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:Abdullah (1966) found expressions for the fundamental (F0) and first harmonic (F1) of the vibrational modes in Hertz to be
F0 4U2 C2 25 2 16 2C 2 R, 3 and
F1 4U2 C2 81 2 16 2C 2 R, 4 where C is the speed of sound in meters per second (a value of 330 m s 1 is used in making the estimates below), U…
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