I'm solving problem 3.D in H. Georgi Lie Algebra etc for fun where one is to compute the matrix elements of the direct product σ2⊗η1 where [σ2]ij and [η1]xy are two different Pauli matrices in two different two dimensional spaces.
Defining the basis in our four dimensional tensor product space |1⟩=|i=1⟩|x=1⟩|2⟩=|i=1⟩|x=2⟩|3⟩=|i=2⟩|x=1⟩|4⟩=|i=2⟩|x=2⟩
Now we know that when we multiply representations, the generators add in the sense of
[J1⊗2a(g)]jyix=[J1a]jiδyx+δji[J2a]yx,
Using all of this I find that in the basis of (1) the matrix representation of the tensor product is given by
σ2⊗η1=(01−i0100−ii0010i10)
(The bold 1 is just notation, see below!) I am not asking you to redo the calculations for me but does (3) make sense?
Appendix. My calculations were done in the following fashion [using equation (2)]: ⟨1|σ2⊗η1|1⟩=⟨j=1,y=1|σ2⊗η1|i=1,x=1⟩=[σ2]11δ11+δ11[η1]11=0.
So are my calculations (4),(5) totally wrong?
The Pauli matrices σ1=(0110)σ2=(0−ii0)σ3=(100−1).
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