tensors in QM

tensors.html

http://math.ucr.edu/home/baez/photon/tensor.htm:

 <blockquote>Both direct sum and tensor product are standard ways of <b>putting together little Hilbert spaces to form big ones.</b> They are used for different purposes. Suppose we have two physical systems.... Roughly speaking, if ... a physical system's ... states are either of A OR of B, its Hilbert space will be [a] direct sum.... If we have a system whose states are states of A AND states of B, its Hilbert space will be [a] tensor product....
<pre>
<b>MEASURE SPACE</b>   disjoint union  Cartesian product
<b>HILBERT SPACE</b>   direct sum      tensor product
</pre>
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