In pg136 to pg137 in the book, we look at the phonon dispersion spectrum of diamond. There are 6 distinct branches in a general oblique direction, which come from:
(2 atoms in the primitive unit cell) * (3 dimensions)
It's pointed out that along certain directions, some of the branches are overlapping and degenerate. Say that we wanted to work out how many distinct branches are along the (1,0,0) direction - how would we do that? There seem to be 4 in the picture, but I can't see how to find that...