1. Where is a research frontier?;Ackerman;Annals of the American Geographers,1963
2. Christaller Central Place Structures: an introductory statement. Studies in Geography No. 22;Alao,1977
3. A dynamic model of growth in a central place system;Allen;Geographical Analysis,1979
4. Qualitative Theory of Second-order Dynamic Systems;Andronov,1973