On the Averaging Principle
  Typically, models with a heterogeneous property are considerably harder to analyze than the corresponding homogeneous models, in which the heterogeneous property is replaced with its average value. In this study we show that any outcome of a heterogeneous model that satisfies the two properties of differentiability and interchangibility is O(\epsilon^2) equivalent to the outcome of the corresponding homogeneous model, where \epsilon is the level of heterogeneity. We then use this averaging principle to obtain new results in queueing theory, game theory (auctions), and social networks (marketing). 
Gadi Fibich and Arieh Gavious and Eilon Solan
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