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# Quick Green's function q watch

1. Just a really quick question. I'm given:

y''+(k^2)y=2kcoskx

I have to find the solution given the conditions: y(0)=y'(0)=0

This is an initial value problem, and I get:

G(x,z)=0 (x<z)
and G(x,z)=sink(x-z)/k (x>z)

Right from here I should easily be able to find the solution by carrying out the integral:

INT(from 0 to x) [2coskz*sink(x-z)dz] but even though the obtained answer satisfies the boundary conditions, I've been told that this is not quite correct. Something like G(x-z) and G(z,z)=0 was mentioned, but I can't see how this will give me the correct answer. In any case, if you can tell me why my approach is incorrect, that would be very helpful.

Thanks alot.
2. You want y(x) = integral of f(z) G(x,z) dz, but you've got integral of f(x) G(x,z) dz.

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Updated: January 21, 2010
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