  The expressions for average densities of currents and charges induced by a
weak electromagnetic field in spatially inhomogeneous systems are obtained. The
case of finite temperatures is considered. It is shown that average values are
separated into "basic" and "additional" parts. The former depends on electric
fields, and the latter depends on derivatives of electric fields on
coordinates. Semiconductor quantum wells, wires or dots may be considered as
spatially inhomogeneous systems.
