# Question on discrete commutation relation in QFT

user1139
Homework Statement:
The statement is in title
Relevant Equations:
The equations are given below
Given the commutation relation

$$\left[\phi\left(t,\vec{x}\right),\pi\left(t,\vec{x}'\right)\right]=i\delta^{n-1}\left(\vec{x}-\vec{x}'\right)$$

and define the Fourier transform as

$$\tilde{\phi}(t,\vec{k})=\frac{1}{\sqrt{L^{n-1}}}\int_{\,0}^{\,L}\phi(t,\vec{x})e^{-i\vec{k}\cdot\vec{x}}\,\mathrm{d}^{n-1}\vec{x}$$

$$\tilde{\pi}(t,\vec{k})=\frac{1}{\sqrt{L^{n-1}}}\int_{\,0}^{\,L}\pi(t,\vec{x})e^{-i\vec{k}\cdot\vec{x}}\,\mathrm{d}^{n-1}\vec{x}$$

Is it then correct to say the following?

$$\left[\tilde{\phi}(\vec{k}),\tilde{\pi}(\vec{k}')\right]=i\delta_{\vec{k},-\vec{k}'}=i\delta_{-\vec{k},\vec{k}'}$$

i.e. can I use both Kronecker deltas interchangeably?

## Answers and Replies

Science Advisor
Gold Member
2022 Award
Have you tried to derive the commutator? You also should destinguish the operators ##\phi(t,\vec{x})## from ##\tilde{\phi}(\vec{k})##, because otherwise it leads to confusion. If so, where is your problem. Of course ##\delta_{\vec{k},-\vec{k}'}=\delta_{-\vec{k},\vec{k}'}##.