Galaxy number counts to second order and their bispectrum
  We determine the number counts to second order in cosmological perturbation
theory in the Poisson gauge and allowing for anisotropic stress. The
calculation is performed using an innovative approach based on the recently
proposed "geodesic light-cone" gauge. This allows us to determine the number
counts in a purely geometric way, without using Einstein's equation. The result
is valid for general dark energy models and (most) modified gravity models. We
then evaluate numerically the relevant contributions to the number counts
bispectrum. In particular we consider the terms involving the density, redshift
space distortion and lensing.
