On the form of the Elementary Excitation Spectrum in the Vicinity of Singular Points 9

Let us investigate the properties of the spectrum near points where one excitation can decay into two. From the laws of conservation of momentum and energy we then have (6-1) ϵ ( p ) = ϵ ( q ) + ϵ ( | p − q | ) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/20d72e0b-9d39-4175-acc6-a943547edf8a/content/eq141.tif"/>