1. 1Department of Spatial Economics, Vrije Universiteit Amsterdam, Amsterdam, Netherlands; email: h.koster@vu.nl
2. 2Tinbergen Institute, Amsterdam, Netherlands
3. 3Cent. Oper. Res. Econom. (CORE), UC Louvain, Louvain-la-Neuve, Belgium; email: jacques.thisse@uclouvain.be
4. 4Institute of Developing Economies, Chiba, Japan