Abstract. This paper discusses observational relations for cosmological point sources in some specific waveband, and their use in the context of the data provided by the galaxy redshift surveys. By starting from the general theory for observations in relativistic cosmology, developed mainly by G. F. R. Ellis, the equations for colour, K-correction and number counts of cosmological point sources are discussed in the context of curved spacetimes. These observables can be directly related to the underlying curved geometry of cosmological models, and since they were not derived in the framework of any specific cosmology, they are therefore valid for any cosmological model. The hypothesis used in such derivation are discussed, together with some difficulties of the practical use of those observables.
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