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I am studying Zeidler's QFT Volume II, and I have a query on page 808:

It is claimed that

**S**Ψ^+_{

**p**,s} = (s

**k**)Ψ^+_{

**p**,s} when

**p**=p^3

**k**.

I tried my hand at deriving this, but when we write

**S**=S^1

**i**+S^2

**j**+S^3

**k**,

then the S^3

**k**term acting on Ψ^+_{

**p**,s} does give s

**k**Ψ^+_{

**p**,s},

but I don't see why the S^1

**i**and S^2

**j**terms don't give any contribution.

To those who are knowledgeable and happen to have access to the book,

could you please help me out? Many thanks!