A collaborative filtering system recommends to users products that similar users like. Collaborative fi ltering systems influence purchase decisions, and hence have become targets of manipulation by unscrupulous vendors. We provide theoretical and empirical results demonstrating that while...

We characterize environmental quality as a stock, and its rate of deterioration as a flow. We consider a class of problems, which we call "SFQ" problems, in which both stocks and flows can be controlled to promote the quality of a resource stock. Abatement (curbing the flow) and restoration...