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Mathematics  2013 

Universal Padé approximants and their behaviour on the boundary

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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 $\OO$ of the universal function $f$. In another kind the universal approximation is not required at any point of $\partial\OO$ but in this case the universal function $f$ can be taken smooth on $\oO$ and, moreover, it can be approximated by it's Taylor partial sums on every compact subset of $\oO$. Similar generic phenomena hold when the partial sums of the Taylor expansion of the universal function are replaced by some Pad\'{e} approximants of it. In the present paper we show that in the case of Pad\'{e} approximants, if $\OO$ is an open set and $S,T$ are two subsets of $\partial\OO$ that satisfy some conditions, then there exists a universal function $f\in H(\OO)$ which is smooth on $\OO\cup S$ and has some Pad\'{e} approximants that approximate $f$ on each compact subset of $\OO\cup S$ and simultaneously obtain universal approximation on each compact subset of $(\C\sm\oO)\cup T$. A sufficient condition for the above to happen is $\oS\cap\oT=\emptyset$, while a necessary and sufficient condition is not known.

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