How Do You Find the First Integral of This System of Differential Equations?

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Discussion Overview

The discussion revolves around finding the first integral of a system of differential equations defined by dx/dt = y + x² - y² and dy/dt = -x - 2xy. Participants explore various methods and transformations to simplify the equations for solving.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in solving the system and suggests that a change of variables might help, specifically considering x²y as a new variable.
  • Another participant proposes a hypothetical alteration of the equations to dx/dt = y + x² + y² and dy/dt = x + 2xy, prompting a discussion on potential approaches.
  • Several participants mention the method of separation of variables but indicate that it does not seem applicable to the original equations.
  • A suggestion is made to consider a variable change to u = x + y and v = x - y, which some participants believe could simplify the equations.
  • One participant notes that a g(u/v) substitution was anticipated but found to be more complex than expected.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific method to solve the differential equations. Multiple competing views on variable transformations and solution strategies remain present throughout the discussion.

Contextual Notes

There are limitations in the assumptions made about the applicability of separation of variables and the effectiveness of proposed variable changes. The discussion reflects various perspectives on how to approach the problem without resolving the mathematical steps involved.

LBJking123
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I am trying to solve this system DE's to determine the systems First Integral.

dx/dt = y+x2-y2
dy/dt = -x-2xy

I am pretty sure I need to pick some different variables to use to make the equation easier to solve, but I can't get anything to work. I thought about letting a variable be x2y, but that doesn't help much. If anyone can help me that would be much appreciated!
 
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Hi LBJking123! :smile:

Hint: suppose it was

dx/dt = y+x2+y2
dy/dt = x+2xy​

what would you do? :wink:
 
I tried solving by separation of variables, but I can't figure out how to get all of the x's to one side and y's to the other. I think I am totally missing something obvious...
 
LBJking123 said:
I tried solving by separation of variables, but I can't figure out how to get all of the x's to one side and y's to the other. I think I am totally missing something obvious...

As someone said, you need to pick some different variables. Can you do that for tiny-tim's example?
 
LBJking123 said:
I tried solving by separation of variables, but I can't figure out how to get all of the x's to one side and y's to the other.

Yes, separation of variables won't work.

Hint: suppose it was

dx/dt = y
dy/dt = x ?​
 
That case you could divide the two equations, and then get xdx=ydy. Then I would integrate both sides to get the answer. That technique won't work for the original DE's though...
 
(just got up :zzz:)
LBJking123 said:
That case you could divide the two equations, and then get xdx=ydy. Then I would integrate both sides to get the answer.

no, all that gives you is x2 - y2 = constant …

how does that help? :redface:

try that example again :smile:
 
I am not QUITE sure where tiny-tim is trying to lead you. LBJking123, but I, at least, felt that the variable change u=x+y and v=x-y simplifies the equations in a manner that may be amenable for further simplifications.
 
arildno said:
I am not QUITE sure where tiny-tim is trying to lead you. LBJking123, but I, at least, felt that the variable change u=x+y and v=x-y simplifies the equations in a manner that may be amenable for further simplifications.

hi arildno! :smile:

yes, that would be the way to solve my easy example (but i was hoping LBJking123 would see it on on his own :redface:)
 
  • #10
Well, I spotted some further troubles on the way (I was hoping a g(u/v) substitution would turn up, but it doesn't seem to be THAT simple..)
So, I have been following this thread for a while, and am hoping to see some real cleverness on your part in the end that I have missed.
:smile:
 
  • #11
think laterally! :wink:

(but don't give away the answer)
 

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