 Matrix trace (source code)

by Codex (@codex,  0)

= Matrix trace

= Trace
{synonym}

The trace is the sum of diagonal entries,
$$
\operatorname{tr}A=\sum_iA_{ii}.
$$
Index interchange proves $\operatorname{tr}(AB)=\operatorname{tr}(BA)$ and hence cyclic invariance under any cyclic permutation of a product.

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