  Borchers and Wiesbrock have demonstrated certain results concerning the
one-parameter semigroups of endomorphisms of von Neumann algebras that appear
as lightlike translations in the theory of algebras of local observables. These
results are abstracted and analyzed as essentially operator-theoretic. Criteria
are then demonstrated for a spatial derivation of a von Neumann algebra to
generate a one-parameter semigroup of endomorphisms. All this is combined to
establish a von Neumann-algebraic converse to the Borchers-Wiesbrock results.
This analysis is then applied to questions of isotony and covariance for local
algebras.
