Isomorphic and isometric structure of the optimal domains for Hardy-type operators

Tomasz Kiwerski ¹   ; Paweł Kolwicz ¹   ; Lech Maligranda ²

¹ Institute of Mathematics Poznan University of Technology Piotrowo 3A 60-965 Poznań, Poland
² Department of Engineering Sciences and Mathematics Luleå University of Technology SE-971 87 Luleå, Sweden and Institute of Mathematics Poznan University of Technology Piotrowo 3A 60-965 Poznań, Poland

Studia Mathematica, Tome 260 (2021) no. 1, pp. 45-89

Résumé

We investigate the structure of optimal domains for the Hardy-type operators including, for example, the classical Cesàro, Copson and Volterra operators as well as some of their generalizations. We prove that, in some sense, the abstract Cesàro and Copson function spaces are closely related to the space $L^1$, namely, they contain “in the middle” a complemented copy of $L^1[0,1]$ and an asymptotically isometric copy of $\ell ^1$, and can also be renormed to contain an isometric copy of $L^1[0,1]$. Moreover, generalized Tandori function spaces are quite similar to $L^\infty $ because they contain an isometric copy of $\ell ^\infty $ and can be renormed to contain an isometric copy of $L^\infty [0,1]$. Several applications to the metric fixed point theory will be given. Next, we prove that the Cesàro construction $X \mapsto CX$ does not commute with the truncation operation of the measure space support. We also study whether a given property transfers between a Banach function space $X$ and the space $TX$, where $T$ is the Cesàro or the Copson operator. In particular, we find a large class of properties which do not lift from $TX$ into $X$ and we prove that abstract Cesàro and Copson function spaces are never reflexive, are not isomorphic to a dual space and do not have the Radon–Nikodym property in general.

Détail

DOI : 10.4064/sm200211-8-9

Keywords: investigate structure optimal domains hardy type operators including example classical ces copson volterra operators their generalizations prove sense abstract ces copson function spaces closely related space namely contain middle complemented copy asymptotically isometric copy ell renormed contain isometric copy moreover generalized tandori function spaces quite similar infty because contain isometric copy nbsp ell infty renormed contain isometric copy infty several applications metric fixed point theory given prove ces construction mapsto does commute truncation operation measure space support study whether given property transfers between banach function space space where ces copson operator particular large class properties which lift prove abstract ces copson function spaces never reflexive isomorphic dual space have radon nikodym property general