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Published online by Cambridge University Press: 30 March 2016
The question which we have addressed is “what is the clustering of galaxies about extragalactic radio sources and how does it vary with the properties of the sources such as radio luminosity, optical spectral properties, radio morphology, etc”. Only a small fraction of strong radio sources belong to rich clusters and most of the work on weaker associations is of a somewhat subjective nature. Our approach has been to work with complete, statistically defined samples of bright radio sources and to cross-correlate the positions of the sources with the Lick counts of galaxies. In this approach, we never define exactly what we mean by a cluster as group of galaxies - clustering is simply defined as the amplitude of a correlation function. This approach is only possible because the Lick counts of galaxies have been studied in detail by Seldner et al. (1977) who calibrated the Lick counts over the whole of the sky.