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This article summarises the theory of several bounded functional calculi for unbounded operators that have recently been discovered. The extend the Hille--Phillips calculus for (negative) generators $A$ of certain bounded $C_0$-semigroups,…

Functional Analysis · Mathematics 2022-02-08 Charles Batty , Alexander Gomilko , Yuri Tomilov

We present some new relations between the Cauchy-Riemann operator on the real Clifford algebra $\mathbb R_n$ of signature $(0,n)$ and slice-regular functions on $\mathbb R_n$. The class of slice-regular functions, which comprises all…

Complex Variables · Mathematics 2022-04-26 Alessandro Perotti

The foundation of spectral theory on the $S$-spectrum can be traced back to the quaternionic framework of quantum mechanics. The concept of $S$-spectrum for quaternionic operators emerged as the natural spectrum in slice hyperholomorphic…

Spectral Theory · Mathematics 2026-02-05 Francesco Mantovani

The spectral theory on the S-spectrum was introduced to give an appropriate mathematical setting to quaternionic quantum mechanics, but it was soon realized that there were different applications of this theory, for example, to fractional…

Spectral Theory · Mathematics 2022-05-18 Fabrizio Colombo , Antonino De Martino , Stefano Pinton , Irene Sabadini

We prove some formulas relating Cauchy-Riemann operators defined on hypercomplex subspaces of an alternative *-algebra to a differential operator associated with the concept of slice-regularity and to the spherical Dirac operator. These…

Complex Variables · Mathematics 2022-07-22 Alessandro Perotti

The $S$-functional calculus is based on the theory of slice hyperholomorphic functions and it defines functions of $n$-tuples of not necessarily commuting operators or of quaternionic operators. This calculus relays on the notion of…

Functional Analysis · Mathematics 2016-09-21 Fabrizio Colombo , Jonathan Gantner

The Fueter-Sce-Qian mapping theorem is a two steps procedure to extend holomorphic functions of one complex variable to quaternionic or Clifford algebra-valued functions in the kernel of a suitable generalized Cauchy-Riemann operator. Using…

Complex Variables · Mathematics 2021-12-10 Fabrizio Colombo , Antonino De Martino , Irene Sabadini

The Fueter-Sce theorem is one of the most important results in hypercomplex analysis, providing a two-step procedure for constructing axially monogenic functions starting from holomorphic functions of one variable. In the first step, the…

Functional Analysis · Mathematics 2026-01-09 Antonino De Martino , Stefano Pinton

The spectral theory on the $S$-spectrum was born out of the need to give quaternionic quantum mechanics (formulated by Birkhoff and von Neumann) a precise mathematical foundation. Then it turned out that this theory has important…

Functional Analysis · Mathematics 2022-10-11 Fabrizio Colombo , Jonathan Gantner , David P. Kimsey , Irene Sabadini

In this paper we introduce fractional powers of quaternionic operators. Their definition is based on the theory of slice-hyperholomorphic functions and on the $S$-resolvent operators of the quaternionic functional calculus. The integral…

Functional Analysis · Mathematics 2016-05-24 Fabrizio Colombo , Jonathan Gantner

This paper focuses on the development of harmonic and Clifford analysis techniques in the context of some conformally flat manifolds that arise from factoring out a simply-connected domain from $R^n$ by special arithmetic subgroups of the…

Differential Geometry · Mathematics 2007-05-23 R. S. Krausshar , John Ryan , Qiao Yuying

This treatise investigates holomorphic functions defined on the space of bicomplex numbers introduced by Segre. The theory of these functions is associated with Fueter's theory of regular, quaternionic functions. The algebras of quaternions…

Complex Variables · Mathematics 2007-05-23 Stefan Rönn

We introduce the notion of rationality for hyperholomorphic functions (functions in the kernel of the Cauchy-Fueter operator). Following the case of one complex variable, we give three equivalent definitions: the first in terms of…

Functional Analysis · Mathematics 2007-05-23 D. Alpay , M. Shapiro , D. Volok

The $H^\infty$-functional calculus is a two-step procedure, introduced by A. McIntosh, that allows the definition of functions of sectorial operators in Banach spaces. It plays a crucial role in the spectral theory of differential…

Spectral Theory · Mathematics 2025-06-23 Fabrizio Colombo , Francesco Mantovani , Peter Schlosser

We study some harmonic properties of slice regular functions in one and several Clifford variables and give explicit formulas of the iterated Laplacian applied to slice regular functions and to their spherical derivative, which are new also…

Complex Variables · Mathematics 2025-03-07 Giulio Binosi

The decompositions of an element of a finite von Neumann algebra into the sum of a normal operator plus an s.o.t.-quasinilpotent operator, obtained using the Haagerup--Schultz hyperinvariant projections, behave well with respect to…

Operator Algebras · Mathematics 2013-10-10 Ken Dykema , Fedor Sukochev , Dmitriy Zanin

One of the primary objectives of this paper is to establish a generalized Cauchy-Kovalevskaya extension for axially harmonic functions. We demonstrate that the result can be expressed as a power series involving Bessel-type functions of…

Functional Analysis · Mathematics 2025-01-23 Antonino De Martino , Ali Guzmán Adán

The aim of this article is to introduce the H-infinity functional calculus for unbounded bisectorial operators in a Clifford module over the algebra R_n. While recent studies have focused on bounded operators or unbounded paravector…

Functional Analysis · Mathematics 2025-02-17 Francesco Mantovani , Peter Schlosser

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 allows to write realizations in terms of a suitable…

Functional Analysis · Mathematics 2011-10-13 Daniel Alpay , Fabrizio Colombo , Irene Sabadini

We study a Floer-theoretic approach to harmonic maps from the two-torus into non-flat K\"ahler manifolds. Building on the complex-regularized polysymplectic (CRPS) formalism of [BF24], which provides a Hamiltonian description of harmonic…

Symplectic Geometry · Mathematics 2026-03-03 L. Asselle , R. Brilleslijper