Fixed points and local dynamical stability
For an autonomous smooth system ẋ=f(x), equilibria satisfy f(x*)=0 and nearby perturbations obey δẋ≈Jδx, J_ij=∂f_i/∂x_j at x*.
Key knowledge
For an autonomous smooth system ẋ=f(x), equilibria satisfy f(x*)=0 and nearby perturbations obey δẋ≈Jδx, J_ij=∂f_i/∂x_j at x*. Strictly negative eigenvalue real parts imply local asymptotic stability; a positive real part implies instability. Zero real parts require nonlinear analysis; bounded oscillations can be Lyapunov stable without attracting.
Values and equations
f(x*)=0; δẋ≈Jδx; J_ij=(∂f_i/∂x_j)|_x*; δx_mode∝e^(λt)
- x,x*,δx
- State, equilibrium and perturbation, with component-dependent units
- f_i
- Component state rate
- J_ij
- Jacobian entry; unit [x_i]/([x_j] s)
- λ
- Jacobian eigenvalue s⁻¹
- t
- Time s
Conditions and scope
- Continuously differentiable finite-dimensional autonomous flow and a local equilibrium; eigenvalues have inverse-time units after consistent state scaling.
- The linear test is decisive only in the stated strict-sign cases. A picture of linear closed orbits does not establish that a nonlinear equilibrium with imaginary eigenvalues is a center.
Sources · 1
References
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Evidence and review
Classical Mechanics III 8.09 (2014): Chapter 7, nonlinear dynamics: §7.2 equation 7.18 and §7.3.1 printed pages 163–168: Jacobian, eigenvalue solution and separate attracting/Lyapunov definitions. Excludes typo det(M)=ac−bd in 7.42 (correct is ad−bc), reversed stable-spiral captions and unqualified nonlinear-center inference.
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