# Luzin-Privalov theorems

(Redirected from Luzin–Privalov theorems)
1) Let $f(z)$ be a meromorphic function of the complex variable $z$ in a simply-connected domain $D$ with rectifiable boundary $\Gamma$. If $f(z)$ takes angular boundary values zero on a set $E\subset\Gamma$ of positive Lebesgue measure on $\Gamma$, then $f(z)=0$ in $D$. There is no function meromorphic in $D$ that has infinite angular boundary values on a set $E\subset\Gamma$ of positive measure.
2) Let $w=f(z)$ be a meromorphic function in the unit disc $D=\{z:\left|z\right|<1\}$ other than a constant and having radial boundary values (finite or infinite) on a set $E$ situated on an arc $\sigma$ of the unit circle $\Gamma=\{z:\left|z\right|=1\}$ that is metrically dense and of the second Baire category (cf. Baire classes) on $\sigma$. Then the set $W$ of its radial boundary values on $E$ contains at least two distinct points. Metric density of $E$ on $\sigma$ means that every portion of $E$ on $\sigma$ has positive measure. This implies that if the radial boundary values of $f(z)$ on a set $E$ of the given type are equal to zero, then $f(z)=0$ in $D$. Moreover, there is no meromorphic function in the unit disc that takes infinite radial boundary values on a set $E$ of the given type.