Mathematics is the universal language. 


That is, until someone says the word "obvious", or "well known". At which point it becomes relative to the reader.


My question is about a "well known" theorem. My problem is that it is not known to me. But I would like to know.


The following comes from Y. Katznelson and B. Weiss [The Classification of Non-Singular Actions, Revisited](http://math.stanford.edu/~katznel/nonsingact.pdf), J. Ergodic Theory and Dynamical Systems, 11, 1991. Page 4.


It reads:



> 
> Thus the family {$\theta\_k$} defines a Boolean set mapping between the $\sigma$-algebras generated by the ladder sets. If the ladder sets are both "algebra complete", a well known theorem implies that there exists a point mapping $\theta : X \mapsto X^\prime$ which induces a set mapping.
> 
> 
> 


Can someone please tell me which theorem they are referring to here?



Point mapping induces a set mapping
This reference seems to be exhaustive (and exhausting) on the subject:


<http://matwbn.icm.edu.pl/ksiazki/cm/cm2/cm2131.pdf>


