Volgmann, Kati. “Dynamics of Metropolitan Functions in German Polycentric Urban System: Spatial Pattern of Concentration and Functional Specialization”. Raumforschung Und Raumordnung | Spatial Research and Planning, vol. 72, no. 1, Feb. 2014, pp. 21–37, https://doi.org/10.1007/s13147-013-0264-5.