  With the popularity of Location-based Social Networks, Point-of-Interest
(POI) recommendation has become an important task, which learns the users'
preferences and mobility patterns to recommend POIs. Previous studies show that
incorporating contextual information such as geographical and temporal
influences is necessary to improve POI recommendation by addressing the data
sparsity problem. However, existing methods model the geographical influence
based on the physical distance between POIs and users, while ignoring the
temporal characteristics of such geographical influences. In this paper, we
perform a study on the user mobility patterns where we find out that users'
check-ins happen around several centers depending on their current temporal
state. Next, we propose a spatio-temporal activity-centers algorithm to model
users' behavior more accurately. Finally, we demonstrate the effectiveness of
our proposed contextual model by incorporating it into the matrix factorization
model under two different settings: i) static and ii) temporal. To show the
effectiveness of our proposed method, which we refer to as STACP, we conduct
experiments on two well-known real-world datasets acquired from Gowalla and
Foursquare LBSNs. Experimental results show that the STACP model achieves a
statistically significant performance improvement, compared to the
state-of-the-art techniques. Also, we demonstrate the effectiveness of
capturing geographical and temporal information for modeling users' activity
centers and the importance of modeling them jointly.
