Euclid (Greek: Εὐκλείδης -- Euklidis), also known as Euclid of Alexandria, was a Greek mathematician of the Hellenistic period who flourished in Alexandria, Egypt, almost certainly during the reign of Ptolemy I (323 BC-283 BC). His Elements is the most successful textbook in the history of mathematics. In it, the principles of geometry are deduced from a small set of axioms. Furthermore, Euclid's method of proving mathematical theorems by logical reasoning from accepted first principles remains the backbone of mathematics and is responsible for that field's characteristic rigor (see Mathematics). Although best-known for its geometric results, the Elements also includes various results in number theory, such as the connection between perfect numbers and Mersenne primes, the proof of the infinitude of prime numbers, Euclid's lemma on factorization (which leads to the fundamental theorem of arithmetic on uniqueness of prime factorizations), and the Euclidean algorithm for finding the greatest common divisor of two numbers.
Euclid also wrote works on perspective, conic sections, spherical geometry, and possibly quadric surfaces.