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:standing for the mode coupling between two adjacent waveguides and κ κ κ = = + + n n n n in 2 2 1 2 1 2 standing for the mode coupling within the same waveguides, Eqs (17) and (18) can be written as
Category: