Wirtinger derivatives

In complex analysis of one and several complex variables, Wirtinger derivatives (sometimes also called Wirtinger operators ), named after Wilhelm Wirtinger who introduced them in 1927 in the course of his studies on the theory of functions of several complex variables, are partial differential operators of the first order which behave in a very similar manner to the ordinary derivatives with respect to one real variable, when applied to holomorphic functions, antiholomorphic functions or simply differentiable functions on complex domains. These operators permit the construction of a differential calculus for such functions that is entirely analogous to the ordinary differential calculus for functions of real variables.

Early days (1899–1911): the work of Henri Poincaré
Wirtinger derivatives were used in complex analysis at least as early as in the paper, as briefly noted by and by. In the third paragraph of his 1899 paper, Henri Poincaré first defines the complex variable in $$\Complex^n$$ and its complex conjugate as follows


 * $$\begin{cases}

x_k+iy_k=z_k\\ x_k-iy_k=u_k \end{cases} \qquad 1 \leqslant k \leqslant n.$$

Then he writes the equation defining the functions $$V$$ he calls biharmonique, previously written using partial derivatives with respect to the real variables $$x_k, y_q$$ with $$k, q$$ ranging from 1 to $$n$$, exactly in the following way


 * $$\frac{d^2 V}{dz_k \, du_q}=0$$

This implies that he implicitly used $$ below: to see this it is sufficient to compare equations 2 and 2' of. Apparently, this paper was not noticed by early researchers in the theory of functions of several complex variables: in the papers of, (and ) and of  all fundamental partial differential operators of the theory are expressed directly by using partial derivatives respect to the real and imaginary parts of the complex variables involved. In the long survey paper by (first published in 1913), partial derivatives with respect to each complex variable of a holomorphic function of several complex variables seem to be meant as formal derivatives: as a matter of fact when Osgood expresses the pluriharmonic operator and the Levi operator, he follows the established practice of Amoroso, Levi and Levi-Civita.

The work of Dimitrie Pompeiu in 1912 and 1913: a new formulation
According to, a new step in the definition of the concept was taken by Dimitrie Pompeiu: in the paper , given a complex valued differentiable function (in the sense of real analysis) of one complex variable $$g(z)$$ defined in the neighbourhood of a given point $$z_0 \in \Complex,$$ he defines the areolar derivative as the following limit


 * $${\frac{\partial g}{\partial \bar{z}}(z_0)}\mathrel{\overset{\mathrm{def}}{=}}\lim_{r \to 0}\frac{1}{2\pi i r^2} \oint_{\Gamma(z_0,r)} g(z)\mathrm{d}z,$$

where $$\Gamma(z_0,r)=\partial D(z_0,r)$$ is the boundary of a disk of radius $$r$$ entirely contained in the domain of definition of $$g(z),$$ i.e. his bounding circle. This is evidently an alternative definition of Wirtinger derivative respect to the complex conjugate variable: it is a more general one, since, as noted a by, the limit may exist for functions that are not even differentiable at $$z=z_0.$$ According to , the first to identify the areolar derivative as a weak derivative in the sense of Sobolev was Ilia Vekua. In his following paper, uses this newly defined concept in order to introduce his generalization of Cauchy's integral formula, the now called Cauchy–Pompeiu formula.

The work of Wilhelm Wirtinger
The first systematic introduction of Wirtinger derivatives seems due to Wilhelm Wirtinger in the paper in order to simplify the calculations of quantities occurring in the theory of functions of several complex variables: as a result of the introduction of these differential operators, the form of all the differential operators commonly used in the theory, like the Levi operator and the Cauchy–Riemann operator, is considerably simplified and consequently easier to handle. The paper is deliberately written from a formal point of view, i.e. without giving a rigorous derivation of the properties deduced.

Formal definition
Despite their ubiquitous use, it seems that there is no text listing all the properties of Wirtinger derivatives: however, fairly complete references are the short course on multidimensional complex analysis by, the monograph of , and the monograph of which are used as general references in this and the following sections.

Functions of one complex variable
$$ Consider the complex plane $$\Complex \equiv \R^2 = \{(x,y) \mid x, y \in \R \}$$ (in a sense of expressing a complex number $$z = x + iy$$ for real numbers $$x$$ and $$y$$). The Wirtinger derivatives are defined as the following linear partial differential operators of first order:


 * $$\begin{align}

\frac{\partial}{\partial z} &= \frac{1}{2} \left( \frac{\partial}{\partial x} - i \frac{\partial}{\partial y} \right) \\ \frac{\partial}{\partial\bar{z}} &= \frac{1}{2} \left( \frac{\partial}{\partial x} + i \frac{\partial}{\partial y} \right) \end{align}$$

Clearly, the natural domain of definition of these partial differential operators is the space of $C^1$ functions on a domain $$\Omega \subseteq \R^2,$$ but, since these operators are linear and have constant coefficients, they can be readily extended to every space of generalized functions.

Functions of n > 1 complex variables
$$ Consider the Euclidean space on the complex field $$\Complex^n = \R^{2n} = \left\{\left( \mathbf{x}, \mathbf{y} \right) = \left(x_1,\ldots,x_n, y_1, \ldots, y_n\right) \mid \mathbf{x},\mathbf{y} \in \R^n \right\}.$$ The Wirtinger derivatives are defined as the following linear partial differential operators of first order: $$\begin{cases} \frac{\partial}{\partial z_1} = \frac{1}{2} \left( \frac{\partial}{\partial x_1}- i \frac{\partial}{\partial y_1} \right) \\ \qquad \vdots \\ \frac{\partial}{\partial z_n} = \frac{1}{2} \left( \frac{\partial}{\partial x_n}- i \frac{\partial}{\partial y_n} \right) \\ \end{cases}, \qquad

\begin{cases} \frac{\partial}{\partial\bar{z}_1} = \frac{1}{2} \left( \frac{\partial}{\partial x_1}+ i \frac{\partial}{\partial y_1} \right) \\ \qquad \vdots \\ \frac{\partial}{\partial\bar{z}_n} = \frac{1}{2} \left( \frac{\partial}{\partial x_n}+ i \frac{\partial}{\partial y_n} \right) \\ \end{cases}.$$

As for Wirtinger derivatives for functions of one complex variable, the natural domain of definition of these partial differential operators is again the space of $C^1$ functions on a domain $$\Omega \subset \R^{2n},$$ and again, since these operators are linear and have constant coefficients, they can be readily extended to every space of generalized functions.

Relation with complex differentiation
When a function $$f$$ is complex differentiable at a point, the Wirtinger derivative $$\partial f/\partial z$$ agrees with the complex derivative $$df/dz$$. This follows from the Cauchy-Riemann equations. For the complex function $$f(z) = u(z) + iv(z)$$ which is complex differentiable


 * $$\begin{align}

\frac{\partial f}{\partial z} &= \frac{1}{2} \left( \frac{\partial f}{\partial x} - i \frac{\partial f}{\partial y} \right) \\ &= \frac{1}{2} \left( \frac{\partial u}{\partial x} + i \frac{\partial v}{\partial x} -i \frac{\partial u}{\partial y} + \frac{\partial v}{\partial y} \right) \\ &= \frac{\partial u}{\partial z} + i \frac{\partial v}{\partial z} = \frac{d f}{d z} \end{align}$$

where the third equality uses the Cauchy-Riemann equations $$\frac{\partial u}{\partial x} = \frac{\partial v}{\partial y}, \frac{\partial u}{\partial y} = - \frac{\partial v}{\partial x}$$.

The second Wirtinger derivative is also related with complex differentiation; $$\frac{\partial f}{\partial \bar{z}} = 0$$ is equivalent to the Cauchy-Riemann equations in a complex form.

Basic properties
In the present section and in the following ones it is assumed that $$z \in \Complex^n$$ is a complex vector and that $$z \equiv (x,y) = (x_1,\ldots,x_n,y_1,\ldots,y_n)$$ where $$x,y$$ are real vectors, with n ≥ 1: also it is assumed that the subset $$\Omega$$ can be thought of as a domain in the real euclidean space $$\R^{2n}$$ or in its isomorphic complex counterpart $$\Complex^n.$$ All the proofs are easy consequences of $$ and $$ and of the corresponding properties of the derivatives (ordinary or partial).

Linearity
$$ If $$f,g \in C^1(\Omega)$$ and $$\alpha,\beta$$ are complex numbers, then for $$i=1,\dots,n$$ the following equalities hold


 * $$\begin{align}

\frac{\partial}{\partial z_i} \left(\alpha f+\beta g\right) &= \alpha\frac{\partial f}{\partial z_i} + \beta\frac{\partial g}{\partial z_i} \\ \frac{\partial}{\partial\bar{z}_i} \left(\alpha f+\beta g\right) &= \alpha\frac{\partial f}{\partial\bar{z}_i} + \beta\frac{\partial g}{\partial\bar{z}_i} \end{align}$$

Product rule
$$ If $$f,g \in C^1(\Omega),$$ then for $$i= 1,\dots,n$$ the product rule holds


 * $$\begin{align}

\frac{\partial}{\partial z_i} (f\cdot g) &= \frac{\partial f}{\partial z_i}\cdot g + f\cdot\frac{\partial g}{\partial z_i} \\ \frac{\partial}{\partial\bar{z}_i} (f\cdot g) &= \frac{\partial f}{\partial\bar{z}_i}\cdot g + f\cdot\frac{\partial g}{\partial\bar{z}_i} \end{align}$$

This property implies that Wirtinger derivatives are derivations from the abstract algebra point of view, exactly like ordinary derivatives are.

Chain rule
This property takes two different forms respectively for functions of one and several complex variables: for the n > 1 case, to express the chain rule in its full generality it is necessary to consider two domains $$\Omega'\subseteq\Complex^m$$ and $$\Omega\subseteq\Complex^p$$ and two maps $$g: \Omega'\to\Omega $$ and $$f:\Omega \to \Omega$$ having natural smoothness requirements.

Functions of one complex variable
$$ If $$f,g \in C^1(\Omega),$$ and $$g(\Omega) \subseteq \Omega,$$ then the chain rule holds


 * $$\begin{align}

\frac{\partial}{\partial z} (f\circ g) &= \left(\frac{\partial f}{\partial z}\circ g \right) \frac{\partial g}{\partial z} + \left(\frac{\partial f}{\partial\bar{z}}\circ g \right) \frac{\partial\bar{g}}{\partial z} \\ \frac{\partial}{\partial\bar{z}} (f\circ g) &= \left(\frac{\partial f}{\partial z}\circ g \right)\frac{\partial g}{\partial\bar{z}}+ \left(\frac{\partial f}{\partial\bar{z}}\circ g \right) \frac{\partial\bar{g}}{\partial\bar{z}} \end{align}$$

Functions of n > 1 complex variables
$$ If $$g \in C^1(\Omega',\Omega)$$ and $$f \in C^1(\Omega,\Omega''),$$ then for $$i= 1,\dots,m$$ the following form of the chain rule holds


 * $$\begin{align}

\frac{\partial}{\partial z_i} \left(f\circ g\right) &= \sum_{j=1}^n\left(\frac{\partial f}{\partial z_j}\circ g \right) \frac{\partial g_j}{\partial z_i} + \sum_{j=1}^n\left(\frac{\partial f}{\partial\bar{z}_j}\circ g \right) \frac{\partial \bar{g}_j}{\partial z_i} \\ \frac{\partial}{\partial\bar{z}_i} \left(f\circ g\right) &= \sum_{j=1}^n\left(\frac{\partial f}{\partial z_j}\circ g \right) \frac{\partial g_j}{\partial\bar{z}_i} + \sum_{j=1}^n\left(\frac{\partial f}{\partial\bar{z}_j}\circ g \right)\frac{\partial \bar{g}_j}{\partial\bar{z}_i} \end{align}$$

Conjugation
$$ If $$f\in C^1(\Omega),$$ then for $$i=1,\dots,n$$ the following equalities hold


 * $$\begin{align}

\overline{\left(\frac{\partial f}{\partial z_i}\right)} &= \frac{\partial \bar{f}}{\partial \bar{z}_i} \\ \overline{\left(\frac{\partial f}{\partial \bar{z}_i}\right)} &= \frac{\partial \bar{f}}{\partial z_i} \end{align}$$

Historical references

 * . "On a boundary value problem" (free translation of the title) is the first paper where a set of (fairly complicate) necessary and sufficient conditions for the solvability of the Dirichlet problem for holomorphic functions of several variables is given.
 * . "Areolar derivative and functions of bounded variation" (free English translation of the title) is an important reference paper in the theory of areolar derivatives.
 * . "Studies on essential singular points of analytic functions of two or more complex variables" (English translation of the title) is an important paper in the theory of functions of several complex variables, where the problem of determining what kind of hypersurface can be the boundary of a domain of holomorphy.
 * . "On the hypersurfaces of the 4-dimensional space that can be the boundary of the domain of existence of an analytic function of two complex variables" (English translation of the title) is another important paper in the theory of functions of several complex variables, investigating further the theory started in.
 * . "On the functions of two or more complex variables" (free English translation of the title) is the first paper where a sufficient condition for the solvability of the Cauchy problem for holomorphic functions of several complex variables is given.
 * , available at DigiZeitschriften.
 * , available at DigiZeitschriften. In this important paper, Wirtinger introduces several important concepts in the theory of functions of several complex variables, namely Wirtinger's derivatives and the tangential Cauchy-Riemann condition.
 * , available at DigiZeitschriften.
 * , available at DigiZeitschriften. In this important paper, Wirtinger introduces several important concepts in the theory of functions of several complex variables, namely Wirtinger's derivatives and the tangential Cauchy-Riemann condition.
 * , available at DigiZeitschriften. In this important paper, Wirtinger introduces several important concepts in the theory of functions of several complex variables, namely Wirtinger's derivatives and the tangential Cauchy-Riemann condition.
 * , available at DigiZeitschriften. In this important paper, Wirtinger introduces several important concepts in the theory of functions of several complex variables, namely Wirtinger's derivatives and the tangential Cauchy-Riemann condition.
 * , available at DigiZeitschriften. In this important paper, Wirtinger introduces several important concepts in the theory of functions of several complex variables, namely Wirtinger's derivatives and the tangential Cauchy-Riemann condition.
 * , available at DigiZeitschriften. In this important paper, Wirtinger introduces several important concepts in the theory of functions of several complex variables, namely Wirtinger's derivatives and the tangential Cauchy-Riemann condition.

Scientific references

 * . Introduction to complex analysis is a short course in the theory of functions of several complex variables, held in February 1972 at the Centro Linceo Interdisciplinare di Scienze Matematiche e Loro Applicazioni "Beniamino Segre".
 * . "Elementary introduction to the theory of functions of complex variables with particular regard to integral representations" (English translation of the title) are the notes form a course, published by the Accademia Nazionale dei Lincei, held by Martinelli when he was "Professore Linceo".
 * ISBN 978-0-387-97195-7. A textbook on complex analysis including many historical notes on the subject.
 * . Notes from a course held by Francesco Severi at the Istituto Nazionale di Alta Matematica (which at present bears his name), containing appendices of Enzo Martinelli, Giovanni Battista Rizza and Mario Benedicty. An English translation of the title reads as:-"Lectures on analytic functions of several complex variables – Lectured in 1956–57 at the Istituto Nazionale di Alta Matematica in Rome".
 * . "Elementary introduction to the theory of functions of complex variables with particular regard to integral representations" (English translation of the title) are the notes form a course, published by the Accademia Nazionale dei Lincei, held by Martinelli when he was "Professore Linceo".
 * ISBN 978-0-387-97195-7. A textbook on complex analysis including many historical notes on the subject.
 * . Notes from a course held by Francesco Severi at the Istituto Nazionale di Alta Matematica (which at present bears his name), containing appendices of Enzo Martinelli, Giovanni Battista Rizza and Mario Benedicty. An English translation of the title reads as:-"Lectures on analytic functions of several complex variables – Lectured in 1956–57 at the Istituto Nazionale di Alta Matematica in Rome".
 * . "Elementary introduction to the theory of functions of complex variables with particular regard to integral representations" (English translation of the title) are the notes form a course, published by the Accademia Nazionale dei Lincei, held by Martinelli when he was "Professore Linceo".
 * ISBN 978-0-387-97195-7. A textbook on complex analysis including many historical notes on the subject.
 * . Notes from a course held by Francesco Severi at the Istituto Nazionale di Alta Matematica (which at present bears his name), containing appendices of Enzo Martinelli, Giovanni Battista Rizza and Mario Benedicty. An English translation of the title reads as:-"Lectures on analytic functions of several complex variables – Lectured in 1956–57 at the Istituto Nazionale di Alta Matematica in Rome".
 * . Notes from a course held by Francesco Severi at the Istituto Nazionale di Alta Matematica (which at present bears his name), containing appendices of Enzo Martinelli, Giovanni Battista Rizza and Mario Benedicty. An English translation of the title reads as:-"Lectures on analytic functions of several complex variables – Lectured in 1956–57 at the Istituto Nazionale di Alta Matematica in Rome".