http://course.baidu.com/view/04fdc0202f60ddccda38a0b5.html
http://rec-sys.net/forum.php?mod=viewthread&tid=381&extra=
http://www.philippe-fournier-viger.com/spmf/index.php?link=documentation.php#example8
http://my.oschina.net/leejun2005/blog/136792