Higher dimensional generalizations of some theorems on normality of meromorphic functions

Abstract

In [Israel J. Math, 2014], Grahl and Nevo obtained a significant improvement for the well-known normality criterion of Montel. They proved that for a family of meromorphic functions $\mathcal F$ in a domain $D\subset \mathbb C,$ and for a positive constant $\epsilon$, if for each $f\in \mathcal F$ there exist meromorphic functions $a_f,b_f,c_f$ such that $f$ omits $a_f,b_f,c_f$ in $D$ and $$\min\{\rho(a_f(z),b_f(z)), \rho(b_f(z),c_f(z)), \rho(c_f(z),a_f(z))\}\geq \epsilon,$$ for all $z\in D$, then $\mathcal F$ is normal in $D$. Here, $\rho$ is the spherical metric in $\widehat{\mathbb C}$. In this paper, we establish the high-dimensional versions for the above result and for the following well-known result of Lappan: A meromorphic function $ f$ in the unit disc $\triangle:=\{z\in\mathbb C: |z|<1\}$ is normal if there are five distinct values $a_1,\dots,a_5$ such that $$\sup\{(1-|z|^2)\frac{ |f '(z)|}{1+|f(z)|^2}: z\in f^{-1}\{a_1,\dots,a_5\}\} < \infty.$$