All Figures

	Figure 1: Stereographic projection of a transequatorial sky region (ranging from up to 20 degs and from up to 80 degs) containing the ten selected DPOSS fields listed in Table 1. In order to trace the global investigated area, all the detected clusters (without any cut in magnitude) are represented.
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In the text

	Figure 2: Upper panel: comparison of the Poissonian distribution (solid line) and the distribution of xs (histogram) in a simulation with galaxies randomly distributed in the sky. Lower panel, as upper panel, but for actual observations: some of the nearest neighbours are physically linked (related) to the groups and produce an excess of neibourghs at small x.
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In the text

	Figure 3: The smoothed two dimensional density map of the number density of galaxies for a field centered at and (DPOSS plate n. 610). The smoothing has been performed by a Gaussian 2D filter with width ( ) typical for a cluster core at redshift 0.1-0.2.
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In the text

	Figure 4: MF of simulated (filled circles) and detected (empty squares) groups. On the horizontal axis there is the number of galaxies in each group, that is the richness.
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In the text

	Figure 5: Simulated (dots) and detected (triangles) structures. Left: groups. Right: clusters. Circles highlight simulated groups/clusters which have been detected.
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In the text

	Figure 6: The richness vs. the core radius of the simulated clusters (open circles). The crosses mark each retrieved cluster.
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In the text

	Figure 7: Cluster number in the one plate area is plotted as a function of estimated richness. The continuous histogram represents the number of clusters given in input to the simulation; the dash shaded histogram represents the retrieved clusters and the dot shaded histogram the spurious detections. The richness bin grows exponentially as 2n/2 (see Sect. 5).
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In the text

	Figure 8: Richness of the simulated vs. the detected clusters. The errors are inversely proportional to the signal to noise ratio for the detection.
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In the text

	Figure 9: The multiplicity function for galaxy structures ranging from small groups (filled triangles) to rich clusters (filled circles). We remove clusters in the richness range where detection efficiency is low.
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In the text

	Figure 10: Overplot of the MF of USGC2 groups (empty circles) on the multiplicity functions obtained from the DPOSS data.
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In the text