sentences:  Lorenz's gauge is used.The scattering of an electromagnetic wave on a PEC body is represented via a boundary condition on the PEC body, namely
  
  with formula_2 representing unitary normal to the PEC surface, formula_3 representing incident electric field intensity, and formula_4 representing scattered electric field intensity defined as
  
  with formula_6 being imaginary unit, formula_7 being angular frequency, formula_8 being vector potential
  
  formula_10 being vacuum permeability, formula_11 being scalar potential
  
  formula_13 being vacuum permittivity, formula_14 being scalar Green's function
  
  and formula_16 being wavenumber.The integro-differential operator formula_17 is the one to be diagonalized via characteristic modes.