Mathematics is the queen of sciences. She is pure, noble and attractive, and also has a distinct character in comparison with subjects in sciences such as physics: its permanent relevance and eternal validness ofits theories and theorems. Whatever was once proved will stay true forever.Mathematics is a vast subject, and many new concepts, theories and results spring up like mushrooms after spring rain. Similarly, there is also a large number of new mathematics books appearing in libraries and on bookshelves. Probably due to the usefulness of mathematics and its foundational nature, there seems to be more books in mathematics than in other subjects. On the other hand, only a limited number, or even a few, of them stand out and are appreciated and used by many people. The best test on the quality of books is the test of time.In this series ofbooks Classical Topics in Mathe'matics, we have selected books written by leading experts on topics which are well-tested by time. We believe that in spite of the passage of time, their power and value have not diminished, and hence they bear the trademark of the classical mathematics.The first volumes of this series consist of an annotated version of Klein's masterpiece Lectures on, the icosahedron, and the solution, of equations of the fifth deg'ree, and the first English translation of Klein and Fricke's four big volumes on modular functions and automorphic functions. For this series, we have tried to pick books which share or reflect Klein's vision of the grand unity of mathematics.The publication of this series of books is consistent with the motto of the Higher Education Press: to provide high quality books on the essential mathematics to the world mathematics community at an affordable price.