A prime function of many major World Wide Web applications is Online Service Allocation (OSA), the function of matching individual consumers with particular services/goods (which may include loans or jobs as well as products) each with its own producer. In the applications of interest, consumers...

Recommender systems often recommend several products to a user at the same time, but with little consideration of the relationships among the recommended products. We argue that relationships such as substitutes and complements are crucial, since the utility of one product may depend on whether...