0.9 The trace
The trace is an operation which turns outer products into inner products,
You can show, using this definition or otherwise, that the trace is cyclic18 (
To recover the first definition from the second, we argue as follows:
Note that “cyclic” does not mean the same thing as “permutation invariant”! It is not true in general that
, but only that , i.e. we can only cyclically permute the operators.↩︎