Trace Operator¶
Sum of the diagonal entries of a matrix
\[
\text{Tr}(A) = \sum_{i}A_{ii}
\]
Properties:
\[\begin{split}
tr(A) = tr(A^T) \\
tr(A + B) = tr(A) + tr(B) \\
tr(AB) = tr(BA) \\
\end{split}\]
This is true even if A and B have different shapes (until they can be multiplied from booth sides)