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 above equation is the starting point of absolute perspective pose estimation [8, 14, 13, 9]: given a set of 3D–2D point correspondences (xi ↔ Xi), one can recover the 3D rigid body transformation (R, t) : W → C acting between the world coordinate frame W and the camera coordinate frame C.
Category: