Fischer
# Ernst Sigismund Fischer

### Born: 12 July 1875
in Vienna, Austria

Died: 14 Nov 1954
in Cologne, Germany

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**Ernst Fischer** studied at Vienna under Mertens from 1894. He spent 1899 in Berlin, then studied at Zurich and Göttingen with Minkowski. From 1902 he was assistant to E Waelsch at the University of Brünn (now Brno), becoming professor there after a few years.
From 1911 until 1920, Fischer was professor at Erlangen, then from 1920 he worked at Cologne.

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. This led to the concept of a Hilbert space. F Riesz published a similar result in the same year.

The theorem, now called the Riesz-Fischer theorem, is one of the great achievements of the Lebesgue theory of integration.

**Reference (One book/article)**

JOC/EFR December 1996