A note on structured pseudospectra of block matrices

In this note we consider the question of equivalence of pseudospectra and structured pseudospectra of block matrices. The structures we study are all so called double structures; that is, the blocks of the given matrix are of the same structure as the block matrix. The approach is based on that of non-block matrices, which are also briefly studied, and the use of distance to singularity. We also list some open problems and conjectures.