Page "Quantum field theory" Paragraph 50

It turns out that a different definition of creation and annihilation must be used for describing fermions.

According to the Pauli exclusion principle, fermions cannot share quantum states, so their occupation numbers N < sub > i </ sub > can only take on the value 0 or 1.

The fermionic annihilation operators c and creation operators are defined by their actions on a Fock state thus