Recurrent solutions of neutral differential-difference systems
  Results of Bohr-Neugebauer type are obtained for recurrent functions : If $y$ is a bounded uniformly continuous solution of a linear neutral difference-differential system with recurrent right-hand side, then $y$ is recurrent if $c_0 \not \subset X$ ; also analogues and extensions to half lines are given. For this, various subclasses "$rec$" are introduced which are linear (the set REC of all recurrent functions is not), invariant, closed etc. Also, analogues of the Bohl-Bohr-Amerio-Kadets and Esclangon- Landau results for REC are obtained. 
Bolis Basit and Hans G\"unzler
math.DS