Английская Википедия:Argument (complex analysis)
Шаблон:Short description Шаблон:Redirect-distinguish Шаблон:For Шаблон:Other uses
In mathematics (particularly in complex analysis), the argument of a complex number Шаблон:Mvar, denoted Шаблон:Math, is the angle between the positive real axis and the line joining the origin and Шаблон:Mvar, represented as a point in the complex plane, shown as <math>\varphi</math> in Figure 1. By convention the positive real axis is drawn pointing rightward, the positive imaginary axis is drawn pointing upward, and complex numbers with positive real part are considered to have an anticlockwise argument with positive sign.
When any real-valued angle is considered, the argument is a multivalued function operating on the nonzero complex numbers. The principal value of this function is single-valued, typically chosen to be the unique value of the argument that lies within the interval Шаблон:Open-closed.[1][2] In this article the multi-valued function will be denoted Шаблон:Math and its principal value will be denoted Шаблон:Math, but in some sources the capitalization of these symbols is exchanged.
Definition
An argument of the complex number Шаблон:Math, denoted Шаблон:Math, is defined in two equivalent ways:
- Geometrically, in the complex plane, as the 2D polar angle <math>\varphi</math> from the positive real axis to the vector representing Шаблон:Mvar. The numeric value is given by the angle in radians, and is positive if measured counterclockwise.
- Algebraically, as any real quantity <math>\varphi</math> such that <math display="block">z = r (\cos \varphi + i \sin \varphi) = r e^{i\varphi}</math> for some positive real Шаблон:Mvar (see Euler's formula). The quantity Шаблон:Mvar is the modulus (or absolute value) of Шаблон:Mvar, denoted |Шаблон:Mvar|: <math display="block">r = \sqrt{x^2 + y^2}.</math>
The names magnitude, for the modulus, and phase,[3][1] for the argument, are sometimes used equivalently.
Under both definitions, it can be seen that the argument of any non-zero complex number has many possible values: firstly, as a geometrical angle, it is clear that whole circle rotations do not change the point, so angles differing by an integer multiple of Шаблон:Math radians (a complete circle) are the same, as reflected by figure 2 on the right. Similarly, from the periodicity of [[sine|Шаблон:Math ]] and [[cosine|Шаблон:Math]], the second definition also has this property. The argument of zero is usually left undefined.
Alternative definition
The complex argument can also be defined algebraically in terms of complex roots as: <math display="block">\arg(z) = \lim_{n\to\infty} n\cdot \operatorname{Im}{\sqrt[n]{z/|z|}}</math> This definition removes reliance on other difficult-to-compute functions such as arctangent as well as eliminating the need for the piecewise definition. Because it's defined in terms of roots, it also inherits the principal branch of square root as its own principal branch. The normalization of <math>z</math> by dividing by <math>|z|</math> isn't necessary for convergence to the correct value, but it does speed up convergence and ensures that <math>\arg(0)</math> is left undefined.
Principal value
Because a complete rotation around the origin leaves a complex number unchanged, there are many choices which could be made for <math>\varphi</math> by circling the origin any number of times. This is shown in figure 2, a representation of the multi-valued (set-valued) function <math>f(x,y)=\arg(x+iy)</math>, where a vertical line (not shown in the figure) cuts the surface at heights representing all the possible choices of angle for that point.
When a well-defined function is required, then the usual choice, known as the principal value, is the value in the open-closed interval Шаблон:Open-closed, that is from Шаблон:Math to Шаблон:Math radians, excluding Шаблон:Math rad itself (equiv., from −180 to +180 degrees, excluding −180° itself). This represents an angle of up to half a complete circle from the positive real axis in either direction.
Some authors define the range of the principal value as being in the closed-open interval Шаблон:Closed-open.
Notation
The principal value sometimes has the initial letter capitalized, as in Шаблон:Math, especially when a general version of the argument is also being considered. Note that notation varies, so Шаблон:Math and Шаблон:Math may be interchanged in different texts.
The set of all possible values of the argument can be written in terms of Шаблон:Math as:
- <math>\arg(z) = \{\operatorname{Arg}(z) + 2\pi n \mid n \in \mathbb Z\}.</math>
Computing from the real and imaginary part
Шаблон:Main If a complex number is known in terms of its real and imaginary parts, then the function that calculates the principal value Шаблон:Math is called the [[atan2|two-argument arctangent function, Шаблон:Math]]:
- <math>\operatorname{Arg}(x + iy) = \operatorname{atan2}(y,\, x)</math>.
The Шаблон:Math function is available in the math libraries of many programming languages, sometimes under a different name, and usually returns a value in the range Шаблон:Open-closed.[1]
In some sources the argument is defined as <math>\operatorname{Arg}(x + iy) = \arctan(y/x),</math> however this is correct only when Шаблон:Math, where <math>y/x</math> is well-defined and the angle lies between <math>-\tfrac\pi2</math> and <math>\tfrac\pi2.</math> Extending this definition to cases where Шаблон:Math is not positive is relatively involved. Specifically, one may define the principal value of the argument separately on the half-plane Шаблон:Math and the two quadrants with Шаблон:Math, and then patch the definitions together:
- <math>\operatorname{Arg}(x + iy) = \operatorname{atan2}(y,\, x) =
\begin{cases}
\arctan\left(\frac y x\right) &\text{if } x > 0, \\[5mu]
\arctan\left(\frac y x\right) + \pi &\text{if } x < 0 \text{ and } y \ge 0, \\[5mu]
\arctan\left(\frac y x\right) - \pi &\text{if } x < 0 \text{ and } y < 0, \\[5mu]
+\frac{\pi}{2} &\text{if } x = 0 \text{ and } y > 0, \\[5mu]
-\frac{\pi}{2} &\text{if } x = 0 \text{ and } y < 0, \\[5mu]
\text{undefined} &\text{if } x = 0 \text{ and } y = 0.
\end{cases}</math>
See atan2 for further detail and alternative implementations.
Identities
One of the main motivations for defining the principal value Шаблон:Math is to be able to write complex numbers in modulus-argument form. Hence for any complex number Шаблон:Mvar,
- <math>z = \left| z \right| e^{i \operatorname{Arg} z}.</math>
This is only really valid if Шаблон:Mvar is non-zero, but can be considered valid for Шаблон:Math if Шаблон:Math is considered as an indeterminate form—rather than as being undefined.
Some further identities follow. If Шаблон:Math and Шаблон:Math are two non-zero complex numbers, then
- <math>\begin{align}
\operatorname{Arg}(z_1 z_2) &\equiv \operatorname{Arg}(z_1) + \operatorname{Arg}(z_2) \pmod{\mathbb{R}/2\pi\mathbb{Z}}, \\
\operatorname{Arg}\left(\frac{z_1}{z_2}\right) &\equiv \operatorname{Arg}(z_1) - \operatorname{Arg}(z_2) \pmod{\mathbb{R}/2\pi\mathbb{Z}}.
\end{align}</math>
If Шаблон:Math and Шаблон:Mvar is any integer, then[1]
- <math>\operatorname{Arg}\left(z^n\right) \equiv n \operatorname{Arg}(z) \pmod{\mathbb{R}/2\pi\mathbb{Z}}.</math>
Example
- <math>\operatorname{Arg}\biggl(\frac{-1- i}{i}\biggr) = \operatorname{Arg}(-1 - i) - \operatorname{Arg}(i) = -\frac{3\pi}{4} - \frac{\pi}{2} = -\frac{5\pi}{4}</math>
Using the complex logarithm
From <math>z = |z| e^{i \operatorname{Arg}(z)}</math>, we get <math>i \operatorname{Arg}(z) = \ln \frac{z}{|z|}</math>, alternatively <math>\operatorname{Arg}(z) = Im [ \ln \frac{z}{|z|} ] = Im [ \ln z ]</math>. As we are taking the imaginary part, any normalisation by a real scalar will not affect the result. This is useful when one has the complex logarithm available.
Extended argument
The extended argument of a number z (denoted as <math>\overline{\arg}(z)</math>) is the set of all real numbers congruent to <math>\arg (z)</math> modulo 2<math>\pi</math>.[4]<math display="block">\overline{\arg}(z) = \arg (z) + 2k\pi, \forall k \in \mathbb{Z}</math>
References
Bibliography
External links
- ↑ 1,0 1,1 1,2 1,3 Шаблон:Cite web
- ↑ Шаблон:Cite web
- ↑ Dictionary of Mathematics (2002). phase.
- ↑ Шаблон:Cite web
