estimated mean value of 0.1, they derived $\bar{\xi}(20)=0.35\pm 0.09$. Stars, galaxy clusters or other spurious contaminants could affect their results.

@mullis01 derived the clustering properties of 217 AGN found in the North Ecliptic Pole (NEP) survey, a connected area of $\sim 81$ deg$^2$ covered by ROSAT observations. The sample spans the redshift interval $0\le z\le 3.889$, with $\bar{z}=0.408$. A $3.8\sigma$ clustering detection was obtained, corresponding to an integrated TPCF of $\bar{\xi}(20)=0.36\pm0.15$. This result confirms that X-ray selected AGN are spatially clustered in a manner similar to that of optically/UV selected AGN.

Notice that @bm93, @gs94, @carrera and @akylas used an EdS cosmology to compute the clustering properties of their samples, while @mullis01 adopted a $\Lambda$ model.

Finally, it is interesting to compare the clustering properties of AGN and normal galaxies at low-$z$, using our results and recent results by @2dfgal. Our value for the AGN correlation strength ($\bar{\xi}(20)\sim 0.461\pm 0.237$) appears slightly lower than the typical value for the brighter early-type galaxies ($\bar{\xi}(20)=0.70^{+0.11}_{-0.08}$), or at most consistent, indicating that these two classes have not experienced a completely different evolutionary history, but could represent two distinct phases during the processes of formation and evolution of the same objects. This gives an 