On propagation of high-energy photon in a medium in presence of an external field

The polarization tensor is calculated which originates from interaction of a photon with the electron-positron field. The effects of multiple scattering of electrons and positrons in a medium side by side with an external field are included. The corresponding general representation of polarization tensor is found in the form of two-dimentional integral. The both effects may be essential for propagation of high-energy photons in oriented single crystals.