关于最大熵模型 http://cseweb.ucsd.edu/~elkan/254/ari_talk.pdf http://uet.vnu.edu.vn/~thuyhq/papers/06_NNH_International%20Conference%20on%20Internet%20Information%20Retrieval_IRC2006_143_149.pdf http://www.stanford.edu/class/cs124/lec/Maximum_Entropy_Classifiers.pdf