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 problem is completely solved in the i.i.d case, however, it is not easy to check effectively, since it has been proved in Blondel et al. (2000) that computing the Lyapunov exponents is an NP-hard problem.
Category: