Source: Forrester, Jay W., World Dynamics. 1973 second ed. 1971, Waltham, MA: Pegasus Communications. P. 144, Reproduced by permission of Jay W. Forrester.
Source: Forrester, Jay W., Urban Dynamics. 1969, Waltham, MA: Pegasus Communications. P. 285. Reproduced by permission of Jay W. Forrester.
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