Person: Fischer, Ernst Sigismund
Ernst Fischer is best known for the RieszFischer theorem in the theory of Lebesgue integration.
Mathematical Profile (Excerpt):
 Ernst was educated in Vienna, and he studied at the University of Vienna under Mertens from 1894.
 From 1911 until 1920, Fischer was professor at the University of Erlangen, appointed to fill the chair left vacant in the previous year when Paul Gordan retired.
 When Fischer arrived in Erlangen it was natural for Noether to work with him.
 She refers in her papers at this time again and again to conversations with Fischer.
 Fischer is best known for one of the highpoints of the theory of Lebesgue integration, called the RieszFischer Theorem.
 From 1920 Fischer worked at the University of Cologne, remaining there until he retired in 1938.
 Let us note again the major result, the RieszFischer Theorem, for which he is best known as Weyl noted in the above quote.
 In 1907 Ernst Fischer studied orthonormal sequences of functions and gave necessary and sufficient conditions for a sequence of constants to be the Fourier coefficients of a square integrable function.
 The theorem, now called the RieszFischer theorem, is one of the great achievements of the Lebesgue theory of integration.
 Fischer went on to study Hadamard determinants, publishing his results in 1908 in the Archiv der Mathematik und Physik, and Sylvester determinants, publishing a paper in Crelle's Journal in the following year.
Born 12 July 1875, Vienna, Austria. Died 14 November 1954, Cologne, Germany.
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Origin Austria
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References
Adapted from other CC BYSA 4.0 Sources:
 Oâ€™Connor, John J; Robertson, Edmund F: MacTutor History of Mathematics Archive