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.
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 incommensurable
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.
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.
The crater
Theaetetus on the Moon is named after
him.
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