“…Resonance poles defined with the resolvent and resonance poles as singularities of the function coincide [ 19 ]. The Friedrichs model has been generalized so as to describe more complex models showing resonances, although not all of the new approaches are exactly solvable due to their increasingly complexity [ 12 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 ]. Even for a construction of resonances in relativistic quantum field theory [ 74 , 75 ], the literature on the subject is far from being complete.…”