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:Then there have been a wide range of research covering a variety of different schemes for undeniable signatures over the past 15 years [7, 1, 9, 8, 19, 14, 18, 25, 4, 17, 16, 22, 3, 26, 27].
Category: