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:In alternans, Cai and Vm are coupled either positively (large Cai transient prolongs APD of the same beat) or negatively (large Cai transient shortens APD of the same beat) (Kennedy et al., 2017).
Category: