On the boundedness of a singular integral operator in the space $C^\alpha(\overline G)$

The article considers an operator of the form
$$ [Au](x)=\int_G\frac{f(x,s)}{|x-y|^m}u(y)\,dy, $$
where $G$ is a bounded domain in $\mathbf R^m$ with a smooth boundary, $x\in G$, $S\in\Omega$, $\Omega=\{s: s\in\mathbf R^m,|s|=1\}$, $u(y)\in C^\alpha(\overline G)$, $0<\alpha<1$. It is proved that if the function $f(x,s)$ satisfies a Hölder condition with exponent $\lambda$, $\alpha<\lambda<1$, and the condition
\begin{equation} \int_{\Omega_1}f(x,s)\,ds=0\qquad x\in G \end{equation}
(where $\Omega_1$ is any polysphere), then the operator is bounded from $C^\alpha(\overline G)$ to $C^\alpha(\overline G)$. Moreover, if $f(x,s)=g(s)$, then in order that the operator $A$ should be defined and bounded from $C^\alpha(\overline G)$ to $C^\alpha(\overline G)$ the condition (1) is necessary.
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