French mathematician Pierre de Fermat (1601–1665) made many contributions to early methods leading to differential calculus; he was also considered by some to be the founder of modern number theory (see “Math Basics”) and did much to establish coordinate geometry, eventually leading to Cartesian coordinates. He supposedly proved a theorem eventually called “Fermat’s last theorem.” It states that the equation xⁿ + yⁿ = zⁿ has no non-zero integer solutions for x, y, and z when n is greater than 2. But there is no proof of Fermat’s “proof,” making most mathematicians skeptical about his supposed discovery.