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:The absolute pose estimation of a perspective camera from n 2D–3D point correspondences is known in the literature as the Perspective n Point (PnP) problem, which has been widely studied in the last few decades [6, 13, 14, 8].
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