On extremal and perfect σ-algebras for flows

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Faculty of Mathematics and Informatics, Institute of Mathematics, Nicholas Copernicus University, Chopina 12/18, Toruń, Polanf
Wrocław Technical University, Wybrzeże Wyspiańskiego 27, 87-100 Toruń, Poland 50-370 Wrocław, Poland

It is shown that there exists a flow on a Lebesgue space with finite entropy and an extremal σ-algebra of it which is not perfect.

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