Towards a quaternionic function theory linked with the Lamé's wave functions

Over the past few years, considerable attention has been given to the role played by the Lamé's Wave Functions (LWFs) in various problems of mathematical physics and mechanics. The LWFs arise via the method of separation of variables for the wave equation in ellipsoidal coordinates. The present paper introduces the Lamé's Quaternionic Wave Functions (LQWFs), which extend the LWFs to a non‐commutative framework. We show that the theory of the LQWFs is determined by the Moisil‐Theodorescu type operator with quaternionic variable coefficients. As a result, we explain the connections between the solutions of the Lamé's wave equation, on one hand, and the quaternionic hyperholomorphic and anti‐hyperholomorphic functions on the other. We establish analogues of the basic integral formulas of complex analysis such as Borel‐Pompeiu's, Cauchy's, and so on, for this version of quaternionic function theory. We further obtain analogues of the boundary value properties of the LQWFs such as Sokhotski‐Plemelj formulae, the ‐hyperholomorphic extension of a given Hölder function and on the square of the singular integral operator. We address all the text mentioned earlier and explore some basic facts of the arising quaternionic function theory. We conclude the paper showing that the spherical, prolate, and oblate spheroidal quaternionic wave functions can be generated as particular cases of the LQWFs. Copyright © 2015 John Wiley & Sons, Ltd.     

Quaternionic spherical wave functions

The scalar spherical wave functions (SWFs) are solutions to the scalar Helmholtz equation obtained by the method of separation of variables in spherical polar coordinates. These functions are complete and orthogonal over a sphere, and they can, therefore, be used as a set of basis functions in solving boundary value problems by spherical wave expansions. In this work, we show that there exists a theory of functions with quaternionic values and of three real variables, which is determined by the Moisil–Theodorescu‐type operator with quaternionic variable coefficients, and which is intimately related to the radial, angular and azimuthal wave equations. As a result, we explain the connections between the null solutions of these equations, on one hand, and the quaternionic hyperholomorphic and anti‐hyperholomorphic functions, on the other. We further introduce the quaternionic spherical wave functions (QSWFs), which refine and extend the SWFs. Each function is a linear combination of SWFs and products of ‐hyperholomorphic functions by regular spherical Bessel functions. We prove that the QSWFs are orthogonal in the unit ball with respect to a particular bilinear form. Also, we perform a detailed analysis of the related properties of QSWFs. We conclude the paper establishing analogues of the basic integral formulae of complex analysis such as Borel–Pompeiu's and Cauchy's, for this version of quaternionic function theory. As an application, we present some plot simulations that illustrate the results of this work. Copyright © 2016 John Wiley & Sons, Ltd.     

Schur Functions and Their Realizations in the Slice Hyperholomorphic Setting

In this paper we start the study of Schur analysis in the quaternionic setting using the theory of slice hyperholomorphic functions. The novelty of our approach is that slice hyperholomorphic functions allow to write realizations in terms of a suitable resolvent, the so called S-resolvent operator and to extend several results that hold in the complex case to the quaternionic case. We discuss reproducing kernels and positive definite functions in this setting and we show how they can be obtained using the extension operator and the slice hyperholomorphic product. We define Schur multipliers, and find their co-isometric realization in terms of the associated de Branges–Rovnyak space.     

Schatten class and Berezin transform of quaternionic linear operators

In this paper, we introduce the Schatten class and the Berezin transform of quaternionic operators. The first topic is of great importance in operator theory, but it is also necessary to study the second one, which requires the notion of trace class operators, a particular case of the Schatten class. Regarding the Berezin transform, we give the general definition and properties. Then we concentrate on the setting of weighted Bergman spaces of slice hyperholomorphic functions. Our results are based on the S‐spectrum of quaternionic operators, which is the notion of spectrum that appears in the quaternionic version of the spectral theorem and in the quaternionic S‐functional calculus. Copyright © 2016 John Wiley & Sons, Ltd.     

Krein–Langer Factorization and Related Topics in the Slice Hyperholomorphic Setting

We study various aspects of Schur analysis in the slice hyperholomorphic setting. We present two sets of results: first, we give new results on the functional calculus for slice hyperholomorphic functions. In particular, we introduce and study some properties of the Riesz projectors. Then we prove a Beurling–Lax type theorem, the so-called structure theorem. A crucial fact which allows to prove our results is the fact that the right spectrum of a quaternionic linear operator and the point S-spectrum coincide. Finally, we study the Krein–Langer factorization for slice hyperholomorphic generalized Schur functions. Both the Beurling–Lax type theorem and the Krein–Langer factorization are far-reaching results which have not been proved in the quaternionic setting using notions of hyperholomorphy other than slice hyperholomorphy.     

BMO‐ and VMO‐spaces of slice hyperholomorphic functions

In this paper we continue the study of important Banach spaces of slice hyperholomorphic functions on the quaternionic unit ball by investigating the BMO‐ and VMO‐spaces of slice hyperholomorphic functions. We discuss in particular conformal invariance and a refined characterization of these spaces in terms of Carleson measures. Finally we show the relations with the Bloch and Dirichlet space and the duality relation with the Hardy space . The importance of these spaces in the classical theory is well known. It is therefore worthwhile to study their slice hyperholomorphic counterparts, in particular because slice hyperholomorphic functions were found to have several applications in operator theory and Schur analysis.     

On the Structure of the Taylor Series in Clifford and Quaternionic Analysis

The Fueter variables form a basis of the space of (quaternionic or Cliffordian) hyperholomorphic homogeneous polynomials of degree one, and their symmetrized products give the respective bases of spaces of hyperholomorphic homogeneous polynomials for any degree k. In the present paper we introduce new bases, i.e., new types of hyperholomorphic variables which lead to the Taylor-type series expansions reflecting the structure of the set of all (quaternionic or Cliffordian algebra-valued) hyperholomorphic functions.     

Adaptive Fourier decomposition of functions in quaternionic Hardy spaces

We study decompositions of functions in the Hardy spaces into linear combinations of the basic functions in the orthogonal rational systems B_n, which are obtained in the respective contexts through Gram–Schmidt orthogonalization process on shifted Cauchy kernels. Those lead to adaptive decompositions of quaternionic‐valued signals of finite energy. This study is a generalization of the main results of the first author's recent research in relation to adaptive Takenaka–Malmquist systems in one complex variable. Copyright © 2011 John Wiley & Sons, Ltd.     

The Slice Hyperholomorphic Bergman Space on $$\varvec{\mathbb {B}_{R}}$$: Integral Representation and Asymptotic Behavior

The aim of the present paper is threefolds. Firstly, we complete the study of the weighted hyperholomorphic Bergman space of the second kind on the ball of radius R centred at the origin. The explicit expression of its Bergman kernel is given and can be written in terms of special hypergeometric functions of two non-commuting (quaternionic) variables. Secondly, we introduce and study some basic properties of an associated integral transform, the quaternionic analogue of the so-called second Bargmann transform for the holomorphic Bergman space. Finally, we establish the asymptotic behavior as R goes to infinity. We show in particular that the reproducing kernel of the weighted slice hyperholomorphic Bergman space gives rise to its analogue for the slice hyperholomorphic Bargamann–Fock space.     

A Survey on the (Hyper-) Derivatives in Complex,Quaternionic and Clifford Analysis

The complex derivative serves as one of the definitions for holomorphic functions but also as an important characteristic of the latter having algebraic, topologic and analytic aspects. The goal of the paper is to explain that in the framework of quaternionic and Clifford analyses there exists the hyperderivative of a hyperholomorphic function which extends to the corresponding situations a series of fundamental properties of its complex antecedent.     

On the Derivatives of Quaternionic Functions Along Two-Dimensional Planes

In a previous paper we introduced the concept of a two-dimensional directional derivative of a quaternionic function along a two-dimensional plane. In this paper we provide a deeper analysis of its properties, as well as of its relations with hyperholomorphic functions, with holomorphic maps of two complex variables and with Cullen-regular functions. Received: October, 2007. Accepted: February, 2008.     

Espaces de Branges Rovnyak et fonctions de Schur : le cas hyper-analytique

We extend to the hyperholomorphic case the notion of Schur functions and the corresponding realization theory. We introduce the notion of characteristic operator function for coisometric colligations between Hilbert spaces of hyperholomorphic functions. We show that every Schur function is the characteristic operator function of a coisometric colligation and vice-versa. To cite this article: D. Alpay et al., C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

Cauchy Integral Formulae in Quaternionic Hermitean Clifford Analysis

The theory of complex Hermitean Clifford analysis was developed recently as a refinement of Euclidean Clifford analysis; it focusses on the simultaneous null solutions, called Hermitean monogenic functions, of two Hermitean Dirac operators constituting a splitting of the traditional Dirac operator. In this function theory, the fundamental integral representation formulae, such as the Borel?CPompeiu and the Clifford?CCauchy formula have been obtained by using a (2 ×?2) circulant matrix formulation. In the meantime, the basic setting has been established for so-called quaternionic Hermitean Clifford analysis, a theory centred around the simultaneous null solutions, called q-Hermitean monogenic functions, of four Hermitean Dirac operators in a quaternionic Clifford algebra setting. In this paper we address the problem of establishing a quaternionic Hermitean Clifford?CCauchy integral formula, by following a (4?× 4) circulant matrix approach.     

On the Bergmann Kernel Function in Hyperholomorphic Analysis

The hyperholomorphic Bergmann kernel function ß for a domain is introduced as the special quaternionic derivative of the Green function for . It is shown that ß is hyperholomorphic, Hermitian symmetric and reproduces hyperholomorphic functions.We obtain an integral representation of ß as a sum of two integrals. One of them gives a smooth function, and the other describes the behaviour of ß near a boundary. To investigate the hyperholomorphic Bergmann function for some fixed class of hyperholomorphic functions we have to use not only the properties of just this class but also those of some other classes. The second fact is completely unpredictable from the complex analysis point of view.The connection between the hyperholomorphic Bergmann projector (the integral operator with the kernel ß) and some classical multidimensional singular integral operators is established.     

Bloch,Besov and Dirichlet Spaces of Slice Hyperholomorphic Functions

In this paper we begin the study of some important Banach spaces of slice hyperholomorphic functions, namely the Bloch, Besov and weighted Bergman spaces, and we also consider the Dirichlet space, which is a Hilbert space. The importance of these spaces is well known, and thus their study in the framework of slice hyperholomorphic functions is relevant, especially in view of the fact that this class of functions has recently found several applications in operator theory and in Schur analysis. We also discuss the property of invariance of these function spaces with respect to Möbius maps by using a suitable notion of composition.     

On the Navier–Stokes equations with free convection in three‐dimensional unbounded triangular channels

The quaternionic calculus is a powerful tool for treating the Navier–Stokes equations very elegantly and in a compact form, through the evaluation of two types of integral operators: the Teodorescu operator and the quaternionic Bergman projector. While the integral kernel of the Teodorescu transform is universal for all domains, the kernel function of the Bergman projector, called the Bergman kernel, depends on the geometry of the domain. In this paper, we use special variants of quaternionic‐holomorphic multiperiodic functions in order to obtain explicit formulas for unbounded three‐dimensional parallel plate channels, rectangular block domains and regular triangular channels. Copyright © 2007 John Wiley & Sons, Ltd.     

On the optimal control method in quaternionic analysis

Gaveau?s optimal control method for real and complex Monge–Ampere operators is generalized to that for quaternionic Monge–Ampere operator. It is also applied to investigate quaternionic regular functions: the characterization of the Silov boundary of a smooth quaternionic pseudoconvex domain.     

Fluid Flow Equations with Variable Viscosity in Quaternionic Setting

For a large class of fluids the relation between shear stress and shear velocity is not longer a constant. The viscosity μ is now a function which depends on the position, the time and the shear-velocity. In our paper we will deal with a class of fluids with variable viscosity functions which correspond to fluid flow equations that permit a representation of the solution by the aid of a quaternionic operator calculus.     

The domain of definition of the quaternionic Monge–Ampère operator

In this paper, we give an equivalent characterization of the domain of definition for the quaternionic Monge–Ampère operator, by using the theory of quaternionic closed positive current we established in [17–19]. This domain of definition for the complex Monge–Ampère operator was introduced by Blocki [7,8].