Dynamical systems

• What Quantity Is Conserved in the Lotka–Volterra System?
  For the standard Lotka–Volterra predator–prey equations, a conserved quantity is d x − c ln x + b y − a ln y; here is its derivation.
• How to Use Variation of Parameters for a 3×3 System
  Use a fundamental matrix to solve a nonhomogeneous 3×3 linear ODE system by variation of parameters, with a worked example.