Theaetetus (ca. 417 B.C. – 369 B.C.) of Athens, son of Euphronius, of the Athenian deme Sunium, was a classical 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. For other uses, see Athens (disambiguation). ... In biology, a deme (rhymes with team) is another word for a local population of organisms of one species that actively interbreed with one another and share a distinct gene pool. ... Cape Sounion, looking out to the Aegean islands The cape of Sounion or Sounio, previously known as Sunium (in ancient Greek Σούνιον) is located 65 kilometres south-east of Athens, in Attica. ... In mathematics, an irrational number is any real number that is not a rational number — that is, it is a number which cannot be expressed as a fraction m/n, where m and n are integers. ... The frontispiece of Sir Henry Billingsleys first English version of Euclids Elements, 1570 Euclids Elements (Greek: ) is a mathematical and geometric treatise, consisting of 13 books, written by the Hellenistic mathematician Euclid in Alexandria circa 300 BC. It comprises a collection of definitions, postulates (axioms), propositions (theorems... In geometry, a Platonic solid is a convex regular polyhedron. ...

Theaetetus, like Plato, was a student of the Greek mathematician Theodorus of Cyrene. Cyrene was a prosperous Greek colony on the coast of North Africa, in what is now Libya, on the eastern end of the gulf of Sidra. Theodorus had explored the theory of incomensuarable quantities, and Theaetetus continued those studies with great enthusiasm; specifically, he classified various forms of irrational numbers according to the way they are expressed as square roots. This theory is presented in great detail in Book X of Euclid's Elements. Theodorus of Cyrene was an Greek mathematician of the 5th century BC who was admired by Plato, who mentions him in several sources. ...

Theaetetus was one of the few Greek mathematicians who were actually natives of Athens. Most Greek mathematicians of antiquity came from the numerous Greek cities scattered around the Ionian coast, the Black Sea and the whole Mediterranean basin. Likewise, most Greek scientists came from the scattered Greek cities and not from Athens. Athens, and later Alexandria were centers of attraction because of the philosophical schools of Plato (the Academy) and Aristotle (the Lyceum), and the renowned Museum and Great Library. The Academy of Plato operated in Athens for almost 600 years, and served as educational center even for some of the early fathers of the Christian church.

He evidently resembled Socrates in the snubness of his nose and bulging of his eyes. This and most of what we know of him comes from Plato, who named a dialogue after him, the Theaetetus. He apparently died from wounds and dysentery on his way home after fighting in an Athenian battle at Corinth, now widely presumed to have occurred in 369 BC. This page is about the ancient Greek philosopher. ... PLATO was one of the first generalized Computer assisted instruction systems, originally built by the University of Illinois (U of I) and later taken over by Control Data Corporation (CDC), who provided the machines it ran on. ... The Theætetus (Θεαίτητος) is one of Platos dialogues concerning the nature of knowledge. ... Dysentery (formerly known as flux or the bloody flux) is the term for tenesmus (painful straining to pass stool), cramping, and frequent, small-volume severe diarrhea associated with blood in the feces. ... For other uses, see Athens (disambiguation). ... Corinth, or Korinth (Greek: Κόρινθος, Kórinthos; see also List of traditional Greek place names) is a Greek city-state, on the Isthmus of Corinth, the narrow stretch of land that joins the Peloponnesus to the mainland of Greece. ...

The Theaetetus crater on the Moon is named after him. Theaetetus is a lunar crater that is located to the southeast of Cassini crater near the eastern edge of Mare Imbrium. ... Apparent magnitude: up to -12. ...