各位朋友，最近正在试着研究中国城市的人口密度模型，请各位帮忙提供如下文献的全文，特别是第4和第5个，他们相当重要 ，方便的话请发送到我的邮箱，谢谢了！

1. Kim, K. H. and J. F. McDonald (1987). "Sufficient Conditions for Negative Exponential Densities - a Further Analysis." Journal of Regional Science 27(2): 295-298.

2. McDonald, J. F. (1975). "Some Causes of Decline of Central Business District Retail Sales in Detroit." Urban Studies 12(2): 229-233.

3. McDonald, J. F. (1987). "The Identification of Urban Employment Subcenters." Journal of Urban Economics 21(2): 242-258.

4. McDonald, J. F. (1989). "Econometric Studies of Urban-Population Density - a Survey." Journal of Urban Economics 26(3): 361-385.

5. McDonald, J. F. and H. W. Bowman (1976). "Some Tests of Alternative Urban-Population Density Functions." Journal of Urban Economics 3(3): 242-252.

6. McDonald, J. F. and D. P. McMillen (1990). "Employment Subcenters and Land Values in a Polycentric Urban Area - the Case of Chicago." Environment and Planning A 22(12): 1561-1574.