ABSTRACT
We see that σ_. , which w ill be re fe rred to as the lk
amplitude of the longitudinal mode (para lle l to a), behaves
ferred to as the amplitudes of transverse modes (perpendic-
ular to a). In the lim it k 0, G^(k) equals χ (given by
(3.25)), but G^(k) becomes m/h, which blows up in the
l im it h 0. The result G^(0) = m/h is a consequence of
isotropy in spin space, as w il l be discussed in Sec. IX. 7.
