In

^{''iθ''}) is taken from

^{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 ''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
$$w\; =\; g(z)\; =\; \backslash left(z^2\; -\; 1\backslash right)^.$$
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).

^{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
$$z^2\; +\; n\; =\; 0\; \backslash quad\; \backslash iff\; \backslash quad\; z\; =\; \backslash pm\; i\backslash sqrt,$$
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
$$f(z)\; =\; 1\; +\; \backslash cfrac.$$
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.

'j''' is used instead of the usual '''i to represent the imaginary component.
Another related use of the complex plane is with the

^{2} and the norm-squared $x^2\; +\; y^2$ are both ^{2}, and that case generates algebras that differ from those derived from R. In any case, the algebras generated are composition algebras; in this case the complex plane is the point set for two distinct composition algebras.

''BibNum''

or English version, click 'à télécharger'/small> {{Complex numbers Complex analysis Complex numbers Classical control theory

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 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 a ...

s, with a Cartesian coordinate system
A Cartesian coordinate system (, ) in a plane (geometry), plane is a coordinate system that specifies each point (geometry), point uniquely by a pair of number, numerical coordinates, which are the positive and negative numbers, signed distance ...

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 measurement, measure a ''continuous'' one-dimensional quantity such as a distance, time, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small var ...

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 (algebra), square of an imagina ...

s.
The complex plane allows a geometric interpretation of complex numbers. Under addition
Addition (usually signified by the Plus and minus signs#Plus sign, plus symbol ) is one of the four basic Operation (mathematics), operations of arithmetic, the other three being subtraction, multiplication and Division (mathematics), division. ...

, they add like vectors. The multiplication
Multiplication (often denoted by the Multiplication sign, cross symbol , by the mid-line #Notation and terminology, dot operator , by juxtaposition, or, on computers, by an asterisk ) is one of the four Elementary arithmetic, elementary Op ...

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 (mathematics), point on a plane (mathematics), plane is determined by a distance from a reference point and an angle from a reference direction ...

—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 (mathematics), sign. Namely, , x, =x if is a positive number, and , x, =-x if x is negative number, negative (in which cas ...

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

Incomplex 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
In mathematics, a real number is a number that can be used to measurement, measure a ''continuous'' one-dimen ...

. 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
In geometry, a coordinate system is a system that uses one or more numbers, or coordinates, to uniquely determine the Position (geometry), position of the Point (geometry ...

.
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 (mathematics), point on a plane (mathematics), plane is determined by a distance from a reference point and an angle from a reference direction ...

as
$$(x,\; y)\; =\; (r\backslash cos\backslash theta,\; r\backslash sin\backslash theta)\backslash qquad(r,\; \backslash theta)\; =\; \backslash left(\backslash sqrt,\; \backslash quad\; \backslash arctan\backslash frac\backslash right).$$
In the Cartesian plane it may be assumed that the arctangent
In mathematics, the inverse trigonometric functions (occasionally also called arcus functions, antitrigonometric functions or cyclometric functions) are the inverse functions of the trigonometric functions (with suitably restricted Domain of a fu ...

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 that c ...

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
$$z\; =\; x\; +\; iy\; =\; ,\; z,\; \backslash left(\backslash cos\backslash theta\; +\; i\backslash sin\backslash theta\backslash right)\; =\; ,\; z,\; e^$$
where
$$,\; z,\; =\; \backslash sqrt;\; \backslash quad\; \backslash theta\; =\; \backslash arg(z)\; =\; \backslash frac\backslash ln\backslash frac\; =\; -i\backslash ln\backslash frac.$$
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''Euler's formula
Euler's formula, named after Leonhard Euler, is a mathematics, mathematical formula in complex analysis that establishes the fundamental relationship between the trigonometric functions and the complex number, complex exponential function. Euler ...

. 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 space, Hausdorff pre-Hilbert space) is a real vector space or a complex vector space with an operation (mathematics), operation called an inner product. The inner product of two ve ...

of complex numbers and is given by $\backslash Re(w\backslash overline)$; 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 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
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 i ...

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 – 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 of ''f''(''z'') as lying in the ''z''-plane, while referring to the range of ''f''(''z'') as a set of points in the ''w''-plane. In symbols we write
$$z\; =\; x\; +\; iy;\; \backslash qquad\; f(z)\; =\; w\; =\; u\; +\; iv$$
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 $\backslash mathbb$.Argand diagram

Argand diagram refers to a geometric 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 (1768–1822), although they were first described by Norwegian–Danish land surveyor and mathematician Caspar Wessel (1745–1818). Argand diagrams are frequently used to plot the positions of the zeros and poles 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 asphere
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
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 ...

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 (mathematics), pencil of parallel l ...

'' – 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. 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 (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 ...

, 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 $$w\; =\; f(z)\; =\; \backslash pm\backslash sqrt\; =\; z^.$$ Before we can treat this relationship as a single-valued function, 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 $$y\; =\; g(x)\; =\; \backslash sqrt\; =\; x^$$ 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 $$z\; =\; re^\backslash quad\backslash text\backslash quad\; w=z^\; =\; \backslash sqrt\backslash ,e^\backslash qquad(0\backslash leq\backslash theta\backslash leq\; 2\backslash pi).$$ 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''Restricting the domain of meromorphic functions

Ameromorphic function
In the mathematical field of complex analysis, a meromorphic function on an open set, open subset ''D'' of the complex plane is a function (mathematics), function that is holomorphic function, holomorphic on all of ''D'' ''except'' for a set of is ...

is a complex function that is holomorphic and therefore analytic everywhere in its domain except at a finite, or countably infinite
In mathematics, a Set (mathematics), set is countable if either it is finite set, finite or it can be made in one to one correspondence with the set of natural numbers. Equivalently, a set is ''countable'' if there exists an injective function fro ...

, 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 th ...

, defined by
$$\backslash Gamma\; (z)\; =\; \backslash frac\; \backslash prod\_^\backslash infty\; \backslash left;\; href="/html/ALL/s/left(1+\backslash frac\backslash right)^e^\backslash right.html"\; ;"title="left(1+\backslash frac\backslash right)^e^\backslash right">left(1+\backslash frac\backslash right)^e^\backslash right$$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 (letter), gamma ().
It is defined as the limit of a sequence, limiting difference between th ...

, and has simple poles at 0, −1, −2, −3, ... because exactly one denominator in the infinite product 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 that is not necessarily zero, by the residue theorem. 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 byinfinite series
In mathematics, a series is, roughly speaking, a description of the operation of addition, 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 generalizat ...

, or by continued fractions. 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
$$f(z)\; =\; \backslash sum\_^\backslash infty\; \backslash left(z^2\; +\; n\backslash right)^.$$
Since ''z''Gluing the cut plane back together

We have already seen how the relationship $$w\; =\; f(z)\; =\; \backslash pm\backslash sqrt\; =\; z^$$ 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 $$f(z)\; =\; z^\; \backslash quad\backslash Rightarrow\backslash quad\; f\text{'}\; (z)\; =\; \backslash tfrac\; z^$$ 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 $$w\; =\; g(z)\; =\; \backslash left(z^2\; -\; 1\backslash right)^,$$ 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 two-dimensional surfaces ofgenus
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

Incontrol 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 ...

, 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 (mathematics), function of a Real number, real Variable (mathematics), variable (usually t, in the ''time ...

''. 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 that converts a Function (mathematics), function of a Real number, real Variable (mathematics), variable (usually t, in the ''time ...

, hence the name 's' plane. Points in the s-plane take the form $s\; =\; \backslash sigma\; +\; j\backslash omega$, where ''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 of its open-loop magnitude and phase response as a function of frequency (or loop transfer function
In engineering
Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range ...

) in the complex plane.
The ''z-plane'' is a discrete-time 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 number, real or complex numbers, into a complex frequency-domain (z-domain or z-plane) representation.
It can be considered as a d ...

s are used instead of the Laplace transformation.
Quadratic spaces

The complex plane is associated with two distinct quadratic spaces. For a point in the complex plane, the squaring function ''z''quadratic form
In mathematics, a quadratic form is a polynomial with terms all of Degree of a polynomial, degree two ("form (mathematics), form" is another name for a homogeneous polynomial). For example,
:4x^2 + 2xy - 3y^2
is a quadratic form in the variables ...

s. 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 mathema ...

on the complex plane. These distinct faces of the complex plane as a quadratic space arise in the construction of algebras over a field with the Cayley–Dickson process. That procedure can be applied to any field, 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''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)-dimensionalMinkowski space
In mathematical physics, Minkowski space (or Minkowski spacetime) () is a combination of Three-dimensional space, three-dimensional Euclidean space and time into a four-dimensional manifold where the spacetime interval between any two Event (rel ...

, also known as the split-complex plane, is a "complex plane" in the sense that the algebraic split-complex number
In algebra, a split complex number (or hyperbolic number, also perplex number, double number) has two real number components and , and is written z=x+yj, where j^2=1. The ''conjugate'' of is z^*=x-yj. Since j^2=1, the product of a number wi ...

s can be separated into two real components that are easily associated with the point in the Cartesian plane.
#The set of dual numbers 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

* Complex coordinate space *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 Cartesian plane, ''xy''-plane scatter dia ...

* Riemann sphere
* s-plane
In mathematics, the Laplace transform, named after its discoverer Pierre-Simon Laplace (), is an integral transform that converts a Function (mathematics), function of a Real number, real Variable (mathematics), variable (usually t, in the ''time ...

* In-phase and quadrature components
In electrical engineering, a sinusoid with angle modulation can be decomposed into, or synthesized from, two Amplitude modulation, amplitude-modulated sinusoids that are offset in Phase (waves), phase by one-quarter cycle (90 degrees or /2 radian ...

* Real line
In elementary mathematics, a number line is a picture of a graduated straight line
Line most often refers to:
* Line (geometry)
In geometry, a line is an infinitely long object with no width, depth, or curvature. Thus, lines are One-dim ...

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''BibNum''

or English version, click 'à télécharger'/small> {{Complex numbers Complex analysis Complex numbers Classical control theory