How Do You Solve This PDE Using Separation of Variables?

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Discussion Overview

The discussion revolves around solving a partial differential equation (PDE) using the method of separation of variables. Participants explore the implications of lacking boundary or initial conditions on the ability to determine specific solutions, particularly focusing on the coefficients in a Fourier series representation.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant suggests that without boundary conditions, it is unclear how to proceed with solving for the coefficients A_n in the series representation of the solution.
  • Another participant emphasizes the necessity of boundary conditions, stating that conditions like u(x_0,y)=g(y) or u(x,y_0)=g(x) are essential to evaluate the equation and find the coefficients using Fourier series.
  • A third participant expresses uncertainty about the role of the separation constant (λ) and whether it could help in addressing the boundary conditions, indicating a struggle with the problem due to the absence of these conditions.
  • Another participant questions the initial approach, asserting that without boundary conditions, the PDE theory implies there are infinitely many solutions, thus making it impossible to determine a unique value for the constant.

Areas of Agreement / Disagreement

Participants generally agree on the necessity of boundary conditions to proceed with the solution, but there is disagreement on the implications of the absence of these conditions and how it affects the determination of the solution.

Contextual Notes

The discussion highlights limitations related to the lack of boundary conditions, which are critical for defining the constants in the solution. The dependence on specific conditions for the application of Fourier series is also noted.

Inigma
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I have a battle with the following direct partial integration and separation of variables toffee:

I have to solve,
u(x,y)=\sum_{n=1}^{∞}A_n sin\lambda x sinh \lambda (b-y)

If there were no boundary or initial conditions given, do I assume that λ is \frac{n\pi}{L} and do I then solve A_n? If I am going in the wrong direction here, please point me in the right direction... thanks!
 
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As far as I know, there is no way to solve this further without boundary conditions. You need a condition of the type u(x_0,y)=g(y) or u(x,y_0)=g(x). By evaluating the equation with the boundary condition, you can use Fourier series to find the coefficient A_n.
 
Meldraft:
Thank you for your contribution.
I agree and am also expecting boundary conditions... I want to carry on using the Fourier approach to solve An but due to the lack of boundary conditions, I find it impossible unless I miss something. I am not sure if the Eigen separation constant (λ) may lead to solving the 'boundary conditions" in some degree based on the arrangement of the equation (hyperbolic)... I am stumped...
 
I am not sure that I understand what your question is. You have a general solution. If you had boundary conditions you could define the constant, but as it is, there is really nothing more you can do. Are you trying to solve it without applying a boundary condition? If so, PDE theory suggests that there are infinite possible solutions, so you cannot possibly get a unique value for your constant, without applying a boundary condition!
 

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