Universal Padé approximants and their behaviour on the boundary

Abstract

There are several kinds of universal Taylor series. In one such kind the universal approximation is required at every boundary point of the domain of definition ${\varOmega }$ of the universal function f. In another kind the universal approximation is not required at any point of $\partial {\varOmega }$ but in this case the universal function f can be taken smooth on $\overline{\varOmega }$ and, moreover, it can be approximated by its Taylor partial sums on every compact subset of $\overline{\varOmega }$. Similar generic phenomena hold when the partial sums of the Taylor expansion of the universal function are replaced by some Padé approximants of it. In the present paper we show that in the case of Padé approximants, if ${\varOmega }$ is an open set and S, T are two subsets of $\partial {\varOmega }$ that satisfy some conditions, then there exists a universal function $f\in H({\varOmega })$ which is smooth on ${\varOmega }\cup S$ and has some Padé approximants that approximate f on each compact subset of ${\varOmega }\cup S$ and simultaneously obtain universal approximation on each compact subset of $(\mathbb {C}{\backslash }\overline{\varOmega })\cup T$. A sufficient condition for the above to happen is $\overline{S}\cap \overline{T}=\emptyset$, while a necessary and sufficient condition is not known.