The tendency of economic activity to congregate in a restricted number of central places, encouraged by functional linkages, and external and agglomeration economies. See Hanson in G. L. Clark et al., eds (2000). Falcioğlu and Akgüngör (2008) Eur. Plan. Studs 16, 2, in a study of centralization in Turkey, confirm the Krugman hypothesis (1991, J. Pol. Econ. 99) that industries become more concentrated with economic integration. Rodríguez-Pose and Zademach (2003) Urb. Studs 40, 10 find that proximity appears to play a distinctive role in the geography of mergers and acquisitions in Germany.