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Published online by Cambridge University Press: 04 August 2017
The concept of a redshift limit for quasars was first mentioned a decade ago by Schmidt (1970, 1972) and Sandage (1972) who independently realized that faint, high redshift quasars were not being found in the numbers expected from the redshift and magnitude distribution of bright quasars. Subsequently the limit moved out in redshift to z ∼ 3.5 from z ∼ 2.5 after the discovery of OQ172 (z = 3.53, Wampler et al. 1973) and numerous other quasars with z > 3. It should be said at the outset that the limit concept is in need of definition. While it would be most interesting if an absolute limit existed, i.e., no quasars beyond a certain redshift, in reality the limit value is more likely to be the redshift at which the quasar space density turns down significantly.