@article{Colonius1984,
  author    = {Fritz Colonius},
  title     = {On approximate and exact null controllability of delay systems},
  journal   = {Systems \& Control Letters},
  volume    = {5},
  number    = {3},
  pages     = {209 -- 211},
  doi       = {10.1016/s0167-6911(84)80105-x},
  year      = {1984},
}