PDE with an inequality constrain

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Discussion Overview

The discussion revolves around solving an elliptic partial differential equation (PDE) of the form Δu - k * u = 0, subject to inequality constraints 0 ≤ u(x,y) ≤ 1.0, where k is a positive constant. Participants explore methods for addressing the problem, including boundary conditions and potential substitutions.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant asks for help in solving the elliptic PDE with specified inequality constraints.
  • Another participant clarifies the notation, suggesting that ∇2u - k*u = 0 is equivalent to the original equation.
  • A participant notes that the usual method of separation of variables may yield multiple solutions due to the lack of fully specified boundary conditions, questioning how to characterize those that meet the inequality constraints.
  • The original poster mentions having some Dirichlet boundary conditions but seeks advice on formulating the PDE with inequality constraints.
  • One participant proposes a potential substitution, u = exp(-v²), to keep u within the desired range, while acknowledging the challenge of finding a suitable substitution that allows for solving the PDE.

Areas of Agreement / Disagreement

Participants express differing views on how to approach the problem, with no consensus on a specific method for incorporating the inequality constraints into the PDE formulation.

Contextual Notes

The discussion highlights the complexity of solving the PDE under the given constraints and the potential for multiple solutions due to the nature of the boundary conditions.

BlackTulip
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Hi everybody,

For part of my research, I need to solve an elliptic PDE like:

Δu - k * u = 0,

subject to : 0≤ u(x,y) ≤ 1.0

where k is a positive constant.



Can anyone tell me how I can solve this problem?


Thanks in advance for your help.
 
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Do you mean ∇2u - k*u = 0?
 
Well, the usual approach is separation of variables to obtain a general solution. Normally one has completely specified boundary conditions, whereas you only have bounds, so there will be multiple solutions, and its not obvious how to characterise those that satisfy the bounds. Is that where you're stuck?
 
I do have some Dirichlet boundary conditions. My problem is that how to formulate this PDE with some inequality constraints. Do you have a suggestion for that?

Thanks for your attention.
 
No idea if this is feasible, but how if you were to replace u by a function that could only be in that range, e.g. substitute u = exp(-v2)? Tricky part might be finding such a substitution that still allows you to solve the PDE.
 

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