Title: The Inverse q-Numerical Range Problem and Connections to the Davis-Wielandt Shell and the Pseudospectra of a Matrix
Abstract: Numerical ranges and related sets provide insights into the behavior
of iterative algorithms for solving systems of equations and computing eigenvalues.
Inverse numerical range problems attempt to enhance these insights. We generalize the
inverse numerical range problem, as proposed by Uhlig, to the inverse
$q$-numerical range problem, and propose an algorithm for solving the
problem that relies on convexity. To determine an approximation to
the boundary of the $q$-numerical range, as needed by our algorithm,
we must approximate the top of the Davis-Wielandt shell, a
generalization of the numerical range. We found that the Davis-Wielandt
shell is in a sense conjugate to the the extreme singular values of the
resolvent of a matrix. Knowing the Davis-Wielandt shell allows for the
approximation of the $q$-numerical range, the pseudospectra and the
Davis-Wielandt shell for any allowed M\"{o}bius transformation of a matrix.
We provide some examples illustrating these connections, as well as
how to solve the inverse $q$-numerical range problem.