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:These models are the radial-modesof-vibration model of Abdullah (1966), the tornado/ boundary interaction model of Tatom et al. (1995), and the corotating vortex model (Powell 1964; Georges 1976; Mitchell et al. 1992). The infrasonic data seemed most consistent with the model of Abdullah, as is shown in a later section. Abdullah (1966) found expressions for the fundamental (F0) and first harmonic (F1) of the vibrational modes in Hertz to be
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