Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:…and Kockelkorn, 2006a, Damala and Kockelkorn, 2006b, Damala,
2007a, Damala, 2007b, Oppermann and Sprecht, 1998, Santoro et al., 2007) and “nomadic
devices” (Brelot et al., 2005, Damala et al., 2005, Hsi, 2004, Vlahakis et al., 2005) are other
variations often enough used in the relevant literature.
Category: