Georg Friedrich Bernhard Riemann (17 September 1826 near Hanover 20 July 1866 Selasca, Italy) was a German mathematician. He had a short life and did not write down very much about his discoveries, but the things he discovered were all extremely important and had a revolutionary effect on mathematics. He contributed to many fields of mathematics, such as analysis, geometry, mathematical physics and number theory. Today, many people see him as a great mathematician. He was among the first mathematicians that worked on complex analysis. The kind of geometry he started (which is called Riemannian geometry today) is one of the bases of the theory of relativity, developed by Albert Einstein.