Norms of polynomials of the Volterra operator
We compute the operator norm of real-quadratic polynomials of the Volterra operator. This is used to test whether the Crouzeix conjecture holds for the Volterra operator.
math.FA↗
arXiv subjects
Nathan Walsh
Publications and source records attributed to Nathan Walsh.
A four-mean theorem and its application to pseudospectra
Let $N\ge 4$. We show that, if $x_1,\dots,x_N$ and $y_1,\dots,y_N$ are $N$-tuples of strictly positive numbers whose arithmetic, geometric and harmonic means agree, then \[ \max_j x_j <(N-2)\max_j y_j \quad\text{and}\quad \min_j x_j <(N-2)\min_j y_j. \] This is used to show that, if $N\ge4$ and $A,B$ are $N\times N$ matrices with super-identical pseudospectra, then, for every polynomial $p$, we have \[ \|p(A)\|< \sqrt{N-2}\|p(B)\|, \] unless $p(A)=p(B)=0$. This improves a previously known inequality to the point of being sharp, at least for $N=4$.
math.FA↗
Spectral sets, extremal functions and exceptional matrices
Let $A$ be a square matrix and let $\Omega$ be an open set in the plane containing the spectrum of $A$. We consider the problem of maximizing the operator norm $\|f(A)\|$ amongst all holomorphic functions $f$ from $\Omega$ into the closed unit disk. If $f_0$ is extremal for this problem and if $\|f_0(A)\|>1$, then it turns out that the matrix $f_0(A)$ has special properties, among them the fact that its principal left and right singular vectors are mutually orthogonal. We study this class of exceptional matrices $f_0(A)$. In particular, we are interested in the extent to which they are characterized by the aforementioned orthogonality property.
math.SP↗