Outer measure preserving ergodic transformations generate the Carathéodory definition of measurable sets

Abstract

It is known that there are specific examples of ergodic transformations on measure spaces for which the calculation of the outer measure of transformation invariant sets leads to a condition closely resembling Carathéodory's condition for sets to be measurable. It is then natural to ask what functions are capable of `generating', that is leading to, the Carathéodory definition in the same way. The present work answers this question by showing that the property of generating Carathéodory's definition holds for the general class of outer measure preserving ergodic transformations on measure spaces. We further show that the previously found specific examples of functions generating Carathéodory's definition fall into this family of transformations.