the frequency of lattice vibration depends on the wave vector
but since professor Huang
[1]
discussed the long wave vibration for optical branch in an ionic crystal
the frequency of longitudinal optical phonons has been considered to be constant.The author has discussed the effect of the dispersion of the longitudinal optical phonon on the property of a polaron. In this paper
the effect of the dispersive properties of longitudinal optical phonon on the properties of an exciton is further discussed.