This monograph has arisen out of a number of attempts spanning almost
five decades to understand how one might examine the evolution of
densities in systems whose dynamics are described by differential delay
equations. Though the authors have no definitive solution to the
problem, they offer this contribution in an attempt to define the
problem as they see it, and to sketch out several obvious attempts that
have been suggested to solve the problem and which seem to have failed.
They hope that by being available to the general mathematical community,
they will inspire others to consider-and hopefully solve-the problem.
Serious attempts have been made by all of the authors over the years and
they have made reference to these where appropriate.