| | Kuratowski: Introduction to Set Theory |
 | | The object of set theory is to investigate the properties of sets from the most general point of view; generality is an essential aspect of the theory of sets. |
 | | From this follows the important corollary that of two different well ordered sets one is of power equal to that of a subset of the other; in the terminology of cardinal numbers this means that for two distinct cardinal numbers corresponding' to well ordered sets, one is always smaller than the other. |
 | | The stimulus to the investigations from which the theory of sets grew, was given by problems of analysis, the establishing of the foundations of the theory of irrational numbers, the theory of trigonometric series, etc, However, the further development of set theory went initially in an abstract direction, little connected with other branches of mathematics. |
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