  Resonances, also known as quasinormal modes (QNM) in the non-Hermitian case,
play a ubiquitous role in all domains of physics ruled by wave phenomena,
notably in continuum mechanics, acoustics, electrodynamics, and quantum theory.
The non-Hermiticity arises from the system losses, whether they are material
(Joule losses in electromagnetism) or linked to the openness of the problem
(radiation losses). In this paper, we focus on the latter delicate matter when
considering bounded computational domains mandatory when using e.g. Finite
Elements. Dispersive perfectly matched layers and absorbing boundary conditions
are studied.
