 Let us now turn to Cantor’s other main legacy, the study of transfinite numbers. By 1908 Hausdorff was working on uncountable order types and introduced the Generalized Continuum Hypothesis \((2^{\aleph_{a}} = \aleph_{a+1})\). He was also the first to consider the possibility of an “exorbitant” cardinal, namely a weakly inaccessible, i.e., a regular cardinal that is not a successor (a cardinal \(\alpha\) is called regular if decomposing \(\alpha\) into a sum of smaller cardinals requires \(\alpha\)-many such numbers). Few years later, in the early 1910s, Paul Mahlo was studying hierarchies of such large cardinals in work that pioneered what was to become a central area of set theory; he obtained a succession of inaccessible cardinals by employing a certain operation that involves the notion of a stationary subset; they are called Mahlo cardinals. But the study of large cardinals developed slowly. Meanwhile, Hausdorff’s textbook Grundzüge der Mengenlehre (1914) introduced two generations of mathematicians into set theory and general topology.