In this paper we study invatiant intervals)the global asymptotic stability and theoscillation of the positive solution of the second order nonlinear difference equation Yn+1=(?),and discuss invariant intervals,the global asymptotic stability of the positivesolution of the the second order rational difference equation Xn+1=(?),where parameters a,b,A,B,C,α,β,γ>0,and C≥B.In addition,we consider the second order nonlinear differential equation(Gompertzmodel existing reserve area with delay)andand discuss the combined effects of harvesting and time delay on these models.weobtain the global asymptotic stability,the sufficient-necessary condition for the existenceof the positive solution,and the optimal harvesting policy without delay;otherwise,somesufficient conditions which ensure the stability of the positive solution with delay are provided,where parameters r,s,Q,σ1,σ2,K,L,τ>0,and 0≤E≤Emax.
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