# Homework Help: Green function for forced harmonic oscillator

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1. Oct 27, 2014

### Judas503

1. The problem statement, all variables and given/known data
The problem requires to solve the integration to find $G(t)$ after $G(\omega)$ is found via Fourier transform. We have $$G(\omega)= \frac{1}{2\pi}\frac{1}{\omega _{0}^2 - \omega ^2}$$

2. Relevant equations
As mentioned previously, the question asks to find $G(t)$

3. The attempt at a solution
It is obvious that calculus of residues is required. To account for causality ($G(t<0)=0$), the poles at $\omega=\pm \omega_{0}$ are shifted to the lower half plane by $i\epsilon$ and integrated along the contour in the lower half plane. Then,
$$G(t)=-\frac{1}{2\pi}\int_{-\infty}^{+\infty}\frac{e^{-i\omega t}}{(\omega +i\epsilon)^2 -\omega _{0} ^2}d\omega$$
After calculating the residues at $\omega =\pm \omega _{0} - i\epsilon$, I found
$$G(t)=\frac{i}{2\omega_{0}}\sin \omega_{0}t$$

2. Oct 27, 2014

### vela

Staff Emeritus
The presence of $i$ suggests it's not. G(t) should be real, shouldn't it?

The sine makes sense. The system is at rest and then you impart an impulse, causing it to oscillate. It has to be sine because it starts from x=0.

3. Oct 28, 2014

### RUber

It looks like the i and the 2 might both get absorbed into the $\sin \omega_0 t =\frac{e^{i \omega_0 t }-e^{-i \omega_0 t }}{2i}$ term.

4. Oct 29, 2014

### Judas503

Yes, sorry! I forgot to put the "i" in the exponential form of sine. That should clear the problem.