In the present edition we have made changes in Chapter 1, mainly as a result of comments by Professor A. S. Besicovitch. Some theorems are stated more explicitly, a few proofs are added, and some are shortened. We are indebted to him for an elementary proof of the theorem of bounded convergence for Riemann integrals, which appears in the notes. In Chapter 6 the proof of Poisson's equation has been improved. In Chapter 17 we have discussed the Airy integral for complex argument in more detail, and have given conditions for uniformity of approximation for asymptotic solutions of Green's type for complex argument. In Chapter 23 we have added some remarks on the analytic continuation of the solutions, and a note applies them to the parabolic cylinder functions. We should like to express our thanks to several readers for drawing our attention to errors and misprints.本书为英文版。