Expected Value Partial Trance Density Matrix

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Hey I am currently studying Quantum Mechanics and I have difficulty grasping a concept.

I don't understand the following step in the derivation:

\langle X_{A} \rangle=tr\left[\left(X_{A}\otimes I_{B}\right)\rho_{AB}\right]
=tr_{A}\left[X_{A} tr_{B}\left[\rho_{AB}\right]\right]

Thanks
 
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I believe 'tr' stands for 'trace', not 'trance'.
 
yes of course
 
SteamKing said:
I believe 'tr' stands for 'trace', not 'trance'.

Note if you're computing it while listening to techno music :).
 

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