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. 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. Wirtinger derivatives were used in complex analysis at least as early as in the paper (Poincaré 1899), as briefly noted by Cherry & Ye (2001, p. 31) and by Remmert (1991, pp. 66–67). As a matter of fact, in the third paragraph of his 1899 paper, Henri Poincaré first defines the complex variable in C n {displaystyle mathbb {C} ^{n}} and its complex conjugate as follows Then he writes the equation defining the functions V {displaystyle V} he calls biharmonique, previously written using partial derivatives with respect to the real variables x k , y q {displaystyle x_{k},y_{q}} with k , q {displaystyle k,q} ranging from 1 to n {displaystyle n} , exactly in the following way This implies that he implicitly used definition 2 below: to see this it is sufficient to compare equations 2 and 2' of (Poincaré 1899, p. 112). Apparently, this paper was not noticed by early researchers in the theory of functions of several complex variables: in the papers of Levi-Civita (1905), Levi (1910) (and Levi 1911) and of Amoroso (1912) 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 Osgood (1966) (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 express the pluriharmonic operator and the Levi operator, he follows the established practice of Amoroso, Levi and Levi-Civita. According to Henrici (1993, p. 294), a new step in the definition of the concept was taken by Dimitrie Pompeiu: in the paper (Pompeiu 1912), given a complex valued differentiable function (in the sense of real analysis) of one complex variable g ( z ) {displaystyle g(z)} defined in the neighbourhood of a given point z 0 ∈ C , {displaystyle z_{0}in mathbb {C} ,} he defines the areolar derivative as the following limit where Γ ( z 0 , r ) = ∂ D ( z 0 , r ) {displaystyle Gamma (z_{0},r)=partial D(z_{0},r)} is the boundary of a disk of radius r {displaystyle r} entirely contained in the domain of definition of g ( z ) , {displaystyle 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 Henrici (1993, p. 294), the limit may exist for functions that are not even differentiable at z = z 0 . {displaystyle z=z_{0}.} According to Fichera (1969, p. 28), the first to identify the areolar derivative as a weak derivative in the sense of Sobolev was Ilia Vekua. In his following paper, Pompeiu (1913) uses this newly defined concept in order to introduce his generalization of Cauchy's integral formula, the now called Cauchy–Pompeiu formula. The first systematic introduction of Wirtinger derivatives seems due to Wilhelm Wirtinger in the paper Wirtinger 1926 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. 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 Andreotti (1976, pp. 3–5), the monograph of Gunning & Rossi (1965, pp. 3–6), and the monograph of Kaup & Kaup (1983, p. 2,4) which are used as general references in this and the following sections. Definition 1. Consider the complex plane C ≡ R 2 = { ( x , y ) ∣ x , y ∈ R } . {displaystyle mathbb {C} equiv mathbb {R} ^{2}={(x,y)mid x,yin mathbb {R} }.} The Wirtinger derivatives are defined as the following linear partial differential operators of first order: