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:However, unlike Cochrane and Saa-Requejo (2000), the authors do not use a Sharpe ratio criterion to restrict the price interval for assets in incomplete markets, but a generalized version of the coherent risk measure first introduced by Artzner et al. (1999). Carr et al. (2001) argue that economic agents will not only invest in any arbitrage opportunity, but also invest in any acceptable opportunity.
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