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:(12)
A theoretical expression for the variation of with strain is given in Varghese and Ophir (1997b) as:
2 1 s 1 e lko 2 1 s 2 1 e lko 22 s2 1 s 2 1
e lko 2 2 2 s 2 1 s 2 1 (13)
where ko is the wave number (given by ko 2 / o where o is the central wavelength), and l is the correlation length.
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