In this paper we are interested in finding solutions to Riemann-Hilbert boundary value problems, for short Riemann-Hilbert problems, with variable coefficients in the case of axially monogenic functions defined over the upper half unit ball centred at the origin in four-dimensional Euclidean space. Our main idea is to transfer Riemann-Hilbert problems for axially monogenic functions defined over the upper half unit ball centred at the origin of four-dimensional Euclidean spaces into Riemann-Hilbert problems for analytic functions defined over the upper half unit disk of the complex plane. Furthermore, we extend our results to axially symmetric null-solutions of perturbed generalized Cauchy-Riemann equations.
Mathematics SubjectClassification. Primary: 30E25, 35Q15, 31A25, 31B20; Secondary: 31B10, 35J56, 35J58.2494 M. Ku et al.Adv. Appl. Clifford Algebras boundary value problems, their study is closely connected with the theory of singular integral equations [17,30] and has a wide range of applications in other fields, such as in the theory of cracks and elasticity [22,30], in quantum mechanics and of statistical physics [9] as well as in the theory of linear and nonlinear partial differential equations [14], in the theory of orthogonal polynomials and asymptotic analysis [12], and in the theory of time-frequency analysis [1]. In addition, their methods and related problems have been extended to null-solutions to complex partial differential equations (PDEs) on the complex plane like poly-analytic functions, meta-analytic functions, and poly-harmonic functions, see, e.g. [5]. Moreover, also recently, the boundary value theory of null-solutions to complex PDEs has been explored on special domains of the complex plane, like the unit disk, upper half plane, half disks, circular rings, triangles, and sectors, and so on, see, e.g. [5,6,36].In parallel, there have been many attempts of generalizing the classical boundary value theory of Riemann-Hilbert problems into higher dimensions, mainly by considering two principal ways, i.e., the theory of several complex variables and Clifford analysis, in particular quaternionic analysis. The latter is an elegant generalization of the classical theory of complex analysis. It concentrates on the study of the theory of so-called monogenic functions, and refines real harmonic analysis in the sense that its principal operator, the Dirac or generalized Cauchy-Riemann operator, factorizes the higher-dimensional Laplace operator, see, e.g. [7,13,18]. In the setting of quaternionic and Clifford analysis, Riemann-Hilbert problems were discussed by many authors, see, e.g. ], the authors studied Riemann-Hilbert problems with constant coefficients. Their solutions were given explicitly in terms of Cauchy-type integral operators together with power series expansions. Moreover, these problems are closely connected with applications, like the theory of fluid mechanics [18] and signal processing in higher dimensions [16,32]. To the authors' knowledge in Refs. [20,21] the authors made the first ...