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Quasianalytical classes of functions in convex domains
R. S. Yulmukhametov
Abstract:
Let
$D$ be a bounded convex domain lying in the left-hand half-plane, with
$0\in\overline D$. A class
$H(D,M_n)$, consisting of functions analytic in
$D$ and satisfying the inequalities
$$
\max_{z\in D}|f^{(n)}(z)|\leqslant C_fM_n,\qquad n=0,1,\dots,
$$
is said to be quasianalytic at
$z=0$ if
$H(D,M_n)$ contains no functions that vanish with all their derivatives at
$z=0$.
Let $h(\varphi)=\max_{\lambda\in D}\operatorname{Re}\lambda e^{i\varphi}$ and
$h(\varphi)=0$,
$\varphi\in[\sigma_-,\sigma_+]$, and let
\begin{gather*}
\Delta_+(\alpha)=\sqrt{\alpha-\sigma_+}\biggl(h'(\alpha)+\int^\alpha_{\sigma_+}h(\theta)\,d\theta\biggr),\qquad\sigma_+<\alpha<\frac\pi2,
\\
\Delta_-(\alpha)=-\sqrt{\sigma_--\alpha}\biggl(h'(\alpha)+\int_{\sigma_-}^\alpha h(\theta)\,d\theta\biggr),\qquad-\frac\pi2<\alpha<\sigma_-,
\\
v_1(x)=\exp\int_{x_1}^x\frac{2\pi-\Delta_+^{-1}(y)+\Delta_-^{-1}(y)}{-\pi+\Delta_+^{-1}(y)-\Delta_-^{-1}(y)}\cdot\frac{dy}y,\qquad x\to0,\quad x_1>0.
\end{gather*}
It is shown that the condition
$$
\int_1^\infty\frac{\ln T(r)}{v(r)\cdot r^2}\,dr=+\infty,
$$
where
$T(r)=\sup r^nM_n^{-1}$ is the trace function of the sequence
$(M_n)$, and
$v(r)$ is the inverse of
$v_1(x)$, is necessary and sufficient for the quasianalyticity of
$H(D,M_n)$.
This theorem generalizes the classical Denjoy–Carleman theorem. In the case when
$D=\bigl\{z:|\arg z|<\frac\pi{2\gamma}\bigr\}$ the theorem follows from Salinas's results of 1955. For
$D=\{z:|z+1|=1\}$ the theorem was proved by Korenblyum in 1965.
Bibliography: 9 titles.
UDC:
517.548.3
MSC: 26E10,
30B60,
30E10 Received: 30.04.1985