GHOST PLASMON AND NEGATIVE DIELECTRIC FUNCTION

We show explicitly that a negative value of the static dielectric func tion necessarily means the existence of what might be called a ''ghost plasmon,'' a pole of the screened response chi(SC)(q, omega) on the i maginary axis of omega. The ghost plasmon is a collective state exhaus ting most of the conductivity sum rule. This is demonstrated for a two -dimensional electron gas with a microscopic calculation.