ABSTRACT
The above discussion can be genera lized when the
quantity o f in terest depends also on a wave vec tor k, for
example, E(k) with k ^ 0. There is then another expan-
d-4 sion param eter uk , which d iverges fo r k -► 0 .
The above discussion can be genera lized when the
quantity o f in terest depends also on a wave vec tor k, for
example, E(k) with k ^ 0. There is then another expan-
d-4 sion param eter uk , which d iverges fo r k -► 0 .