Serge Lang (French:[l]; May 19, 1927 September 12, 2005) was a French-born American mathematician. He is known for his work in number theory and for his mathematics textbooks, including the influential Algebra. He was a member of the Bourbaki group.Lang was born in Paris in 1927, and moved with his family to California as a teenager, where he graduated in 1943 from Beverly Hills High School. He subsequently graduated from the California Institute of Technology in 1946, and received a doctorate from Princeton University in 1951. He held faculty positions at the University of Chicago and Columbia University (from 1955, leaving in 1971 in a dispute). At the time of his death he was professor emeritus of mathematics at Yale University.Lang studied under Emil Artin at Princeton University, writing his thesis on quasi-algebraic closure. Lang then worked on the geometric analogues of class field theory and diophantine geometry. Later he moved into diophantine approximation and transcendence theory, proving the SchneiderLang theorem.A break in research while he was involved in trying to meet 1960s student activism halfway caused him (by his own description) difficulties in picking up the threads afterwards. He wrote on modular forms and modular units, the idea of a 'distribution' on a profinite group, and value distribution theory.He made a number of conjectures in diophantine geometry: MordellLang conjecture, BombieriLang conjecture, Lang's integral point conjecture, LangTrotter conjecture, Lang conjecture on Gamma values, Lang conjecture on analytically hyperbolic varieties.He introduced the Lang map and the LangSteinberg theorem (cf. Lang's theorem) in algebraic groups.He introduced the KatzLang finiteness theorem.He was a prolific writer of mathematical texts, often completing one on his summer vacation. Most are at the graduate level. He wrote calculus texts and also prepared a book on group cohomology for Bourbaki.Lang's Algebra, a graduate-level introduction to abstract algebra, was a highly influential text that ran through numerous updated editions. His Steele prize citation stated, "Lang's Algebra changed the way graduate algebra is taught...It has affected all subsequent graduate-level algebra books." It contained ideas of his teacher, Artin; some of the most interesting passages in Algebraic Number Theory also reflect Artin's influence and ideas that might otherwise not have been published in that or any form.