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In page History of mathematics:

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Non-standard analysis, introduced by Abraham Robinson, rehabilitated the infinitesimal approach to calculus, which had fallen into disrepute in favour of the theory of limits, by extending the field of real numbers to the Hyperreal numbers which include infinitesimal and infinite quantities.[citation needed] An even larger number system, the surreal numbers were discovered by John Horton Conway in connection with combinatorial games.[citation needed]