To expand search, see The Pythagoreans. Laterally related topics: The Pentagram and Nichomachus of Gerasa.
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Jones, Phillip S. Irrationals or Incommensurables. I. Their discovery, and a "Logical Scandal". Mathematics Teacher 49 (1956), 123--27.
The discovery of irrationals. Discusses an appealing theory, due to Kurt von Fritz, that the discovery of irrationals grew out of a study of the pentagram. Von Fritz is in support of the traditional theory that discovery or irrationals was due to Hippasus of Metapontum. Reprinted in Swetz, Frank J., From Five Fingers to Infinity. Closely related topics: Irrationals and The Pentagram.