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][1], 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?

  [1]: http://math.stanford.edu/~katznel/nonsingact.pdf