The one-dimensional monatomic harmonic crystal (section 5.3) is in fact a particular case of the diatom ic model described in section 5.4, for which the two atoms are identical. To prove th is, show that the expression for the diatomic harmonic crystal can be transformed into an expression similar to the monatomic crystal. Solve Eq. ( 5.22 ) in the l im it M 1 =M2 =M. What considerations do you have to take into account to do this?