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Published online by Cambridge University Press: 31 October 2008
A classical theorem of Cantor states that the class of all sub-classes of a given class has a cardinal greater than that of the given class. This theorem is here established in a sharpened form, which was suggested to me by a question set by Professor J. M. Whittaker, F.R.S, in the 1950 examination for the Honours B.Sc. Degree at Liverpool.
1 The question was as follows:— “Let A be any class and let T be the class of all subclasses of A which contain more than one member. If A has more than two-members, prove that T has a greater cardinal than A.”