German-born American mathematician and educator who made significant advances in the calculus of variations.

Courant received his secondary education in Germany and Switzerland and his doctorate from the University of Göttingen in 1910 under David Hilbert. For the next four years he taught mathematics at Göttingen. Then, at age 26, his teaching and research were interrupted by World War I. A few years after the war, Courant returned to Göttingen, where he founded and, from 1920 until 1933, was director of the university's mathematics institute. While at the institute he wrote a two-volume treatise with Hilbert (Hilbert, David), Methods of Mathematical Physics (1953–62; originally published in German, 1924–37), a work that furthered the evolution of quantum mechanics.

With the rise of the Nazis in 1933, Courant fled from Germany to England. The following year he was appointed professor of mathematics at New York University, a position he held until his retirement in 1958. Courant wanted to duplicate at NYU a mathematical institute similar to the one that he had directed at Göttingen. Under his leadership, Courant, with mathematicians James J. Stoker and Kurt O. Friedrichs, established one of America's most prestigious institutes of applied mathematics; upon his retirement, the institute was named in his honour. Courant also wrote a two-volume elementary work on applied calculus, Differential and Integral Calculus (1934; originally published in German, 1927–29), and, with H. Robbins, a general work for the layman, What Is Mathematics? (1941).