Instabilities in Mimetic Matter Perturbations
  We study cosmological perturbations in mimetic matter scenario with a general
higher derivative function. We calculate the quadratic action and show that
both the kinetic term and the gradient term have the wrong sings. We perform
the analysis in both comoving and Newtonian gauges and confirm that the
Hamiltonians and the associated instabilities are consistent with each other in
both gauges. The existence of instabilities is independent of the specific form
of higher derivative function which generates gradients for mimetic field
perturbations. It is verified that the ghost instability in mimetic
perturbations is not associated with the higher derivative instabilities such
as the Ostrogradsky ghost.
