站在巨人的肩膀上眺望远方
reference：
http://blog.csdn.net/zouxy09/article/details/8537620
https://en.wikipedia.org/wiki/Expectation%E2%80%93maximization_algorithm#As_a_maximization-maximization_procedure
http://www.cnblogs.com/jerrylead/archive/2011/04/06/2006936.html
https://www.zhihu.com/question/27976634
http://www.jianshu.com/p/1121509ac1dc
http://blog.pluskid.org/?p=81
https://chenrudan.github.io/blog/2015/12/02/emexample.html
http://ai.stanford.edu/~chuongdo/papers/em_tutorial.pdf
http://rstudio-pubs-static.s3.amazonaws.com/1001_3177e85f5e4840be840c84452780db52.html
《统计学习方法》