The Application of Mutual Energy Theorem in Expansion of Radiation Field   in Spherical Waves
  In recent years the shperical wave expansion method has been widely applied to the theory and calculation of electromagnetic fields. But the inner product exist in reference[1] is defined on the Banach space[2]. Through redefining the inner product this article limits the wave expansion method to Hibert space[3]. For this reason the mutual energy theorem is introduced. 
Shuang-Ren Zhao
physics.class-ph