Only when a user u expresses a preference for topic t do we compute an element of the sparse matrix r u,t , Here, #(u, t, +1) and #(u, t, −1) represent the numbers of occurrences of instances (u, t, +1) and (u, t, −1), respectively.
an element r u,t approaches +1 as the user u favors the topic t, and −1 otherwise.