Reference
Amazon's Item-to-Item Collaborative Filtering.
http://www.cs.umd.edu/~samir/498/Amazon-Recommendations.pdf
More about clustering analysis
http://www.statmethods.net/advstats/cluster.html
About the package
instaR
https://github.com/pablobarbera/instaR/tree/master/instaR
Online courses
https://www.coursera.org/course/clusteranalysis
https://class.coursera.org/ml-003/lecture/100
Celebrity users and the location hashtags were obtained from here:
https://socialblade.com/instagram/top/100/followers
http://www.popsugar.com/celebrity/Celebrities-Using-Instagram-21244293?stream_view=1#photo-23154687
http://top-hashtags.com/hashtag/monument/