the clustering of QSOs.

The Data
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It is paradoxical that in the era of 2QZ and Sloan Digital Sky Survey (SDSS), with thousands of faint QSOs discovered up to the highest redshifts, there are still relatively few bright QSOs known at low redshift. One of the main reasons, as shown in previous papers [@GC2000; @DSS2k2], is the rather low surface density of low-$z$ and bright QSOs, of the order of few times $10^{-2}$ per deg$^{2}$. This corresponds to a very small number of objects also in the case of the 750 deg$^{2}$ of the complete 2QZ and 1000 deg$^{2}$ of the SDSS (during commissioning phase). One more reason, not less important than the previous one, is that with the optical information only it is difficult to efficiently isolate bright QSOs from billions of stars in large areas. As a consequence, a survey based on different selection criteria is required. In @GC2000 [@DSS2k2] we have used the X-ray emission, a key feature of the AGN population.

The AERQS is divided in three sub-samples, two in the northern hemisphere (USNO and GSC), described in @GC2000 (hereafter Paper I), and the DSS sample in the southern hemisphere, described in @DSS2k2 (Paper II). 