What is the solution to a 3rd order nonlinear ODE?

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    Nonlinear Ode
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Discussion Overview

The discussion centers around the third-order nonlinear ordinary differential equation (ODE) given by y''' + 2y''y - 3y'^2 = 0. Participants explore the existence of an analytic solution and share insights on the nature of the solutions, including the implications of the equation being autonomous.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant, Max, seeks help in finding an analytic solution to the ODE, expressing that they do not require a full explanation, just hints or the solution with three arbitrary constants.
  • Another participant argues that the presence of three arbitrary constants would only be the case if the ODE were linear, suggesting that the ODE has only trivial solutions due to its autonomous nature.
  • A different participant proposes a specific solution of the form y = 6/(x+a), although this is met with a correction regarding the assumptions about solutions meeting the y-axis.
  • Another participant confirms that the ODE does have an analytic general solution, but notes that it is complex and involves hypergeometric functions, recommending the use of Maple for implicit solutions.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the solutions to the ODE. While some assert that only trivial solutions exist, others propose specific forms of solutions and acknowledge the complexity of the general solution. No consensus is reached regarding the nature and form of the solutions.

Contextual Notes

There are unresolved assumptions regarding the nature of the solutions, particularly in relation to the ODE being autonomous and the implications for the existence of non-trivial solutions. The discussion also highlights the complexity of the general solution involving hypergeometric functions.

Max0526
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Hi, everybody.
I have an ODE:
[tex] y'''+2y''y-3y'^2=0[/tex]
I know that it has an analytic solution, but I cannot get it (yet).
Can anybody help me?
(I don't need a full explanation how to solve it, just some hints or just the solution with 3 arbitrary constants).
Thanks beforehand,
Max.
 
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Max0526 said:
Hi, everybody.
I have an ODE:
[tex] y'''+2y''y-3y'^2=0[/tex]
I know that it has an analytic solution, but I cannot get it (yet).
Can anybody help me?
(I don't need a full explanation how to solve it, just some hints or just the solution with 3 arbitrary constants).
Thanks beforehand,
Max.


Three constants? That would be the case if the ODE was linear.

There is another problem here. The ODE is autonomous, and all functions of the form y=const. are solutions, plus the solutions cannot meet each other. So this ODE has only trivial solutions.
 
How about:
[tex]y = \frac{6}{x+a}[/tex]
 
How about ...you re right?
I mistakingly assumed that every solution has to meet the y axis.
 
Max0526 said:
Hi, everybody.
I have an ODE:
[tex] y'''+2y''y-3y'^2=0[/tex]
I know that it has an analytic solution, but I cannot get it (yet).
Can anybody help me?
(I don't need a full explanation how to solve it, just some hints or just the solution with 3 arbitrary constants).
Thanks beforehand,
Max.

You are right, this ODE has an analytic general solution, but it is not so simple and includes hypergeom functions and so on. To see the solution in implicit form use Maple with

ode:=diff(y(x),x,x,x)+2*diff(y(x),x,x)*y(x)-3*diff(y(x),x)^2=0;

ans:=dsolve(ode);
 

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