An Estimator for Lensing Potential from Galaxy Number Counts
  We derive an estimator for the lensing potential from galaxy number counts
which contains a linear and a quadratic term. We show that this estimator has a
much larger signal-to-noise ratio than the corresponding estimator from
intensity mapping. This is due to the additional lensing term in the number
count angular power spectrum which is present already at linear order. We
estimate the signal-to-noise ratio for future photometric surveys. Particularly
at high redshifts, $z\gtrsim 1.5$, the signal to noise ratio can become of
order 30. Therefore, the number counts in photometric surveys would be an
excellent means to measure tomographic lensing spectra.
