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:…(Xu, 1997, 2001b); (b) an adaptive algorithm for Gaussian factor analysis and a criterion for determining the dimension of
principal subspace (Xu, 1998b); (c) a learned parametric model for Independent Component Analysis (ICA) that works on sources mixed with super-Gaussian and subGaussian…
Category: