Normal ordering is a standard mathematical operation used in quantum field theory to resolve operator ordering ambiguities, making citations unnecessary as it is trivially true by definition.
The claim states that normal ordering is a valid operation in quantum field theory. This is a foundational, axiomatic definition taught in every standard textbook on QFT and utilized routinely in calculations (as referenced tangentially in paper [0]). It is a piece of common knowledge within the field, requiring no external citation to validate its truth.