arXiv subjects
Publications and source records attributed to Kenta Kojin.
In this paper, we focus on an indefinite structure lying behind the Bergman kernel on the open unit disk. In particular, an invariant distance, birational maps and an indefinite kernel are constructed from the Bergman kernel, and we deal with their interaction.
We show that any complete Nevanlinna-Pick space whose multiplier algebra has isometric Gelfand transform (or commutative C*-envelope) is essentially the Hardy space on the open unit disk.
In this short note, we will give a generalization of the indefinite Schwarz-Pick inequality due to Seto [8]. Our approach is based on a connection between complex geometry and the geometry of reproducing kernel Hilbert spaces, which was crucially used in our previous work [6].
In this paper, we will characterize those sets, over which every irreducible complete Nevanlinna--Pick space enjoys that its multiplier and supremum norms coincide. Moreover, we will prove that, if there exists an irreducible complete Nevanlinna--Pick space of holomorphic functions on a reduced complex space $X$ whose multiplier algebra is isometrically equal to the algebra of bounded holomorphic functions (we will say that such a space is of $\textbf{Hardy type}$ in this paper), then $X$ must be biholomorphic to the unit disk minus a zero analytic capacity set. This means that the Hardy space is characterized as a unique irreducible complete Nevanlinna--Pick space of Hardy type.
We study a Schwarz-Pick type inequality for the Schur-Agler class $SA(B_δ)$. In our operator theoretical approach, von Neumann's inequality for a class of generic tuples of $2\times 2$ matrices plays an important role rather than holomorphy. In fact, the class $S_{2, gen}(B_Δ)$ consisting of functions that satisfy the inequality for those matrices enjoys \begin{equation*} d_{\mathbb{D}}(f(z), f(w))\le d_Δ(z, w) \;\;(z,w\in B_Δ, f\in S_{2, gen}(B_Δ)). \end{equation*} Here, $d_Δ$ is a function defined by a matrix $Δ$ of abstract functions. Later, we focus on the case when $Δ$ is a matrix of holomorphic functions. We use the pseudo-distance $d_Δ$ to give a sufficient condition on a diagonalizable commuting tuple $T$ acting on $\mathbb{C}^2$ for $B_Δ$ to be a complete spectral domain for $T$. We apply this sufficient condition to generalizing von Neumann's inequalities studied by Drury and by Hartz-Richter-Shalit.
In this paper, we prove a noncommutative (nc for short) analog of Schwarz lemma for the nc Schur-Agler class and prove that the regular nc Schur-Agler class and the regular free Herglotz class are homeomorphic. Moreover, we give a characterization of regular free Herglotz functions. As an application, we will show that any regular free Herglotz functions can uniformly be approximated by regular Herglotz free polynomials.
This short note refines nc Oka-Weil theorem by using a characterization of free compact nc sets based on the notion of dilation hulls. A consequence of it is that any free holomorphic function can be represented as a free polynomial on each free compact nc set.