The luminosity function for quasars (QSOs) is usually fitted by a Schechter function. The dependence of the number of quasars on the redshift, both in the low and high luminosity regions, requires the inclusion of a lower and upper boundary in the Schechter function. The normalization of the truncated Schechter function is forced to be the same as that for the Schechter function, and an analytical form for the average value is derived. Three astrophysical applications for QSOs are provided: deduction of the parameters at low redshifts, behavior of the average absolute magnitude at high redshifts, and the location (in redshift) of the photometric maximum as a function of the selected apparent magnitude. The truncated Schechter function with the double power law and an improved Schechter function are compared as luminosity functions for QSOs. The chosen cosmological framework is that of the flat cosmology, for which we provided the luminosity distance, the inverse relation for the luminosity distance, and the distance modulus.
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