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## Summary:

- What is the space energy propagator?

## Main Question or Discussion Point

This is section 16.3 of QFT for the Gifted Amateur. I understand the concept of the spacetime propagator ##G^+(x, t, x', t')##, but the following propagator is introduced without any explanation I can see:

$$G^+(x, y, E) = \sum_n \frac{i\phi_n(x)\phi_n^*(y)}{E - E_n}$$

It would be good to have an explanation of what this is and what role it plays. Thanks.

$$G^+(x, y, E) = \sum_n \frac{i\phi_n(x)\phi_n^*(y)}{E - E_n}$$

It would be good to have an explanation of what this is and what role it plays. Thanks.