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 a previous work (17), we studied the tumor-immune system dynamics in the presence of effector T (TE) cells that can potentially reject or induce the rejection of the tumor, and TR cells, by formulating a simple mathematical model for the interaction between a solid tumor and the lymphocytes on its adjacent lymph node.
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