On the computation of $\pi$-flat outputs for differential-delay systems
  We introduce a new definition of $\pi$-flatness for linear differential delay systems with time-varying coefficients. We characterize $\pi$- and $\pi$-0-flat outputs and provide an algorithm to efficiently compute such outputs. We present an academic example of motion planning to discuss the pertinence of the approach. 
F. Antritter and F. Cazaurang and J. L\'evine and J. Middeke
math.OC