Theaetetus (mathematician)
Theaetetus (c. 417 B.C. – 369 B.C.) was a Greek mathematician of Geometry. 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 polyhedra.
He was described as having a short nose and protruding eyes. Much of what we know of him comes from Plato.
The Theaetetus crater on the Moon is named after him.