Figure 19: Number density of X-ray-selected AGN as a function of redshift for sources with erg s−1, as published by Brusa et al. . Lines show AGN luminosity functions [39, 79, 84–86] while individual measurements are shown by green circles  and black squares . The red star at was obtained from the optical luminosity function assuming no evolution in . The black-shaded area shows the observations of optically bright SDSS QSOs .