Many information sources embody recommender systems as a way of personal-
izing their content for users [73]. Recommender systems have the effect of guiding
users in a personalized way to interesting or useful objects in a large space of possi-
ble options [17]. Recommendation algorithms use input about a customer’s interests
to generate a list of recommended items. At Amazon.com, recommendation algo-
rithms are used to personalize the online store for each customer, for example show-
ing programming titles to a software engineer and baby toys to a new mother [50].