Hirsch and Smale Stability Definition Confusion

In summary, the conversation is about studying Dynamical Systems from Hirsch and Smale's book "Differential Equations, Dynamical Systems, and Linear Algebra." The book is filled with typos, but is otherwise great. The reader obtained the book from their university library and found that the previous reader had fixed most of the typos with pen/pencil, but had gotten lazy a third of the way in. They are currently stuck on a definition and are unsure if there is an error or if they are misreading it. The definition is about stable equilibriums and uses the notation of U and U_1. The other person in the conversation clarifies that the notation is correct and that the point is that solutions starting in
  • #1
l'Hôpital
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So, I'm studying Dynamical Systems from Hirsch and Smale's "Differential Equations, Dynamical Systems, and Linear Algebra." For those who are acquainted with the book, the book is filled with typos. However, otherwise, it's great. I obtained this book from my University library and it appeared the reader before was very troubled by the typos and so he fixed most of them with pen/pencil. However, it seems the typo fixer got lazy a third of the way in, so I've kinda sort of fixed the errors I found. However, I'm currently stuck in one, that I'm just a little confused on whether it's an error or just a misunderstanding of mine. Straight from Hirsch and Smale:

Definition 1: Suppose [tex]\bar{x} \in W [/tex] is an equilibrium of the differential equation

(1) [tex]x' = f(x)[/tex]

where [tex]f : W \rightarrow E [/tex] is a [tex]C^1[/tex] map from an open set W of the vector space [tex]E[/tex] into [tex]E[/tex].

Then [tex]\bar{x}[/tex] is a stable equilibrium if for every neighborhood [tex]U[/tex] of [tex]\bar{x}[/tex] in [tex]W[/tex] there is a neighborhood [tex]U_1[/tex] of [tex]\bar{x}[/tex] in [tex]U[/tex] such that every solution [tex]x(t)[/tex] with [tex]x(0)[/tex] in [tex]U_1[/tex] is defined and in [tex]U[/tex] for all [tex] t > 0[/tex]. (See Fig. A.)

Fig A:
http://i50.photobucket.com/albums/f348/XavvaX/Smale1.png?t=1274719313

Should it be U instead of U_1 in the definition and vice versa? I say this because x(t) is not defined in U for all t > 0, but rather, defined in U_1. However, I don't know if I'm just misreading the definition and/or the picture. However, in the other picture describing asymptotic stability, they also use the same notation.

Fig B:
http://i50.photobucket.com/albums/f348/XavvaX/Smale2.png?t=1274720012
 
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  • #2
No, what they give is correct. There is nothing said about x(t) being defined in U or in U1. It says that x(0) is defined in U1. The point is that solutions which start in some set U1 may go into a larger set, U, but don't get too far away.
 
  • #3
I understand the intuitive idea, but your very posts confuses me. If the solution starts in U_1 and go to a set bigger than that, then it couldn't be U, since U is in U_1 (as per Fig A). Unless you mean to say that it starts U instead of U_1, which seems to go back to my initial problem.
 

FAQ: Hirsch and Smale Stability Definition Confusion

What is the Hirsch and Smale stability definition and why is it confusing?

The Hirsch and Smale stability definition is a mathematical concept used to analyze the stability of dynamical systems. It can be confusing because it involves complex mathematical equations and terminology.

What are the main differences between the Hirsch and Smale stability definition and other stability definitions?

The Hirsch and Smale stability definition differs from other stability definitions in its use of Lyapunov functions, which are mathematical functions that measure the rate of change of a system over time. It also takes into account the behavior of the system in a neighborhood around an equilibrium point.

How is the Hirsch and Smale stability definition used in scientific research?

The Hirsch and Smale stability definition is commonly used in fields such as physics, biology, and engineering to analyze the stability of complex systems. It is also used in the study of chaotic systems and has applications in control theory and optimization.

What are some common misconceptions about the Hirsch and Smale stability definition?

One common misconception is that the Hirsch and Smale stability definition only applies to linear systems. In reality, it can be applied to nonlinear systems as well. Another misconception is that it guarantees stability, when in fact it only provides a necessary condition for stability.

Are there any limitations to the Hirsch and Smale stability definition?

Like any mathematical model, the Hirsch and Smale stability definition has its limitations. It may not accurately capture the behavior of highly complex systems or systems with external influences. It also relies on certain assumptions about the system, which may not always hold true in real-world scenarios.

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