since, in order to view the shape changes in our wave packet we are presented with the taylor expansion of the frequency
ω(k) = ω(k0) + (k − k0)dω/dk + 1/2*(k − k0)^2 (d^2ω/dk^2)
we are told that only the third term that is the
1/2*(k − k0)^2 (d^2ω/dk^2)
contributes to change in shape of the...