Delay differential equation
In mathematics, delay differential equations (DDEs) are a type of differential equation in which the derivative of the unknown function at a certain time is given in terms of the values of the function at previous times. DDEs are also called timedelay systems, systems with aftereffect or deadtime, hereditary systems, equations with deviating argument, or differentialdifference equations. They belong to the class of systems with the functional state, i.e. partial differential equations (PDEs) which are infinite dimensional, as opposed to ordinary differential equations (ODEs) having a finite dimensional state vector. Four points may give a possible explanation of the popularity of DDEs.[1] (1) Aftereffect is an applied problem: it is well known that, together with the increasing expectations of dynamic performances, engineers need their models to behave more like the real process. Many processes include aftereffect phenomena in their inner dynamics. In addition, actuators, sensors, communication networks that are now involved in feedback control loops introduce such delays. Finally, besides actual delays, time lags are frequently used to simplify very high order models. Then, the interest for DDEs keeps on growing in all scientific areas and, especially, in control engineering. (2) Delay systems are still resistant to many classical controllers: one could think that the simplest approach would consist in replacing them by some finitedimensional approximations. Unfortunately, ignoring effects which are adequately represented by DDEs is not a general alternative: in the best situation (constant and known delays), it leads to the same degree of complexity in the control design. In worst cases (timevarying delays, for instance), it is potentially disastrous in terms of stability and oscillations. (3) Delay properties are also surprising since several studies have shown that voluntary introduction of delays can also benefit the control. (4) In spite of their complexity, DDEs however often appear as simple infinitedimensional models in the very complex area of partial differential equations (PDEs).
Differential equations  

Navier–Stokes differential equations used to simulate airflow around an obstruction.  
Classification  
Types


Relation to processes 

Solution  
General topics 

Solution methods 

A general form of the timedelay differential equation for is
where represents the trajectory of the solution in the past. In this equation, is a functional operator from to
Examples
 Continuous delay
 Discrete delay
 for .
 Linear with discrete delays
 where .
 Pantograph equation
 where a, b and λ are constants and 0 < λ < 1. This equation and some more general forms are named after the pantographs on trains.[2][3]
Solving DDEs
DDEs are mostly solved in a stepwise fashion with a principle called the method of steps. For instance, consider the DDE with a single delay
with given initial condition . Then the solution on the interval is given by which is the solution to the inhomogeneous initial value problem
 ,
with . This can be continued for the successive intervals by using the solution to the previous interval as inhomogeneous term. In practice, the initial value problem is often solved numerically.
Example
Suppose and . Then the initial value problem can be solved with integration,
i.e., , where the initial condition is given by . Similarly, for the interval we integrate and fit the initial condition,
i.e.,
Reduction to ODE
In some cases, differential equations can be represented in a format that looks like delay differential equations.
 Example 1 Consider an equation
 Introduce to get a system of ODEs
 Example 2 An equation
 is equivalent to
 where
The characteristic equation
Similar to ODEs, many properties of linear DDEs can be characterized and analyzed using the characteristic equation.[4] The characteristic equation associated with the linear DDE with discrete delays
is
 .
The roots λ of the characteristic equation are called characteristic roots or eigenvalues and the solution set is often referred to as the spectrum. Because of the exponential in the characteristic equation, the DDE has, unlike the ODE case, an infinite number of eigenvalues, making a spectral analysis more involved. The spectrum does however have some properties which can be exploited in the analysis. For instance, even though there are an infinite number of eigenvalues, there are only a finite number of eigenvalues to the right of any vertical line in the complex plane.
This characteristic equation is a nonlinear eigenproblem and there are many methods to compute the spectrum numerically.[5] In some special situations it is possible to solve the characteristic equation explicitly. Consider, for example, the following DDE:
The characteristic equation is
There are an infinite number of solutions to this equation for complex λ. They are given by
 ,
where W_{k} is the kth branch of the Lambert W function.
See also
Notes
 Richard, JeanPierre (2003). "Time Delay Systems: An overview of some recent advances and open problems". Automatica. 39 (10): 1667–1694. doi:10.1016/S00051098(03)001675.
 Griebel, Thomas (20170101). "The pantograph equation in quantum calculus". Masters Theses.
 "The dynamics of a current collection system for an electric locomotive". royalsocietypublishing.org. 1971. doi:10.1098/rspa.1971.0078. Retrieved 20190126.
 Michiels, Niculescu, 2007 Chapter 1
 Michiels, Niculescu, 2007 Chapter 2
References
 Bellman, Richard; Cooke, Kenneth L. (1963). Differentialdifference equations. New YorkLondon: Academic Press. ISBN 9780120848508.
 Driver, Rodney D. (1977). Ordinary and Delay Differential Equations. New York: Springer Verlag. ISBN 0387902317.
 Michiels, Wim; Niculescu, SilviuIulian (2007). Stability and stabilization of timedelay systems. An eigenvalue based approach. doi:10.1137/1.9780898718645. ISBN 9780898716320.
 Briat, Corentin (2015). Linear ParameterVarying and TimeDelay Systems. Analysis, Observation, Filtering & Control. Springer Verlag Heidelberg. ISBN 9783662440490.
 Bellen, A., & Zennaro, M. (2013). Numerical methods for delay differential equations. Oxford university press.
External links
 Skip Thompson (ed.). "DelayDifferential Equations". Scholarpedia.