Gradient-like vector field
In differential topology, a mathematical discipline, and more specifically in Morse theory, a gradient-like vector field is a generalization of gradient vector field. The primary motivation is as a technical tool in the construction of Morse functions, to show that one can construct a function whose critical points are at distinct levels. One first constructs a Morse function, then uses gradient-like vector fields to move around the critical points, yielding a different Morse function.
Definition
Given a Morse function f on a manifold M, a gradient-like vector field X for the function f is, informally:
- away from critical points, X points "in the same direction as" the gradient of f, and
- near a critical point (in the neighborhood of a critical point), it equals the gradient of f, when f is written in standard form given in the Morse lemmas.
Formally:[1]
- away from critical points, [math]\displaystyle{ X \cdot f \gt 0, }[/math]
- around every critical point there is a neighborhood on which f is given as in the Morse lemmas:
- [math]\displaystyle{ f(x) = f(b) - x_1^2 - \cdots - x_{\alpha}^2 + x_{\alpha +1}^2 + \cdots + x_n^2 }[/math]
and on which X equals the gradient of f.
Dynamical system
The associated dynamical system of a gradient-like vector field, a gradient-like dynamical system, is a special case of a Morse–Smale system.
References
- An introduction to Morse theory, Yukio Matsumoto, 2002, Section 2.3: Gradient-like vector fields, p. 56–69
- Gradient-Like Vector Fields Exist, September 25, 2009
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