Differential equations equilibrium points

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Homework Help Overview

The discussion revolves around a system of differential equations defined by x' = x(x² + y²) and y' = y(x² + y²). Participants are tasked with finding equilibrium points, analyzing the behavior of the linearized system, and describing the phase portrait for the nonlinear system.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss finding equilibrium points, with some noting that x = 0 and y = 0 are potential solutions. There is mention of using the Jacobian matrix and evaluating it at the equilibrium point (0,0) to find eigenvalues. Questions arise about the behavior of the phase portrait and the accuracy of the linearized system in describing local behavior.

Discussion Status

Some participants have provided guidance on calculating the Jacobian and finding eigenvalues, while others express a need for further assistance. There is an acknowledgment that linearization may not be applicable for all systems, prompting exploration of which types may present challenges.

Contextual Notes

Participants are working under the constraints of homework guidelines, which may limit the depth of assistance they can provide to one another. There is a focus on understanding the implications of linearization and the characteristics of the system being analyzed.

sana2476
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Homework Statement



x'=x(x^2+y^2)
y'=y(x^2+y^2)

i) Find all the equilibrium points and describe the behavior of the associated linearized system
ii) Describe the phase portrait for the nonlinear system
iii) Does the linearized system accurately describe the local behavior near the equilibrium points?

Homework Equations



Solving for equilibrium, you get: x=0 and x^2+y^2=0. Likewise with second function, you get: y=0 and x^2+y^2=0. But I don't know what the phase portrait will behave.
 
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I know that getting the Jacobian matrix and evaluating it at (0,0). You get your eigenvalues as 0 as well.
 
I really need help...can someone try and help me with this please!
 
sana2476 said:
I really need help...can someone try and help me with this please!

Hello sana. I can't help you directly here as I have limited access to the web. But I can give you really good advice in the interim: The book "Differential Equations" by Blanchard, Devaney and Hall does a really good job of working with these problems: First linearize it by calculating the partials and form the Jacobian matrix, find the eigenvalues and determine the types of fixed points based on the kinds of eigenvalues. Draw the phase portrait. Mathematica ver 7 has an excellent function for drawing in an instant what use to take hours: It's called "StreamPlot" (I think, or something like this) and draws the entire phase portrait in a single operation.

Note linearization fails for some systems. Find out which types of systems these are.
 

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