24.1 Introduction
The problem of recommendation has been identified as the way to help individuals
in a community to find information or items that are most likely to be interesting
to them or to be relevant to their needs [4, 39, 72]. Typically, it assumes that there
is set Users of all the users of a system and set Items of all possible items that
can be recommended to them. Then, the utility function that measures the appropriateness of recommending item i ∈ Items to user u ∈ Users is often defined as
R : Users × Items → R0, where R0 typically is represented by non-negative integers