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toffee

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**The concept of "Rotation in 3-d" in QM**

I'm having difficulty with two ideas and wud appreciate some help:

1)commutation relations:

[tex][L_i, L_j]=i\hbar\epsilon_i_j_kL_k[/tex]

reflect the law of combination of rotation in 3 dimensions.

What is the this law and why must in be satisfied

**whatever be the nature of the wave’functions’ they rotate**?

2)Since rotation in 3 dimensions always obey composition rules the spin operators [tex]S_i [/tex] obey the same commutation relations:

[tex][S_x,S_y]=i\hbar S_z [/tex] and cyclic

what are compostion rules and why does the above hold true?

thanks in advance