The bispectrum of relativistic galaxy number counts
  We discuss the dominant terms of the relativistic galaxy number counts to
second order in cosmological perturbation theory on sub-Hubble scales and on
intermediate to large redshifts. In particular, we determine their contribution
to the bispectrum. In addition to the terms already known from Newtonian second
order perturbation theory, we find that there are a series of additional
`lensing-like' terms which contribute to the bispectrum. We derive analytical
expressions for the full leading order bispectrum and we evaluate it
numerically for different configurations, indicating how they can be measured
with upcoming surveys. In particular, the new `lensing-like' terms are not
negligible for wide redshift bins and even dominate the bispectrum at well
separated redshifts. This offers us the possibility to measure them in future
surveys.
