@article{Lichawski:1989,
  author  = "K. Lichawski and J. Matkowski and J. Miķ",
  title   = "Locally defined operators in the space of differentiable functions",
  journal = "Bulletin of the Polish Academy of Sciences : Mathematics",
  volume  = "Vol. 37",
  number  = "no 1-6",
  pages   = "315--325",
  year    = "1989",
}