# Ernst Sigismund Fischer

circa 1920

Ernst Sigismund Fischer (born July 12, 1875 in Vienna , † November 14, 1954 in Cologne ) was an Austrian mathematician who dealt with analysis and algebra .

## Live and act

Ernst Sigismund Fischer was the son of the composer and music professor Jacob Fischer and studied from 1894 in Vienna (and one semester in Berlin) a. a. with Franz Mertens . In 1899 he received his doctorate there under Leopold Gegenbauer . He then studied with Hermann Minkowski in Zurich and Göttingen . In 1902 he was E. Waelsch's assistant at the German Technical University in Brno . In 1904 he became a private lecturer and in 1910 an associate professor. In 1911 he was a professor in Erlangen as the successor to Paul Gordan , where Emmy Noether was at the same time at the mathematical institute, on which, according to Hermann Weyl, he exerted greater influence than her doctoral supervisor Gordan, since in contrast to this he had a more abstract point of view in the Algebra represented in the sense of the school of David Hilbert . After military service in the First World War , he was a professor in Cologne from 1920, where he was forced to retire in 1938 because of Jewish ancestors.

Fischer is best known for his proof of the Fischer-Riesz theorem, named after him and Frigyes Riesz , about the completeness of the space of square-integrable functions . In his corresponding work from 1907, he also introduced the “ mean convergence ” . With the simultaneous work of Frigyes Riesz (1907) this was an important step in the development of the Hilbert space . They used the Lebesgue integral and their theorem was one of the early successes of Henri Lebesgue's theory of integration . ${\ displaystyle L ^ {2}}$${\ displaystyle L ^ {2}}$

Furthermore, according to him the min-max theorem designated to a representation of the eigenvalues of a symmetric or Hermitian matrix maximum as minimal or Rayleigh quotient indicates

In 1931 he was President of the German Mathematicians Association .