Trace (linear algebra)
This follows immediately from the fact that transposing a square matrix does not affect elements along the main diagonal.
The trace of the Kronecker product of two matrices is the product of their traces:
For example, consider the one-parameter family of linear transformations given by rotation through angle θ,
which clearly has trace zero, indicating that this matrix represents an infinitesimal transformation which preserves area.
The trace is a linear operator, hence it commutes with the derivative:
The trace also plays a central role in the distribution of quadratic forms.
For more properties and a generalization of the partial trace, see traced monoidal categories.
The operation of tensor contraction generalizes the trace to arbitrary tensors.