(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

This is from Ryder's QFT book, second ed. page 37. At the bottom of the page it says that the commutation relations (eqn 2.68?) are satisfied by:

[itex]K = \pm i\frac{\sigma}{2}[/itex]

However, I do not find this to be so. What am I missing?

2. Relevant equations

Here is one of the commutation relations that I think he means.

[itex][K_x,K_y] = -iJ_z[/itex]

3. The attempt at a solution

Using [itex]K = i\frac{\sigma}{2}[/itex], I get:

[itex][K_x,K_y] = [i\frac{\sigma_x}{2},i\frac{\sigma_y}{2}] = \frac{-1}{4}[\sigma_x,\sigma_y] = -\frac{1}{2}\sigma_z = iK_z \neq -iJ_z[/itex]

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# Homework Help: Simple matrix algebra

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