A Fixed Point for which the Stability Matrix is purely Imaginary, (for ).
See also Differential Equation, Fixed Point, Hyperbolic Fixed Point (Differential Equations), Parabolic Fixed Point, Stable Improper Node, Stable Node, Stable Spiral Point, Stable Star, Unstable Improper Node, Unstable Node, Unstable Spiral Point, Unstable Star
Tabor, M. ``Classification of Fixed Points.'' §1.4.b in
Chaos and Integrability in Nonlinear Dynamics: An Introduction. New York: Wiley, pp. 22-25, 1989.