Lagrange stability
# Lagrange stability

Lagrange stability is a concept in the stability theory of dynamical systems, named after Joseph-Louis Lagrange.

For any point in the state space, $x\in M$ in a real continuous dynamical system $(T,M,\Phi )$, where $T$ is $\mathbb {R}$, the motion $\Phi (t,x)$ is said to be positively Lagrange stable if the positive semi-orbit $\gamma _{x}^{+}$ is compact. If the negative semi-orbit $\gamma _{x}^{-}$ is compact, then the motion is said to be negatively Lagrange stable. The motion through $x$ is said to be Lagrange stable if it is both positively and negatively Lagrange stable. If the state space $M$ is the Euclidean space $\mathbb {R} ^{n}$, then the above definitions are equivalent to $\gamma _{x}^{+},\gamma _{x}^{-}$ and $\gamma _{x}$ being bounded, respectively.

A dynamical system is said to be positively-/negatively-/Lagrange stable if for each $x\in M$, the motion $\Phi (t,x)$ is positively-/negatively-/Lagrange stable, respectively.

