Sound speed of scalar field dark energy: weak effects and large
  uncertainties
  The possibility of reconstruction of Lagrangian for the scalar field dark
energy with constant effective sound speed $c_s$ is analyzed. It is found that
such reconstruction can be made with accuracy up to an arbitrary constant. The
value of $c_s$ is estimated together with other dark energy parameters
($\Omega_{de}$, $w_0$, $c_a^2$) and main cosmological ones on the basis of data
including Planck-2013 results on CMB anisotropy, BAO distance ratios from
recent galaxy surveys, galaxy power spectrum from WiggleZ, magnitude-redshift
relations for distant SNe Ia from SNLS3 and Union2.1 compilations, the HST
determination of the Hubble constant. It is shown that no value of $c_s$ from
the range [0,1] is preferred by the used data because of very weak influence of
dark energy perturbations on the large scale structure formation and CMB
temperature fluctuations.
