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 normal velocity of the tumor boundary [21], [18], [22], [13] is: V = −μn · (∇P ) (8) Following [21], [18], [22], [13] and others, assume that λ, λA, λB, nB, and b are uniform.
Category: