If X and Y are ergodic G
spaces, where G is a countable discrete group and X is an extension of Y , we study
the embedding of the group-measure von Neumann algebra corresponding to (Y,G)
into the group-measure von Neumann algebra corresponding to (X,G). Necessary
and sufficient conditions for the existence of a normal faithful conditional
expectation are established. Under appropriate conditions the normalizer of the
subalgebra is determined, and a correspondence between intermediate quotient
actions and intermediate von Neumann algebras is established. A relationship
between normal extensions with relatively discrete spectrum and crossed dual
products of von Neumann algebras by compact second countable groups is
determined.
