# Second quantization

## Main Question or Discussion Point

Hi

Say I have the following two fermionic creation/annihilation operators

$$c^\dagger_ic_j$$

1) Yesterday, my lecturer said that the following is valid

$$c^\dagger_ic_j = \delta_{i,j}c_jc^\dagger_i$$

Can you guys explain to me, where this formula comes from? I originally thought that it was one of the anti-commutator relations, but it cannot come from there.

2) Say I have en expression of the form

$$c_{k+q}^\dagger c_{k-q'} c^\dagger_{k'-q'}c_{k'}$$

If the operators are fermionic, then if I want to have all dagger-operators on the LHS and non-dagger operators on the RHS, then do I have to use anti-commutator relatations in order to rewrite the expression?

Likewise, if they were bosonic operators, then I would have to use commutator relations in order to rewrite the expression?

Best,
Niles.

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Avodyne

2) Yes.

Thank you, it is kind of you to answer so quickly.

A. Neumaier
2019 Award
$$c^\dagger_ic_j = \delta_{i,j}c_jc^\dagger_i$$
probably he meant c^*_ic_j = \delta_{i,j}+c_jc^*_i
which is the CCR for bosons.

Thanks. I have another question related to fermionic operators, so I'll just ask it here. It is regarding the relation

$$\delta (t - t')\left\langle {\left\{ {c_i (t),c_i^\dag (t')} \right\}} \right\rangle = \delta (t - t')$$

The curly brackets denote an anti-commutator. Is there an easy way of showing this? The way I would show this is to look at the case t = t' and the case t != t'.

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A. Neumaier
2019 Award
probably he meant c^*_ic_j = \delta_{i,j}+c_jc^*_i
which is the CCR for bosons.
Sorry, this is not quite true; $$c^*_ic_j = -\delta_{i,j}+c_jc^*_i$$

A. Neumaier
$$\delta (t - t')\left\langle {\left\{ {c_i (t),c_i^\dag (t')} \right\}} \right\rangle = \delta (t - t')$$