1. Chicago: University of Chicago Press, 1929.
2. Pick realized this, but he did not present the mathematics. He also saw that the Varignon frame is not limited to three-point problems.
3. E. M. HooverThe Location of Economic Activity (New York: McGraw-Hill, 1948). A brief summary is offered in my “Location Theory,” in J. R. P. Friedmann and W. Alonso,Regional Development and Planning (Cambridge: Massachusetts Institute of Technology Press, 1964).
4. H. W. Kuhn and R. E. Kuenne, “An Efficient Algorithm for the Numerical Solution of the Generalized Weber Problem in Spatial Economics,”Journal of Regional Science, Winter, 1962.
5. An interesting exposition of these concepts applied to geographic matters is found in W. Warntz, “The Topology of a Socio-Economic Terrain and Spatial Flows,”Papers of the Regional Science Association, XVII, 1966. Other treatments of the mathematical tools presented here may be found in standard texts of mathematics, particularly those for engineers, under sections dealing with analytic geometry and vector analysis.