Why can't n be negative in Laplace's equation?

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Discussion Overview

The discussion centers on the question of why the variable \( n \) cannot be negative in the context of Laplace's equation. Participants explore the implications of negative values for \( n \) in relation to the solutions of the equation, particularly focusing on the behavior of sine functions and the conditions required for solutions to remain valid.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether \( n \) can be negative and suggests that if it were, the negative sign might be absorbed into a constant \( C \).
  • Another participant notes that since \( \sin(-x) = -\sin(x) \), solutions for negative \( n \) would simply repeat those for positive \( n \).
  • A subsequent reply agrees with the previous point but raises the possibility of different constants for negative \( n \).
  • Further contributions clarify that the constant \( C \) is arbitrary and discuss the implications of including negative \( n \) in the solution, particularly regarding the boundary condition that requires \( V \to 0 \) as \( x \to \infty \).
  • Participants reiterate that if \( n \) is negative, it could lead to a violation of the boundary condition due to the inclusion of terms that were previously excluded.

Areas of Agreement / Disagreement

Participants express differing views on the implications of allowing \( n \) to be negative, with some arguing that it leads to redundancy in solutions while others emphasize the violation of boundary conditions. No consensus is reached on whether negative \( n \) can be validly included in the solutions.

Contextual Notes

The discussion highlights the dependence on boundary conditions and the behavior of sine functions in the context of Laplace's equation. Assumptions regarding the arbitrary nature of constants and the implications of negative values for \( n \) remain unresolved.

laser1
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Griffiths Pg 133 4th Edition
1737829932254.png

Why can't n be negative? Is there a reason for this? My thought is that if n is negative, as sine is odd, the negative gets absorbed into C, a constant. Is this correct?

Would it be equally correct to let n be a negative integer?

Thank you
 
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Because ##\sin (-x) = - \sin(x)##, the solutions for negative ##n## simply repeat the solution for positive ##n##.
 
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PeroK said:
Because ##\sin (-x) = - \sin(x)##, the solutions for negative ##n## simply repeat the solutionf for positive ##n##.
But with different constant, though?
 
laser1 said:
But with different constant, though?
The constant ##C## is arbitrary.
 
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The solution of the problem is $$ V(x,y)=(Ae^{k x}+Be^{-k x})(C\sin(ky)+D\cos(ky)) $$ where ## k>0 ## and where the condition (iv) ## V\to0 ## as ## x\to\infty ## requires that A is equal to zero.
If ## n ## is negative ## -k ## will be positive and the part, which has already been excluded from the solution by ## A=0 ##, will be included into the solution again and the condition (iv) will be violated.
 
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Gavran said:
The solution of the problem is $$ V(x,y)=(Ae^{k x}+Be^{-k x})(C\sin(ky)+D\cos(ky)) $$ where ## k>0 ## and where the condition (iv) ## V\to0 ## as ## x\to\infty ## requires that A is equal to zero.
If ## n ## is negative ## -k ## will be positive and the part, which has already been excluded from the solution by ## A=0 ##, will be included into the solution again and the condition (iv) will be violated.
Thank you
 
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