Acoustics of tachyon Fermi gas
  We consider a Fermi gas of free tachyons as a continuous medium and find
whether it satisfies the causality condition. There is no stable tachyon matter
with the particle density below critical value $n_T$ and the Fermi momentum
$k_F<\sqrt{\frac 32}m$ that depends on the tachyon mass $m$. The pressure $P$
and energy density $E$ cannot be arbitrary small, but the situation $P>E$ is
not forbidden. Existence of shock waves in tachyon gas is also discussed. At
low density $n_T<n<3.45n_T$ the tachyon matter remains stable but no shock wave
do survive.
