如果想在这一行发paper或者搞研究的话，或者读phd, 这是必须的。Cong recommendation : If you want to visit in this paper, or engage in the study, or phd time, it is essential. 但是书比较难，要有心理准备。But book more difficult, to be psychologically prepared. 作者2是哥大教授。Columbia is the author of two professors. 他们俩还有一本书我正在读，1998年写的，但是很难，不推荐。They both have a book I was reading, in 1998 wrote, it is difficult and not recommended.