books.google.com - There can be few textbooks of mathematics as well-known as Hardy's Pure Mathematics. Since its publication in 1908, it has been a classic work to which successive generations of budding mathematicians have turned at the beginning of their undergraduate courses. In its pages, Hardy combines the enthusiasm...https://books.google.com/books/about/A_Course_of_Pure_Mathematics.html?id=oEp0kpPpLB4C&q=principal+value&utm_source=gb-gplus-shareA Course of Pure Mathematics