Heinz Hopf
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Heinz Hopf (November 19, 1894 – June 3, 1971) was a mathematician born in Gräbschen, Germany. He attended Dr. Karl Mittelhaus' higher boys' school from 1901 to 1904, and then entered the König-Wilhelm Gymnasium in Breslau. He showed mathematical talent from an early age. In 1913 he entered the Silesian Friedrich Wilhelms University where he attended lectures by Steinitz, Kneser, Max Dehn, Erhard Schmidt, and Rudolf Sturm. When World War 1 broke out in 1914, Hopf eagerly enlisted. He was wounded twice and received the iron cross (first class) in 1918.
In 1920, Hopf moved to Berlin to continue his mathematical education. He studied under Ludwig Bieberbach, receiving his doctorate in 1925. In his dissertation, Connections between topology and metric of manifolds (German Über Zusammenhänge zwischen Topologie und Metrik von Mannigfaltigkeiten), he proved that any Riemannian 3-manifolds of constant curvature is globally isometric to Euclidean, spherical, or hyperbolic space. He also studied the indices of zeros of vector fields on hypersurfaces, and connected their sum to curvature. Some six months later he gave a new proof that the sum of the indices of the zeros of a vector field on a manifold is independent of the choice of vector field and equal to the Euler characteristic of the manifold. This theorem is now called the Poincaré-Hopf theorem.
Hopf spent the year after his doctorate at Göttingen, where David Hilbert, Richard Courant, Carl Runge, and Emmy Noether were working. While there he met Paul Alexandrov and began a lifelong friendship.
In 1926 Hopf moved back to Berlin, where he gave a course in combinatorial topology. He spent the academic year 1927/28 at Princeton University on a Rockefeller fellowship with Alexandrov. Solomon Lefschetz, Oswald Veblen and J.W. Alexander were all at Princeton at the time. In the summer of 1928 Hopf returned to Berlin and began working with Alexandrov, at the suggestion of Courant, on a book on topology. Three volumes were planned, but only one was finished. It was published in 1935.
In October 1928 Hopf married Anja von Mickwitz (1891 - 1967). The next year he declined a job offer from Princeton. In 1931 Hopf took Hermann Weyl's position at Zürich.
Hopf received another invitation to Princeton in 1940, but he declined it. Two years later, however, he was forced to file for Swiss citizenship after his property was confiscated by Nazi authorities.
In 1946/47 and 1955/56 Hopf visited America, staying at Princeton and giving lectures at New York University and Stanford University. He served as president of the International Mathematical Union from 1955 to 1958. He received honorary doctorates from Princeton, Freiburg i. Br., Manchester, Sorbonne at Paris, Brussels, and Lausanne. Some of his students were Alexandre Priessman, Friedrich Hirzebruch, Beno Eckmann, and Hans Freudenthal.