Now I am reading the book Radiation and Scattering of Waves by Leopold B. Felsen and Nathan Marcuvitz. But the Operator operation is so sofisticated, So I cann't do my work ahead . This is the first time that I heard about the concept of decomposition of gradient. What's more, the concept of dyadic Green's Function is also a big problem for me. I am looking forwad to someone who had read through the book recommond some appled books for me .Thanks !