《希尔伯特空间问题集(第2版)》讲述了：This book was written for the active reader. The first part consists of problems, frequently preceded by definitions and motivation, and some-times followed by corollaries and historical remarks. Most of the problems are statements to be proved, but some are questions (is it?, what is?), and some are challenges (construct, determine). The second part, a very short one, consists of hints. A hint is a word, or a paragraph, usually intended to help the reader find a solution. The hint itself is not necessarily a con-densed solution of the problem; it may just point to what I regard as the heart of the matter. Sometimes a problem contains a trap, and the hint may serve to chide the reader for rushing in too recklessly. The third part, the longest, consists of solutions: proofs, answers, or constructions, depending on the nature of the problem