“…In this case the characteristics of undulator radiation are practically independent on channeling oscillations [3,4], and the operational principle of a crystalline undulator is the same as for a conventional one (see, e.g., [58][59][60][61][62]) in which the monochromaticity of radiation is the result of constructive interference of the photons emitted from similar parts of trajectory, see In this case the characteristics of undulator radiation are practically independent on channeling oscillations [3,4], and the operational principle of a crystalline undulator is the same as for a conventional one (see, e.g., [58][59][60][61][62]) in which the monochromaticity of radiation is the result of constructive interference of the photons emitted from similar parts of trajectory, see …”