Solve ODE for Y: yy'' = (y')2 - (y')3

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In summary, the conversation discusses the equation yy'' = (y')2 - (y')3 and the steps taken to solve it. It includes the use of the substitution y' = p(y) and the application of the chain rule. The conversation also addresses a mistake in the solution and concludes with a thank you for pointing it out.
  • #1
manenbu
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Homework Statement



yy'' = (y')2 - (y')3

Homework Equations





The Attempt at a Solution



y' = p(y)
y'' - p'p

yp' = p - p2

dp/p + dp/(1-p) = dy/y

ln|p|+ln|1-p| = ln|y|+c

p-p2 = cy

y' - y'2 = cy

now what?
how do I solve for y?
I think I'm missing some stupid algebra thingy here, but can't figure it out.
 
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  • #2
manenbu said:

Homework Statement



yy'' = (y')2 - (y')3

Homework Equations





The Attempt at a Solution



y' = p(y)
y'' - p'p
What does p(y) mean? I would normally take this to mean "p of y". Do you mean y' = py; i.e. p times y?
What is y'' - p'p? That's not an equation. How does it relate to the equation above it?
manenbu said:
yp' = p - p2

dp/p + dp/(1-p) = dy/y

ln|p|+ln|1-p| = ln|y|+c

p-p2 = cy

y' - y'2 = cy

now what?
how do I solve for y?
I think I'm missing some stupid algebra thingy here, but can't figure it out.
 
  • #3
y' = p(y)
p is a function of y
should be y = p'p, not y - p'p.
y'' = p'(y)y' (chain rule)
y'' = p'(y)p(y) or p'p. :)
 
  • #4
manenbu said:
dp/p + dp/(1-p) = dy/y

ln|p|+ln|1-p| = ln|y|+c

Errm... [itex]\frac{d}{dp}\ln|1-p|=-\frac{1}{1-p}\neq\frac{1}{1-p}[/itex] :wink:
 
  • #5
oh. of course. stupid me. Now it all comes together.
Thanks for pointing this out.

It's always the little stuff that makes it problematic.
Have a nice day! :)
 

1. What is an ODE?

An ODE (ordinary differential equation) is a mathematical equation that relates the rate of change of a function to the function itself. It is often used to model dynamic systems in fields such as physics, engineering, and biology.

2. How do you solve an ODE?

There are various methods for solving ODEs, depending on the type of equation and initial conditions. One common method is separation of variables, where the equation is rearranged and integrated to obtain the solution. Another method is using power series or numerical methods for more complex equations.

3. What does yy'' = (y')2 - (y')3 mean?

This is a second-order ODE, where the function y has a second derivative (y'') and the equation relates it to the first derivative (y') squared and cubed. In other words, the rate of change of the rate of change of y is equal to the square of the rate of change of y minus the cube of the rate of change of y.

4. Can this ODE be solved analytically?

It depends on the specific equation and initial conditions. Some ODEs can be solved analytically using mathematical techniques, while others require numerical methods or cannot be solved at all. This particular ODE may be solvable analytically depending on the specific values and conditions involved.

5. What are some applications of solving ODEs?

ODEs are used to model and understand a wide range of physical, biological, and engineering systems. For example, they can be used to predict the motion of objects, the growth of populations, or the behavior of electrical circuits. They are also essential in the development of computer simulations and mathematical models for scientific research and engineering design.

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