Finding Eigenvectors & Stabilizing 0,0 in System Stability

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Homework Help Overview

The discussion revolves around a dynamical system defined by the equations \(\dot{x}=y^2\) and \(\dot{y}=x^2\). Participants are exploring the classification of the stability of the fixed point at (0,0) and the process of finding eigenvectors for sketching the phase portrait.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to find eigenvectors and classify the stability of the fixed point. Some participants question the necessity of computing eigenvectors, while others suggest that the system's Hamiltonian nature may simplify the analysis.

Discussion Status

Participants are actively discussing the implications of the system being Hamiltonian and how this affects the need for eigenvector computation. There is acknowledgment of the complexity of the problem, with some guidance provided regarding the Hamiltonian approach.

Contextual Notes

There are assumptions regarding the stability of the fixed point based on the positivity of \(x^2\) and \(y^2\) for non-zero values. The discussion also touches on the integration of the system's equations to find the Hamiltonian, which influences the phase portrait.

namu
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For the system

\dot{x}=y2
\dot{y}=x2

Both the eigenvalues are zero. How do I
find the eigenvectors so that I can sketch
the phase portrait and how do I classify
the stability of the fixed point (0,0)?
 
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Well, obviously, both x^2 and y^2 are positive for all non-zero x and y so (0, 0) is unstable.
 
Yes, that is true. Thank you. How do I find the eigenvectors though?
 
It is not necessary to compute eigenvectors. This system is Hamiltonian (conservative). On dividing one equation by the other you get
\begin{equation}
\frac{dx}{dy} = \frac{y^2}{x^2}
\end{equation}
Separating variables and integrating you find the Hamiltonian
\begin{equation}
H(x,y) = \frac{1}{3} (x^3-y^3)
\end{equation}
The level sets \begin{equation}H = constant\end{equation} define the phase portrait.
 
oh my god, that make life so easy. Thank you!
 

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