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:Muller and Schupp [80] proved that a finitely generated graph has finitary end-structure if and only if Γ is isomorphic to the complete transition graph Γ(M) of some pushdown automaton M .
Category: