The effect of dynamical friction on the time evolution of lopsided disks is examined by using linear perturbation theory. The friction is caused by the gravitational interaction of a rotating lopsided pattern with a density wake induced in halos. The density wake is determined by solving the linearized collisionless Boltzmann and Poisson equations by means of the Fourier-Laplace transform. Then, it is found that dynamical friction always damps a lopsided pattern in our halo model. In addition, the damping time is much shorter than a Hubble time, typically 1 Gyr, unless the pattern speed is quite slow. Considering such a short damping time-scale and the observed large fraction of lopsided disks in spirals, say, approx=30%, it will be unlikely that all of the lopsided disks are recently excited. Thus, it is suggested that most of the observed lopsided disks are very slowly rotating patterns. The significance of weakly damped modes that have a slow pattern speed is discussed.
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