Laplace Transforms to solve non IVPs?

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Discussion Overview

The discussion revolves around the applicability of Laplace transforms in solving differential equations, specifically whether they can be used for problems that are not initial value problems, such as those involving conditions at different points in time. Participants explore the nature of the problem presented and the classification of differential equations.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions if the given equation, x' + x = T, can be solved using Laplace transforms, suggesting it may not be an initial value problem due to the conditions provided.
  • Another participant asserts that the problem is indeed an initial value problem because x(0) is given.
  • A different participant argues that the problem is not an initial value problem, clarifying that an initial value problem requires both x and x' to be specified at the same time, while the provided conditions are at different times, thus classifying it as a boundary value problem.
  • There is a correction regarding the order of the differential equation, with one participant mistakenly identifying it as second order and later clarifying it should be x'' + x = T.
  • One participant expresses skepticism about the utility of Laplace transforms, suggesting that they complicate the solution process and that alternative methods, such as variation of parameters, may be more effective.
  • Another participant appreciates the Laplace transform for its ability to convert discontinuities in the time domain to continuity in the s-domain, highlighting its aesthetic appeal and mathematical properties.
  • One participant proposes a method of using Laplace transforms by leaving x'(0) as an unknown and later applying additional conditions to solve for it, suggesting that Laplace transforms can handle similar equations to those solvable by the method of undetermined coefficients.

Areas of Agreement / Disagreement

Participants express differing views on whether the problem qualifies as an initial value problem or a boundary value problem. There is no consensus on the effectiveness or appropriateness of using Laplace transforms for the given problem, with some advocating for their use and others suggesting alternative methods.

Contextual Notes

The discussion includes varying interpretations of what constitutes an initial value problem versus a boundary value problem, and the implications of these classifications on the use of Laplace transforms. There are also differing opinions on the complexity introduced by Laplace transforms in solving differential equations.

Alex6200
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Is it possible to use a laplace transform to solve a problem like

x' + x = T

where x is a function of T and x(0) = 5 and x(4) = 7

Or can you only solve initial value problems?
 
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Isn't that already an initial value problem? You are given x(0). Yes you can use the Laplace transform to solve it.
 
No, that is not an initial value problem. Since it is a second order differential equation, an "initial value problem" would have x and x' given at the same value of t. Since you are given x at two different values of t, that is a "boundary value problem" which is significantly harder than an initial value problem.

I've always felt that "Laplace transform" was over rated. It gives a very "mechanical" way of solving problems that could be done in other ways. A problem like the one given here, to me, cries out for "variation of parameters".
 
Why is this a second order differential equation? It is x' and not x''.
 
Allright, my mistake, it should be

x'' + x = T, or something along those lines.

My question is: can I use Laplace transforms to solve endpoint value problems?
 
HallsofIvy said:
I've always felt that "Laplace transform" was over rated. It gives a very "mechanical" way of solving problems that could be done in other ways. A problem like the one given here, to me, cries out for "variation of parameters".

I really like Laplace transforms. There's just something really cool and dare I say - transcendental - about how discontinuous areas on the t domain become continuous on the s domain. Although I'd suppose that's true of the integration transform too.

Also, it kind of blows my mind how similar T and sin(T) are on the s-domain.
 
HallsofIvy said:
No, that is not an initial value problem. Since it is a second order differential equation, an "initial value problem" would have x and x' given at the same value of t. Since you are given x at two different values of t, that is a "boundary value problem" which is significantly harder than an initial value problem.

I've always felt that "Laplace transform" was over rated. It gives a very "mechanical" way of solving problems that could be done in other ways. A problem like the one given here, to me, cries out for "variation of parameters".
Agreed, the Laplace transform simply complicates matters more by introducing finding the inverse Laplace transform. There are much easier ways to solve ODE's and simpler ways of reducing PDE's.
 
I think you can. Just leave x'(0) to be an unknown, then you should have a function back that is in terms of T and x'(0). Then apply the fact that x(4) = 7 to find the value of x'(0) which gives you the full equation back.

Laplace transforms, I think, solves exactly the same family of equations that the method of undetermined coefficients solves.
 

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