“…Recently, reachable sets have been used to increasing effect, for example, in problems in engineering design, in the study of the vulnerability of renewable resource systems, and in the stability of an ecosystem (for instance, a food chain) against external perturbations. See, for example, Grantham and Vincent (1975), Vincent and Anderson (1979), Goh (1976Goh ( , 1980, Fisher and Goh (1981), Vincent (1981) and Leitmann (1983). However, the theory of reachable sets for both linear and nonlinear .systems remains to be fully developed, and present techniques for calculating reachable set boundaries of systems of order three or more are generally difficult or impossible to apply.…”