Ernst Sigismund Fischer (Vienna, July 12, 1875 - Cologne, 14 November 1954) was a German mathematician born in Austria. He worked with the University of Vienna together with Franz Mertens and the University of Zurich with Hermann Minkowski. In 1910 he became the successor to Paul Gordan at Erlangen University. In the invariant theory he was on the line of David Hilbert. In the years immediately after 1910 he worked closely with Emmy Noether.
Its main field of research was the mathematical analysis, especially orthonormal rows of functions, which laid the foundation for the creation of a Hilbert space concept.
The statement of Riesz-Fischer in Lebesgue integration was named after him
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