Bounded Second Order Differential Equations

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Discussion Overview

The discussion revolves around finding the values of δ for which the solution to the initial-value problem defined by the second-order differential equation y'' − 4y = sin x, with initial conditions y(0) = δ and y'(0) = 0, remains bounded. Participants explore the meaning of boundedness in this context and seek to identify the appropriate values of δ that satisfy this condition.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the meaning of "bounded" in relation to the solution of the differential equation.
  • Another participant defines boundedness as the existence of a real number M such that |y| < M for all x in (-∞, ∞), suggesting that δ should be chosen to prevent y from becoming large.
  • Multiple participants mention a specific value of -1/10 as a potential solution but do not agree on how to derive this value or confirm its validity.
  • There is a call for clarification on the general solution of the differential equation and how it relates to the initial conditions, with emphasis on the need to analyze which components of the solution may grow unbounded as x increases.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the specific values of δ that ensure boundedness. There are competing views on how to approach the problem, particularly regarding the interpretation of the solution and the implications of boundedness.

Contextual Notes

Participants have not provided a complete derivation of the general solution to the differential equation, nor have they fully explored the implications of the initial conditions on the boundedness of the solution. There is also uncertainty about which terms in the solution may lead to unbounded behavior.

Who May Find This Useful

This discussion may be of interest to students and practitioners dealing with second-order differential equations, particularly in the context of initial-value problems and the concept of bounded solutions.

desbro05
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Hello all. I am having a very serious problem. The question states:

Find the value(s) of δ such that the solution of the initial-value problem

y'' − 4y = sin x;

where y(0) = δ and y'(0) = 0


is bounded.

I have no problem "solving" the equation and getting y in terms of x and δ, but what does bounded mean in this case, and what values satisfy this condition?
 
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What did you get for a solution?
Bounded in this case means there exist a real number M such that
|y|<M for all x in (-∞,∞)
so pick the δ that keeps y from being big
 
The solution is -1/10, but I can't figure out how to go about finding this answer.
 
What was your solution (y in terms of x and δ) to
y'' − 4y = sin x;

where y(0) = δ and y'(0) = 0

What must δ be to assure y is never big?
 
desbro05 said:
The solution is -1/10, but I can't figure out how to go about finding this answer.
Well, there's your first problem! The solution to the equation, which is what lurflurf was asking, is not a number, it is a function of x. What did you get as the general solution to the differential equation? What did you get as the solution to this "initial value problem" (it will depend on \delta). Which of the functions in that solution will get "larger and larger" (for x getting larger both positive and negative?). You need to make the cofficients of those functions 0.
 

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