A functional differential equation (FDE) describes the evolution of a dynamical system
for which the rate of change of the state variable depends on not only the current
but also the historical and even future states of the system. FDEs arise naturally in
economics, life sciences, and engineering, and the study of FDEs has been a major
source of inspiration for advancement of nonlinear analysis and infinite-dimensional
dynamical systems. Therefore, FDEs provide an excellent theoretical platform for
developing an interdisciplinary approach to understanding complex nonlinear phenomena
via appropriate mathematical techniques.