Application of Jacobi stability analysis to a first-order dynamical system: relation between nonlinearizability of one-dimensional differential equation and Jacobi stable region

In this study, we discuss Jacobi stability in equilibrium and nonequilibrium regions for a first-order one-dimensional system using deviation curvatures. The deviation curvature is calculated using the Kosambi-Cartan-Chern theory, which is applied to second-order differential equations. The deviation curvatures of the first-order one-dimensional differential equations are calculated using two methods as follows. Method 1 is only differentiating both sides of the equation. Additionally, Method 2 is differentiating both sides of the equation and then substituting the original equation into the second-order system. From the general form of the deviation curvatures calculated using each method, the analytical results are obtained as (A), (B), and (C). (A) Equilibrium points are Jacobi unstable for both methods; however, the type of equilibrium points is different. In Method 1, the equilibrium point is a nonisolated fixed point. Conversely, the equilibrium point is a saddle point in Method 2. (B) When there is a Jacobi stable region, the size of the Jacobi stable region in the Method 1 is different from that in Method 2. Especially, the Jacobi stable region in Method 1 is always larger than that in Method 2. (C) When there are multiple equilibrium points, the Jacobi stable region always exists in the nonequilibrium region located between the equilibrium points. These results are confirmed numerically using specific dynamical systems, which are given by the logistic equation and its evolution equation with the Hill function. From the results of (A) and (B), differences in types of equilibrium points affect the size of the Jacobi stable region. From (C), the Jacobi stable regions appear as nonequilibrium regions where the equations cannot be linearized.