What happens when a boundary condition is included?

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Homework Help Overview

The discussion revolves around the differential equation f''(x) = (-σ^2)f(x) and the implications of including a boundary condition, specifically g(0) = 0. Participants explore the nature of solutions involving sine and cosine functions in relation to this boundary condition.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the validity of sine and cosine as solutions to the differential equation and how the boundary condition affects these solutions. There is a focus on whether f(0) = 0 leads to the conclusion that only sin(σx) is a valid solution, while questioning the conditions under which cos(σx) may also apply.

Discussion Status

The discussion is active, with participants clarifying assumptions about the boundary condition and its implications for the solutions. Some guidance has been offered regarding the conditions under which each function satisfies the boundary condition, though there is no explicit consensus on the broader implications.

Contextual Notes

Participants are operating under the assumption that g(0) = 0 translates to f(0) = 0, which is central to their exploration of the solutions. There is a noted confusion regarding the specific values of σ that would allow cos(σx) to satisfy the boundary condition.

cytochrome
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Homework Statement


Show that sines and cosines are the solutions of the differential equation

f''(x) = (-σ^2)f(x)

What if a boundary condition is included that g(0) = 0?


Homework Equations


f''(x) = (-σ^2)f(x)


The Attempt at a Solution


Plugging in sin(σx) and cos(σx) yields an equality therefore the expression is true.

I'm just confused about the boundary condition.

If g(0) = 0 then only the sin(σx) works, correct?
 
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I am assuming that g(0) = 0 stands for f(0) = 0.

The solution f(x) = sin(σx) satisfies the condition that f(0) = 0 for all values of σ. But the solution f(x) = cos(σx) satisfies the given condition only for particular values of σ.
 
cytochrome said:

Homework Statement


Show that sines and cosines are the solutions of the differential equation

f''(x) = (-σ^2)f(x)

What if a boundary condition is included that g(0) = 0?

Homework Equations


f''(x) = (-σ^2)f(x)

The Attempt at a Solution


Plugging in sin(σx) and cos(σx) yields an equality therefore the expression is true.

I'm just confused about the boundary condition.

If g(0) = 0 then only the sin(σx) works, correct?
I hope you mean f(0) = 0 .

Yes, if f(0) = 0, then only sin(σx) works.

cos(σx) does not work for that boundary condition.

grzz said:
I am assuming that g(0) = 0 stands for f(0) = 0.

The solution f(x) = sin(σx) satisfies the condition that f(0) = 0 for all values of σ. But the solution f(x) = cos(σx) satisfies the given condition only for particular values of σ.
@grzz,

For what value of σ will cos(σ∙0) = 0 ?
 
Thanks SammyS for pointing out my mistake.

The last part of my post i.e.'But the solution f(x) = cos(σx) satisfies the given condition only for particular values of σ' is not correct.
 

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