristic equation) graphically. The equation is normally expressed as a polynomial in the parameter 's' of the
Laplace transform In mathematics, the Laplace transform, named after its discoverer Pierre-Simon Laplace (), is an integral transform In mathematics, an integral transform maps a function from its original function space into another function space via integra ...
, hence the name 's' plane. Points in the s-plane take the form s = \sigma + j\omega, where '''j''' is used instead of the usual '''i to represent the imaginary component. Another related use of the complex plane is with the
Nyquist stability criterion In control theory and stability theory, the Nyquist stability criterion or Strecker–Nyquist stability criterion, independently discovered by the German electrical engineer at Siemens in 1930 and the Swedish-American electrical engineer Harry ...
. This is a geometric principle which allows the stability of a closed-loop feedback system to be determined by inspecting a
Nyquist plot In control theory and stability theory, the Nyquist stability criterion or Strecker–Nyquist stability criterion, independently discovered by the German electrical engineer at Siemens in 1930 and the Swedish-American electrical engineer Har ...
of its open-loop magnitude and phase response as a function of frequency (or loop
transfer function In engineering, a transfer function (also known as system function or network function) of a system, sub-system, or component is a mathematical function that theoretically models the system's output for each possible input. They are widely used ...
) in the complex plane. The ''z-plane'' is a
discrete-time In mathematical dynamics, discrete time and continuous time are two alternative frameworks within which variables that evolve over time are modeled. Discrete time Discrete time views values of variables as occurring at distinct, separate "po ...
version of the s-plane, where
z-transform In mathematics and signal processing, the Z-transform converts a discrete-time signal, which is a sequence of real or complex numbers, into a complex frequency-domain (z-domain or z-plane) representation. It can be considered as a discrete-tim ...
s are used instead of the Laplace transformation.


Quadratic spaces

The complex plane is associated with two distinct
quadratic space In mathematics, a quadratic form is a polynomial with terms all of degree two ("form" is another name for a homogeneous polynomial). For example, :4x^2 + 2xy - 3y^2 is a quadratic form in the variables and . The coefficients usually belong to ...
s. For a point in the complex plane, the squaring function ''z''2 and the norm-squared x^2 + y^2 are both quadratic forms. The former is frequently neglected in the wake of the latter's use in setting a
metric Metric or metrical may refer to: * Metric system, an internationally adopted decimal system of measurement * An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement Mathematics In mathem ...
on the complex plane. These distinct faces of the complex plane as a quadratic space arise in the construction of
algebras over a field In mathematics, an algebra over a field (often simply called an algebra) is a vector space equipped with a bilinear product. Thus, an algebra is an algebraic structure consisting of a set together with operations of multiplication and addition a ...
with the Cayley–Dickson process. That procedure can be applied to any
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grass ...
, and different results occur for the fields R and C: when R is the take-off field, then C is constructed with the quadratic form x^2 + y^2 , but the process can also begin with C and ''z''2, and that case generates algebras that differ from those derived from R. In any case, the algebras generated are
composition algebra In mathematics, a composition algebra over a field is a not necessarily associative algebra over together with a nondegenerate quadratic form that satisfies :N(xy) = N(x)N(y) for all and in . A composition algebra includes an involution ...
s; in this case the complex plane is the point set for two distinct composition algebras.


Other meanings of "complex plane"

The preceding sections of this article deal with the complex plane in terms of a geometric representation of the complex numbers. Although this usage of the term "complex plane" has a long and mathematically rich history, it is by no means the only mathematical concept that can be characterized as "the complex plane". There are at least three additional possibilities. #Two-dimensional complex vector space, a "complex plane" in the sense that it is a two-dimensional vector space whose coordinates are ''complex numbers''. See also: . #(1 + 1)-dimensional
Minkowski space In mathematical physics, Minkowski space (or Minkowski spacetime) () is a combination of three-dimensional Euclidean space and time into a four-dimensional manifold where the spacetime interval between any two events is independent of the inerti ...
, also known as the split-complex plane, is a "complex plane" in the sense that the algebraic split-complex numbers can be separated into two real components that are easily associated with the point in the Cartesian plane. #The set of
dual number In algebra, the dual numbers are a hypercomplex number system first introduced in the 19th century. They are expressions of the form , where and are real numbers, and is a symbol taken to satisfy \varepsilon^2 = 0 with \varepsilon\neq 0. Du ...
s over the reals can also be placed into one-to-one correspondence with the points of the Cartesian plane, and represent another example of a "complex plane".


See also

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Complex coordinate space In mathematics, the ''n''-dimensional complex coordinate space (or complex ''n''-space) is the set of all ordered ''n''-tuples of complex numbers. It is denoted \Complex^n, and is the ''n''-fold Cartesian product of the complex plane \Complex wi ...
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Constellation diagram A constellation diagram is a representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying. It displays the signal as a two-dimensional ''xy''-plane scatter diagram in the ...
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Riemann sphere In mathematics, the Riemann sphere, named after Bernhard Riemann, is a model of the extended complex plane: the complex plane plus one point at infinity. This extended plane represents the extended complex numbers, that is, the complex numbers ...
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s-plane In mathematics, the Laplace transform, named after its discoverer Pierre-Simon Laplace (), is an integral transform that converts a function of a real variable (usually t, in the ''time domain'') to a function of a complex variable s (in the com ...
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In-phase and quadrature components In electrical engineering, a sinusoid with angle modulation can be decomposed into, or synthesized from, two amplitude-modulated sinusoids that are offset in phase by one-quarter cycle (90 degrees or /2 radians). All three functions have the s ...
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Real line In elementary mathematics, a number line is a picture of a graduated straight line (geometry), line that serves as visual representation of the real numbers. Every point of a number line is assumed to correspond to a real number, and every real ...


Notes


References


Works Cited

* * * Reprinted (1973) by Chelsea Publishing Company . *


External links

* * Jean-Robert Argand, "Essai sur une manière de représenter des quantités imaginaires dans les constructions géométriques", 1806, online and analyzed o
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