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center

An equilibrium surrounded by closed trajectories that neither approach nor leave it.

A center is an equilibrium of $\mathbf{x}'=A\mathbf{x}$ with purely imaginary eigenvalues ( $\operatorname{tr}(A)=0$ , $\det(A)>0$ ) -- every nearby trajectory is a closed loop that returns to its starting point, neither spiraling in nor out.