The examples collected at the end of each chapter are of two kinds. Some of them are mere exercises. Some, however, (those marked with an asterisk), are of a very different nature. They are essentially suggestions for such further investigations, as appear to me to be of promise and importance. I have, also in the body of the book, taken the privilege of pointing out further problems which seem to be of interest. Many others will readily suggest themselves. It is my sincere wish that these suggestions may be helpful toward a further development of this fascinating subject.
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