A functional differential equation is one in which the
rate of change of y(x) depends not only on the values of y
for the same time value but also on time values less than
x. In the simplest case, this has the form
y
(x)
=
f (x, y(x), y(x
−
δ)) (1.1)
where δ is a constant delay. Throughout this article they
will be referred to as delay differential equations (DDEs)
or difference differential equations.