This article is cited in
1 paper
Mathematics
On non-uniqueness sets for spaces of holomorphic functions
B. N. Khabibullin,
F. B. Khabibullin Bashkir State University, Ufa
Abstract:
Problems of description of zero subsequences for weight
spaces of holomorphic functions are reduced, according to a general scheme, to solving certain
problems in weight classes of subharmonic functions.
Let
$D$ be a domain in the complex plane
$\mathbb C$. We associate with every at most countable sequence $\Lambda
= \{\lambda_k\}_{k=1,2, \dots} \subset D$, without accumulation points in
$D$, the counting measure
$n_{\Lambda}(S) := \sum_{\lambda_k\in S} 1$.
We denote by
$\mathrm{Hol} (D)$ the vector space of all holomorphic functions in
$D$. For
$0\neq f\in \mathrm{Hol} (D)$, denote by
$\mathrm{Zero}_f$ zero sequence of
$f$ with account of multiplicities.
A sequence
$\Lambda\subset D$ is called the non-uniqueness sequence for a subspace
$H\subset \mathrm{Hol} (D)$, if there exists a nonzero function
$f\in H$ such that
$\Lambda \subset \mathrm{Zero}_f$, i. e. $n_\Lambda (\lambda)\leq n_{\mathrm{Zero}_f}(\lambda)$ for all
$\lambda \in D$. We denote by
$\mathrm{sbh} (D)$ the convex cone of all subharmonic functions in
$D\subset \mathbb{C}$.
For
$-\infty\not\equiv s\in \mathrm{sbh} (D)$ we denote by
$\nu_s$ the Riesz measure of
$s$. A Borel positive measure
$\nu$ is called the submeasure for a subset
$S\subset \mathrm{sbh} (D)$, if there exists a function
$s\in S$,
$s\not\equiv -\infty$, with the Riesz measure
$\nu_s\geq \nu$ on
$D$.
For a (weight) function
$M\colon D\to [-\infty,+\infty]$ we define the weight classes
$\mathrm{sbh}(D;M]:=\{s \in \mathrm{sbh} (D) \colon s\leq M +\mathrm{const} \; \text{on } D \}$ and
$\mathrm{Hol}(D;\exp M]:=\{f\in \mathrm{Hol} (D)\colon |f|\leq \mathrm{const} \cdot \exp M \; \text{on } D \}$, where
$\mathrm{const}$ is a constant.
Let
$S$ be a subset of the extended complex plane
$\mathbb{C}_{\infty}:=\mathbb{C}\cup \{\infty\}$.
Denote by
$\mathrm{clos} S$ and
$\mathrm{bd} S$ the closure and the boundary of
$S$ in
$\mathbb{C}_{\infty}$ resp. Let
$\mathrm{dist} (\cdot , \cdot)$
be the Euclidean distance between two objects (points or subsets) in
$\mathbb{C}$.
Let
$d\colon D\to (0,1]$ be a continuous function such that
$0<d(z)<\mathrm{dist}(z, \mathrm{bd} D)$,
$z\in D$. We will juxtapose to a weight function
$N \colon D \to [-\infty,+\infty]$
its average value of
$N$ over the disk
$\{z'\in \mathbb{C} \colon |z'-z|<r\}$:
\begin{equation*}
B (z,r;N):=\frac{1}{\pi r^2}\int_0^{2\pi}\int_0^r N(z+te^{i\theta}) t\, d t \,d \theta,
\end{equation*}
and some its “lifting”
$N^{\uparrow}\colon D\to [-\infty,+\infty]$ so that
$$
\begin{aligned}
N^{\uparrow}(z)&:= B(z,d(z);N)+\ln\frac{1}{d(z)}, \quad \text{if} \; \mathbb{C}_{\infty}\setminus \mathrm{clos} D\neq \varnothing; \\
&N^{\uparrow}(z):= B\Bigl(z,\frac{1}{(1+|z|)^P};N\Bigr), \quad \text{if} \; D=\mathbb{C},
\end{aligned}
$$
where
$P\geq 0$ is an arbitrary fixed number.
Theorem 1. Let $N,M, M-N\in \mathrm{sbh} (D)$,
$N,M\neq \boldsymbol{-\infty}$,
and
$\Lambda$ be a sequence in $D$. If $\Lambda$ is the non-uniqueness sequence for $\mathrm{Hol}(D;\exp N]$,
then $n_\Lambda+\nu_{M-N}$ is submeasure for $\mathrm{sbh}(D;M]$.
Conversely, if $n_\Lambda+\nu_{M-N}$ is a submeasure for $\mathrm{sbh}(D;M]$ and $N$ is a continuous function on $D$,
then $\Lambda$ is a non-uniqueness sequence for $\mathrm{Hol}(D;\exp N^{\uparrow}]$
with a suitable lifting $N^{\uparrow}$ (see above cases $D=\mathbb{C}$ with an arbitrary fixed $P\geq 0$ and $\mathbb{C}_{\infty}\setminus \mathrm{clos} D\neq \varnothing$).
We also consider an important special case of subharmonic positively homogeneous of degree
$\rho>0$ weight functions
$N, M$ on
$\mathbb{C}$ (see Section 2, Theorem 2).
Keywords:
holomorphic function, zero sequence, subharmonic function, Riesz measure, non-uniqueness sequence.
UDC:
517.53 :
517.574
BBK:
22.161
DOI:
10.15688/jvolsu1.2016.4.8