Theaetetus

Theaetetus of Athens, possibly the son of Euphronius of the Athenian deme Sunium, was a Greek mathematician. His principal contributions were on irrational lengths, which was included in Book X of Euclid's Elements and proving that there are precisely five regular convex polyhedra. A friend of Socrates and Plato, he is the central character in Plato's eponymous Socratic dialogue.