Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2001/iii/paper-52/1/solution

Let be prey and predator abundances. With constant positive demographic and predation coefficients, the Lotka-Volterra equations are
Their positive equilibrium is , . Its Jacobian matrix and eigenvalues are
The linearized populations oscillate with period , with the predator peak following the prey peak. This is a center equilibrium, not an attracting equilibrium. The conclusion is supported beyond linearization by the Logarithmic first integral of the Lotka-Volterra equations: in these units one may use
Direct differentiation gives . Its Hessian matrix is a positive-definite matrix in the positive quadrant, and its minimum is at . Nearby level curves are closed periodic orbits, so there is no damping toward a preferred oscillation amplitude.

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