  The problem of ballistic annihilation for a spatially homogeneous system is
revisited within Boltzmann's kinetic theory in two and three dimensions. Exact
analytical results are derived for the time evolution of the particle density
for some isotropic discrete bimodal velocity modulus distributions. According
to the allowed values of the velocity modulus, different behaviors are
obtained: power law decay with non-universal exponents depending continuously
upon the ratio of the two velocities, or exponential decay. When one of the two
velocities is equal to zero, the model describes the problem of ballistic
annihilation in presence of static traps. The analytical predictions are shown
to be in excellent agreement with the results of two-dimensional molecular
dynamics simulations.
