Lyapunov function

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Definition

Consider a differentiable vectorfield f:X \rightarrow X, x \mapsto f(x), defined on a complete metric space X. A differentiable function V:U \rightarrow \mathbb{R}, defined on an open subset U is called a Lyapunov function for f on U if the inequality: \overset{\circ}{V}(x) := \nabla V(x)^T f(x) \, \leq 0 is satisfied for all x \in U.
\overset{\circ}{V} defined as above is called the orbital differential of V at x.

In other words, a Lypunov function is decreasing along the orbits of points in U that are introduced by the flow corresponding to the vectorfield f.

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