Some Features of Exceptional Points

A short resume of the essentials of exceptional points of second order is given. We then concentrate on a discussion of specific features of exceptional points of third order. While general properties of these singularities have been expounded extensively in the literature, we here concentrate on some specific aspects, that is the occurrence of ‘hidden’ or ‘concealed’ third order exceptional points. They occur under specific circumstances when an apparent second order exceptional point is accompanied by a third eigenvalue being equal to the other two at the singularity.