1.machine learning definition

• Arthur Samuel (1959).Field of study that gives computers the ability to learn without being explicitly programmed.
• Tom Mitchell (1998) Well-posed Learning Problem: A computer program is said to learn from experience E with respect to some task T and some performance measure P, if its performance on T, as measured by P, improves with experience E.
  Tom Mitchell的定义是说计算机程序从现有的经验中去学习怎样根据评价指标去完成任务，随着学习次数的增加，经验增长，任务完成的会更好。下面有一个例子：

Suppose your email program watches which emails you do or do not mark as spam, and based on that learns how to better filter spam. What is the task T in this setting?

1. Classifying emails as spam or not spam.(right answer)

2. Watching you label emails as spam or not spam.

3. The number (or fraction) of emails correctly classified as spam/not spam.

4. None of the above—this is not a machine learning problem.

2. machine learning classification

• supervised learning
• unsupervised learning
• semi-supervised learning
• others：Reinforcement learning, recommender systems.

机器学习主要包括监督学习、无监督学习、强化学习、推荐系统、半监督学习等，其中监督学习和无监督学习是核心。