@techreport{NBERw3607,
 title = "Cities in Space: Three Simple Models",
 author = "Paul Krugman",
 institution = "National Bureau of Economic Research",
 type = "Working Paper",
 series = "Working Paper Series",
 number = "3607",
 year = "1991",
 month = "January",
 URL = "http://www.nber.org/papers/w3607",
 abstract = {Urban agglomerations arise at least in part out of the interaction between economies of scale in production and market size effects. This paper develops a simple spatial framework to develop illustrative models of the determinants of urban location, of the number and size of cities, and of the degree of urbanization. A Central theme is the probable existence of multiple equilibria, and the dependence of the range of potential outcomes on a few key parameters.},
}