Numerical radius and $\ell_p$ operator norm of Kronecker products: inequalities and equalities

Abstract: Suppose $A=[a_{ij}]\in \mathcal{M}n(\mathbb{C})$ is a complex $n \times n$ matrix and $B\in \mathcal{B}(\mathcal{H})$ is a bounded linear operator on a complex Hilbert space $\mathcal{H}$. We show that $w(A\otimes B)\leq w(C),$ where $w(\cdot)$ denotes the numerical radius and $C=[c{ij}]$ with $c_{ij}= w\left(\begin{bmatrix} 0& a_{ij}\ a_{ji}&0 \end{bmatrix} \otimes B\right).$ This refines Holbrook's classical bound $w(A\otimes B)\leq w(A) |B|$ [J. Reine Angew. Math. 1969], when all entries of $A$ are non-negative. If moreover $a_{ii}\neq 0$ $ \forall i$, we prove that $w(A\otimes B)= w(A) |B|$ if and only if $w(B)=|B|.$ We then extend these and other results to the more general setting of semi-Hilbertian spaces induced by a positive operator. In the reverse direction, we also specialize these results to Kronecker products and hence to Schur/entrywise products, of matrices: (1) We show that if $w(A) = | A |$ then $w(A^{\circ m})\leq w^m(A)\ \forall m \geq 1$, and characterize equality here when $A$ is normal. (2) We provide upper and lower bounds for the $\ell_p$ operator norm and the numerical radius of $A\otimes B$ for all $A \in \mathcal{M}_n(\mathbb{C})$, which become equal when restricted to doubly stochastic matrices $A$. Finally, using these bounds we obtain an improved estimation for the roots of an arbitrary complex polynomial.