Abstract
We show that a domain is an extension domain for a Hajłasz–Besov or for a Hajłasz–Triebel–Lizorkin space if and only if it satisfies a measure density condition. We use a modification of the Whitney extension where integral averages are replaced by median values, which allows us to handle also the case \(0<p<1\). The necessity of the measure density condition is derived from embedding theorems; in the case of Hajłasz–Besov spaces we apply an optimal Lorentz-type Sobolev embedding theorem which we prove using a new interpolation result. This interpolation theorem says that Hajłasz–Besov spaces are intermediate spaces between \(L^p\) and Hajłasz–Sobolev spaces. Our results are proved in the setting of a metric measure space, but most of them are new even in the Euclidean setting, for instance, we obtain a characterization of extension domains for classical Besov spaces \(B^s_{p,q}\), \(0<s<1\), \(0<p<\infty \), \(0<q\le \infty \), defined via the \(L^p\)-modulus of smoothness of a function.
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Acknowledgments
The authors thank the referees for carefully reading the paper and valuable comments. The research was supported by the Academy of Finland, Grants No. 135561, 141021 and 272886. Part of the paper was written when the third author was visiting the Université Paris-Sud (Orsay) in Springs 2013 and 2014 and while the authors visited The Institut Mittag–Leffler in Fall 2013. They thank these institutions for their kind hospitality.
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Communicated by Stephan Dahlke.
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Heikkinen, T., Ihnatsyeva, L. & Tuominen, H. Measure Density and Extension of Besov and Triebel–Lizorkin Functions. J Fourier Anal Appl 22, 334–382 (2016). https://doi.org/10.1007/s00041-015-9419-9
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DOI: https://doi.org/10.1007/s00041-015-9419-9