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:[4] mathematically proved the reconstruction of analog signals, with an error as low as required, from the pulses under certain conditions; thus, APC is an alternative to conventional ADC and has sparse representations, at least for certain impulsive classes of signals [5]–[7].
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