We present the solution of the time development of an unstable initial
density perturbation in the linearized Vlasov equation, completing the previous
analysis in the literature. The additional contributions found are usually
damped and can be neglected at large times in the unstable region. The work
clarifies also the problem of the normalization of the solution with respect to
the initial perturbation of the density.Comment: revision of the discussion, different initial perturbation, 9 pages,
4 figures included, uses epsfi
