  A theoretical investigation has been made of the magnetoplasmon excitations
in a quantum wire characterized by a confining harmonic potential and subjected
to a perpendicular magnetic field. We study the (nonlocal, dynamic) inverse
dielectric function to examine the charge-density excitations within a
two-subband model in the framework of Bohm-Pines' random-phase approximation. A
particular stress is put on the (intersubband) magnetoroton excitation which
changes the sign of its group velocity twice before merging with the respective
single-particle continuum. It has already been suggested that the electronic
device based on such magnetoroton excitations can act as an {\it active} laser
medium [see, e.g., Phys. Rev. B {\bf 78}, 153306 (2008)]. Scrutinizing the real
and imaginary parts of the inverse dielectric function provides us with an
important information on the longitudinal and transverse (Hall) resistances of
the system.
