[Math] trace of product of a diagonal and a matrix

matricestrace

I would like to know if anything can be said about the trace of a product of two matrices, where one matrix is a diagonal matrix, i.e.,
$$\text{trace}(DA)=…$$
Are there some bounds in terms of $\text{trace}(D)$ and $\text{trace}(A)$ ?

Best Answer

Note that, when $D$ is diagonal:

$$(DA)_{ii} = D_{ii} A_{ii}$$

So $tr(DA) = \sum_{i=1}^n D_{ii} A_{ii}$. About the best bound you can do for this is the Cauchy-Schwarz inequality, i.e.

$$|tr(DA)| \leq \left ( \sum_{i=1}^n D_{ii}^2 \right )^{1/2} \left ( \sum_{i=1}^n A_{ii}^2 \right )^{1/2}$$

If you want a result in terms of traces, you can use the fact that $\| x \|_2 \leq \| x \|_1$ to get

$$|tr(DA)| \leq tr(|D|) tr(|A|)$$

where $(|D|)_{ij} = |D_{ij}|$ and similar for $|A|$. The first bound is attained when the diagonals of $D$ and $A$ are proportional to each other. The second is attained when these diagonals only have one nonzero entry. So for example you could have $D=A=\begin{pmatrix} 1 & 0 \\ 0 & 0 \\ \end{pmatrix}$, then $|tr(DA)|=1=1 \cdot 1 = tr(|D|) tr(|A|)$.

Note that when $D$ and $A$ both have nonnegative diagonal entries, we get the nice result

$$tr(DA) \leq tr(D) tr(A)$$

which is probably more like what you were looking for.

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