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 boundary conditions of the tumor [18] may be set up by: (n) = n ∞ (6) (P ) = γ κ (7) where the pressure boundary condition (7) reflects the influence of cell–cell adhesion through the parameter γ and κ.
Category: