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We have been conducting a search for protoclusters around known $z>4$ quasars using a multicolor imaging technique (Djorgovski et al. 1991, ASPCS, 21, 325; Smith et al. 1993, ASPCS, 43, 189). We have obtained imaging data on 23, $z>4$ quasar fields using the Palomar 200-inch telescope, the CTIO 4-m telescope, and the ESO NTT. These data are typically complete to $r=24^m$. Assuming $H_0 = 75$km/sec/Mpc and $q_0 = 0.1$, this is equivalent to $M_B = -23^m$, and thus we are probing the AGN luminosity function to the Seyfert 1 level. In a search for high redshift quasars at $r < 21.5$, Schmidt et al. (1991, ASPCS, 21, 109) see evidence for a turnover in the $z>4$ quasar luminosity function at faint magnitudes, while Irwin et al. (1991, ASPCS, 21, 117) see no such decline at $R < 19$. Our survey has covered 0.75 deg$^2$ which, for an effective redshift range of $4.1 < z < 4.9$, corresponds to $6\times10^6$ comoving $Mpc^3$ for the cosmology stated above. We typically find 1 -- 2 candidates per field selected by their colors and image morphology. Most of these objects have turned out to be galaxies at $z<1$, but we still have a few candidates with emission suggestive of them being objects at $z>4$. However, we have found no significant population of objects around these known quasars, suggesting that the decline seen by Schmidt et al. is real, and that it continues some 2 magnitudes deeper than the limits of their survey. We will present detailed completeness estimates and quantitative constraints on the $z > 4$ quasar luminosity function.
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