Breidenbach, Philipp, et al. “Staying Alone or Reaching More Success Together? A Path for More Regional Cooperation in the Ruhrgebiet”. Raumforschung Und Raumordnung | Spatial Research and Planning, vol. 71, no. 2, Apr. 2013, pp. 143–156, https://doi.org/10.1007/s13147-013-0219-x.