Three Critical Points and Type of Local Phaseportrait

In summary, Homework Statement: Determine the three critical points of the system and their type of local phase portrait.
  • #1
aeroguy2008
46
0

Homework Statement



y1'= −4*y1+y2+y1*y2
y2'= −2*y1−y2+y1*y1

Determine the three critical points of the system and their type of local phase portrait (stable node, unstable, saddle point, spiral, center, no node)

Hence I need to get three critical points (x1,y1), (x2,y2) & (x3,y3) and their local phase portraits. Can somebody help pls?

 
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  • #2
Set both y1' and y2' to zero and solve to get three solutions.
 
  • #3
Do you know the definition of "critical point"?
 
  • #4
Is a critical point when I differentiate the function and put it to zero?
 
  • #5
You already have the derivatives!

As dirk mec1 said, set each of those derivatives equal to 0 and solve for y1 and y2. (The way your system is set up, each critical point will be of the form (y1, y2).)

Now, how do you determine the phase portrait type?
 
  • #6
To elaborate even further, a y1' = 0 and y2' = 0 are your nullclines, and the critical points can be thought of as the intersections of these nullclines.
 
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  • #7
Thanks...once I set these two equations I got y values of; y1,1=0, y1,2=-1, y1,3=2...how do i move on from here?
 
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  • #8
Well, what are the y2 values that correspond to each of your y1 values?
 
  • #9
aeroguy2008 said:
Thanks...once I set these two equations I got y values of; y1,1=0, y1,2=-1, y1,3=2...how do i move on from here?

Do you mean that the three values of y1 are 0, -1, and 2? What is the corresponding value of y2 for each y1?

I'm a bit concerned about the whole tenor of this thread. If you are at a point in a course where you are expected to be able to draw local phase portraits, or determine whether a given critical point is a node, center, etc., finding the points themselves should be trivial. Yet you sound like you really have no idea what the problem is asking.
 
  • #10
Well you can be a whole lot concerned. I am just trying to learn something.
 
  • #11
aeroguy2008 said:
Well you can be a whole lot concerned. I am just trying to learn something.

Then I can be a lot less concerned. I was afraid you were taking a course in differential equation (and might have to take the final exam next week)!
 

Related to Three Critical Points and Type of Local Phaseportrait

1. What are the three critical points in a phase portrait?

The three critical points in a phase portrait are the stable point, unstable point, and saddle point. These points represent the behavior of a dynamical system at different equilibrium states.

2. How are the critical points determined in a phase portrait?

The critical points in a phase portrait can be determined by setting the derivatives of the system's variables to zero and solving for the equilibrium points. Alternatively, they can be found by graphing the system's equations and identifying where the curves intersect.

3. What is the significance of the type of critical point in a phase portrait?

The type of critical point in a phase portrait determines the behavior of the system near that point. A stable point indicates that nearby solutions will approach the equilibrium state, while an unstable point means that nearby solutions will move away from the equilibrium. A saddle point represents a mixture of stable and unstable behavior.

4. How can the type of local phase portrait be determined?

The type of local phase portrait can be determined by analyzing the eigenvalues of the Jacobian matrix at the critical point. If all eigenvalues are negative, the point is stable. If all eigenvalues are positive, the point is unstable. If there are both positive and negative eigenvalues, the point is a saddle.

5. What does a phase portrait reveal about a dynamical system?

A phase portrait provides a visual representation of the behavior of a dynamical system over time. It reveals the equilibrium points, their stability, and the direction of movement for solutions near those points. This information is crucial in understanding the long-term behavior of a system and predicting its future states.

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