Clemens, C. and Clemens, A. 1999. Peripheries, boundaries, transition zones: Boundaries and borders as a basis for Spatial Planning around cities. Raumforschung und Raumordnung | Spatial Research and Planning. 57, 5-6 (Sep. 1999), 350–358. DOI:https://doi.org/10.1007/BF03184500.