Ernst Sigismund Fischer (1875-1954)
mathematician

At the University of Erlangen, where Max Noether was a Professor of Mathematics, and where his daughter Emmy received her Ph.D., the "invariant king," Paul Gordan, retired in 1910. A year later, the position was filled by Ernst Fischer. As Emmy Noether had recently written her dissertation under Gordan's direction, it was natural that she continued working along the same lines as Gordan's successor, Fischer.

In his Memorial Address after Emmy Noether's passing in 1935, Hermann Weyl wrote,

Fischer's field was algebra..., in particular the theory of elimination and of invariants. He exerted upon Emmy Noether, I believe, a more penetrating influence than Gordan did. Under his direction the transition from Gordan's formal standpoint to the Hilbert method of approach was accomplished. She refers to her papers at this time again and again to conversations with Fisher.
Fischer is best known for one of the highpoints of the theory of Lebesgue integration, called the Riesz-Fischer Theorem. The theorem is that the space of all square-integrable functions is complete, in the sense that Hilbert space is complete, and the two spaces are isomorphic by means of a mapping based on a complete orthonormal system.

Additional biographical material on Fischer and a list of his publications are given in

M. Pinl, Ernst Sigismund Fischer, Neue Deutsche Biographie 5 (Berlin, 1952- ), 183.

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