==== Goal and Motivation ====
  * understanding interplay between the spatial distribution of population, economic activity and interactions within cities. 
  * Integrated model for the origins of scale-invariant power laws: Zipf's law, Fractal structure, quantity related scaling law. 
  * Policy implication of urban planning [[city:urbanplanning]]

==== Elements ====
  * Fractal distribution of population: possibly (more or less) invariant
  * [[city:physical area:travel budget| constant travel budget]] 
  * hierarchical structure of businesses: 1) frequency of needs and 2) creation of new businesses (associated with innovation and solving societal problems which requires higher interaction and diversity). 
==== Data ====
  * SEDAC MSAs http://sedac.ciesin.columbia.edu/data/collection/usgrid/sets/browse
  * Census County data (at the level of MSAs, county, zip codes). 
==== Literatures ====
  * population density distribution: 
    * one of the oldest and most popular models proposes that population density shows a simple exponential decay with radial distance from he city centre: \rho (r) = \rho_0 exp ^{-br} -- C. Clark "Urban Population Densities", Journal of the Royal Statistical Society vol 114 no. 4 pp.490-496 (1951). 
    * This form is analytically tractable and shows good agreement in many empirically studied cases --by J. C. Martori and J. Surinach, "Classical Models of urban population density", Congress of the European Regional Science Association, 2001
    * Newling suggests modifying this to a quadratic exponential function \rho(r) = \rho_0 exp^{\br-cr^2} in order to add the flexibility to account for the density crater seen towards the centre of some larger cities [B. E. Newling, "The Spatial Variation of Urban population Densities", Geographical Review 59 2 242-252 (1969). 
    * Decaying power-law function: \rho(r) = K r^{-a}: First introduced by Smeed. although there is some analytical awkwardness it has some empirical advantage. by R. J. Smeed, The traffic Problem in Towns (Manchester Statistical Society Papers). Norbury Lockwood: Manchester, 1961. 
    * Combined power-law with gamma function: \rho(r) = K r^{-ar} exp^{-br}: S. Angel and G. M. Hyman, Urban fields: a geometry of movement for regional science. Pion: London (1976). 
    * common characteristics of these models is radial symmetry
    * Batty and Kim (1992) develop an interesting argument in favor of using a power-law by M. Batty and K. S. Kim "form Follows Function: Reforumulating Urban population density functions", Urban studies vol. 29, no. 7 pp. 1043-1069 (1992). They find \alpha, the exponent of the fractal structure of cities. Cities tend to exhibit substantial self-similarity on different spatial scales and often have distinct hierarhical structures-- by M. Batty and P. A. Longley, Fractal Cities: A geometry of Form and Function. Academic Press: London 1994; and by S. Guoqiang, "Fractal dimension and fractal growth of urbanized areas", Int. J. Geographical Information Science vol. 16, no. 5, pp.419-437 (2002). and by L. Yongmei and T. Junmei, "Fractal dimension of a transportation network and its relationship with urban growth: a study of the Dallas-Fort Worth area" Env. and Planning B: Planning and Design vol. 31 pp. 895-911 (2004). 
    * Simulated cities "grown" with a diffusion-limited aggregation (DLA) method were found to behave similarly - an encouraging correspondence between this fractal interpretation and a popular statistical model by M. Batty and P. Longley, "Urban growth and form: scaling, fractal geometry, and diffusion-limited aggregation", Environment and Planing A. vol 21 no. 11 pp.1447-1472 (1989). 
    * Multi-center distribution of London (barthelemey) 

  * Central Place Theory (CPT): This comes down to our point: hierarchal structure of spatial organization which are manifested as scaling and fractality, and temporal evolution (do we want to do this???) in a more quantitative way. 
    * Basic literatures: Christaller 1966 and Beckmann 1958 based on L\"oschian 1954 framework of retailers who have endogenously determined market areas and hence scale of production.  
    * CPT, however, is less to do with scale of economic activities by urbanization and innovation but more of "manufactural supply-demand and spatial competition in **retailing to rural areas**".  
    * our work is a slightly different in the sense that we add the creation of new centres and role of bigger cities in the process of economic developments---newly created businesses should be associated with solving coordination problem probably located in larger cities. 

  * Urban Scaling : 
    * Urban scaling: Bettencout et. al 2007, 2012, 2013
    * Optimization of travel time in response to population density: Eom et. al. 2007(?) PNAS. 

  * Business structure and urbanization: 
    * Youn et. al. 2014: implicitly deals with innovation--creation of new types
    * G. Duranton's papers http://real.wharton.upenn.edu/~duranton/Duranton_Papers/Current_Research/urban_growth.pdf
    * H. Venom's book: Urban development (economic geographer)--specialization: 
    * Hsu's paper and Mori2008: http://www.sciencedirect.com/science/article/pii/S0022053114001392
      * different degree of scale economies with city size
    * Berry paper 1958: http://www.jstor.org/stable/pdfplus/142299.pdf?&acceptTC=true&jpdConfirm=true
    * Nathan Schiff's 2013 (Cities and Product Variety){{:city:schiff2013.pdf|}}: the most recent paper using central place theory and hiearchical structure but more to do with consumer product variety--demand aggregation is a function of population size (scale) and land area (transportation cost)
    * Note: when is called a hierarchical structure, it conventionally means a different occurrence frequency--a large city with relatively rare things often has all of the more common cuisines. This is different definition of Youn et. al. where scaling exponents are utilized as parameters of hierarchical order.
    * The specific cuisines found in each city follow a hierarchical structure in which cities with 

  * Innovation and Cities: http://anglejournal.com/article/2015-06-the-city-and-the-triumph-of-diversity/ 

  * Fundamental Hierarchical Structure of techonological change (there has been debate in cultural evolution; but here we are looking at economic evolution): 
    * Brian Arthur's book 'The Nature of Technology' (use of effects; combination; recursive). 
    * Murdock, Carneiro, Provost (measurement of cultural complexity)
    * More invention activities at the largest cities 
    * More technological capacity with larger population (Rober, Boyd)
    *