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## Homework Statement

I'm trying to follow the solution to a homework problem in QM, and I don't fully understand this step. Where does the factor [tex](2\pi)^3[/tex] come from?

[tex]\int d^3re^{-i\vec{p}\cdot\vec{r}}\int{\frac{d^3p'}{(2\pi)^32E_{p'}}\left(a(\vec{p}')e^{-i(E_{p'}t-\vec{p}'\cdot\vec{r})}+a^{\dagger}(\vec{p}')e^{+i(E_{p'}t-\vec{p}'\cdot\vec{r})}\right) = [/tex]

[tex]=\int{\frac{d^3p'}{(2\pi)^32E_{p'}}\left(a(\vec{p}')e^{-iE_{p'}t}(2\pi)^3\delta(\vec{p}-\vec{p}')+a^{\dagger}(\vec{p}')e^{+iE_{p'}t}(2\pi)^3\delta(\vec{p}+\vec{p}')\right)[/tex]

## Homework Equations

See above.

Any help appreciated. Thanks!