Ernst Sigismund Fischer

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Ernst S. Fischer
ca. 1920
Born	(1875-07-12)12 July 1875 Vienna
Died	14 November 1954(1954-11-14) (aged 79) Cologne
Nationality	Austrian
Alma mater	University of Vienna
Known for	Riesz–Fischer theorem Fischer's inequality
Spouse	Ellis Strauss
Children	Ursula
Scientific career
Fields	Mathematics
Institutions	University of Cologne
Thesis	Zur Theorie der Determinanten  (1899)
Doctoral advisor	Franz Mertens Leopold Gegenbauer

Ernst Sigismund Fischer (12 July 1875 – 14 November 1954) was a mathematician born in Vienna, Austria. He worked alongside both Mertens and Minkowski at the Universities of Vienna and Zurich, respectively. He later became professor at the University of Erlangen, where he worked with Emmy Noether.

His main area of research was mathematical analysis, specifically orthonormal sequences of functions, which laid groundwork for the emergence of the concept of a Hilbert space.

The Riesz–Fischer theorem in Lebesgue integration is named in his honour.

He is the grandson of composer Karl Graedener.^[1][2]