The Herschel-ATLAS: a sample of 500{\mu}m-selected lensed galaxies over
  600 square degrees
  We present a sample of 80 candidate strongly lensed galaxies with flux
density above 100mJy at 500{\mu}m extracted from the Herschel Astrophysical
Terahertz Large Area Survey (H-ATLAS), over an area of 600 square degrees.
Available imaging and spectroscopic data allow us to confirm the strong lensing
in 20 cases and to reject it in one case. For other 8 objects the lensing
scenario is strongly supported by the presence of two sources along the same
line of sight with distinct photometric redshifts. The remaining objects await
more follow-up observations to confirm their nature. The lenses and the
background sources have median redshifts z_L = 0.6 and z_S = 2.5, respectively,
and are observed out to z_L = 1.2 and z_S = 4.2. We measure the number counts
of candidate lensed galaxies at 500{\mu}m and compare them with theoretical
predictions, finding a good agreement for a maximum magnification of the
background sources in the range 10-20. These values are consistent with the
magnification factors derived from the lens modelling of individual systems.
The catalogue presented here provides sub- mm bright targets for follow-up
observations aimed at exploiting gravitational lensing to study with
un-precedented details the morphological and dynamical properties of dusty star
forming regions in z >~ 1.5 galaxies.
