Theaetetus (born c. 417 bc, Athens [Greece]—died 369, Athens) was an Athenian mathematician who had a significant influence on the development of Greek geometry.

Theaetetus was a disciple of Socrates and studied with Theodorus of Cyrene. He taught at some time in Heraclea (located in present-day southern Italy). Plato made Theaetetus the chief subject of two dialogues—Theaetetōs (Theaetetus) and Sophistēs (Sophist)—the former being the major source of information about Theaetetus’s life, including his death in a battle between Athens and Corinth in 369 bc.

Theaetetus made important contributions to the mathematics that Euclid (fl. c. 300 bc) eventually collected and systematized in his Elements. A key area of Theaetetus’s work was on incommensurables (which correspond to irrational numbers in modern mathematics), in which he extended the work of Theodorus by devising the basic classification of incommensurable magnitudes into different types that is found in Book X of the Elements. He also discovered methods of inscribing in a sphere the five Platonic solids (tetrahedron, cube, octahedron, dodecahedron, and icosahedron), the subject of Book XII of the Elements. Finally, he may be the author of a general theory of proportion that was formulated after the numerically based theory of the Pythagoreans (fl. 5th century bc) yet before that of Eudoxus of Cnidus (c. 400–350 bc) as described in Book V of the Elements.

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