$\ast$-Logarithm for Slice Regular Functions

Abstract

In this paper, we study the (possible) solutions of the equation ${\exp }_{\ast }(f)=g$, where $g$ is a slice regular never vanishing function on a circular domain of the quaternions $\mathbb{H}$ and ${\exp }_{\ast }$ is the natural generalization of the usual exponential to the algebra of slice regular functions. Any function $f$ which satisfies ${\exp }_{\ast }(f)=g$ is called a $\ast$-logarithm of $g$. We provide necessary and sufficient conditions, expressed in terms of the zero set of the ``vector'' part $g_v$ of $g$, for the existence of a $\ast$-logarithm of $g$, under a natural topological condition on the domain $\Omega$. By the way, we prove an existence result if $g_v$ has no non-real isolated zeroes; we are also able to give a comprehensive approach to deal with more general cases. We are thus able to obtain an existence result when the non-real isolated zeroes of $g_v$ are finite, the domain is either the unit ball, or $\mathbb{H}$, or $\mathbb{D}$ and a further condition on the ``real part'' $g_0$ of $g$ is satisfied (see Theorem 6.19 for a precise statement). We also find some unexpected uniqueness results, again related to the zero set of $g_v$, in sharp contrast with the complex case. A number of examples are given throughout the paper in order to show the sharpness of the required conditions.

arXiv comment27 pages, 4 figures. Published in Rendiconti Lincei Matematica e Applicazioni

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Cited by in corpus (3)

• On a definition of logarithm of quaternionic functions
• Quaternionic Cartan coverings and applications
• On a continuation of quaternionic and octonionic logarithm along curves and the winding number