General Solution for y''+(1/x)y'=0: Proving Boundedness

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In summary, the general solution for the equation y''+(1/x)y'=0 is y=cln(x)+d, where c and d are constants. It can be shown that only constant solutions are bounded by noticing that ln(x) is unbounded. Similarly, for the equation y''=0, the general solution is y=cx+d and it can be shown that only constant solutions are continuous.
  • #1
sara_87
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Homework Statement



Find the general solution of: y''+(1/x)y'=0
and show that only constant solutions are bounded.

Homework Equations





The Attempt at a Solution



integrating factor say a=e^(int(1/x)dr)=x
so xy''+y'=0. so (xy')'=0
integrate both sides: xy'=c (c is a constant)
integrate again: y=cln(x)+d (d is a constant)

but i don't know how to show that only constant solutions are bounded.
Thank you
 
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  • #2
Hi sara_87! :smile:
sara_87 said:
Find the general solution of: y''+(1/x)y'=0
and show that only constant solutions are bounded.

y=cln(x)+d (d is a constant)

but i don't know how to show that only constant solutions are bounded

But you're there

ln(x) is unbounded, isn't it? :wink:
 
  • #3
Oh right so this means only the solutions y=d can be bounded, right?
Also, there's a similar question:
find the general solution of: y''=0 and show that only constant solutions are continuous.

general solution i found to be: y=cx+d (again c and d are constants)
but in this case, why can only constant solutions be continuous?
 
  • #4
sara_87 said:
find the general solution of: y''=0 and show that only constant solutions are continuous.

general solution i found to be: y=cx+d (again c and d are constants)
but in this case, why can only constant solutions be continuous?

dunno :confused:

must be a misprint :redface:
 
  • #5
:)
It's not a misprint.
Never mind.
Thank you.
 

1. What is the general solution for y''+(1/x)y'=0?

The general solution for this differential equation is y = C1 + C2ln(x), where C1 and C2 are arbitrary constants.

2. How do you prove that the solution is bounded?

To prove boundedness, we need to show that there exists a finite number M such that |y| < M for all values of x. This can be done by showing that the second derivative of y is finite and that the first derivative approaches zero as x approaches infinity.

3. Can you explain the concept of boundedness in this context?

Boundedness in this context means that the function y is limited or restricted within a finite range of values. In other words, y does not approach infinity or negative infinity as x approaches infinity.

4. Why is it important to prove boundedness for this differential equation?

Proving boundedness is important because it ensures that the solution to the differential equation is well-behaved and does not have any extreme or unpredictable behavior. This is especially important in scientific applications where we want to accurately model and predict the behavior of a system.

5. Are there any other methods for proving boundedness?

Yes, there are other methods for proving boundedness such as using the Mean Value Theorem or the Cauchy-Schwarz inequality. However, for this specific differential equation, showing that the second derivative is finite and the first derivative approaches zero as x approaches infinity is the most straightforward and efficient method for proving boundedness.

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