Abstract
In early studies of Hilbert space (by Hilbert, Hellinger, Toeplitz, and others) the objects of chief interest were quadratic forms. Nowadays they play a secondary role. First comes an operator A on a Hilbert space H, and then, apparently as an afterthought, comes the numerical-valued function f ↦ (Af, f) on H. This is not to say that the quadratic point of view is dead; it still suggests questions that are interesting with answers that can be useful.
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© 1982 Springer-Verlag New York Inc
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Halmos, P.R. (1982). Numerical Range. In: A Hilbert Space Problem Book. Graduate Texts in Mathematics, vol 19. Springer, New York, NY. https://doi.org/10.1007/978-1-4684-9330-6_22
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DOI: https://doi.org/10.1007/978-1-4684-9330-6_22
Publisher Name: Springer, New York, NY
Print ISBN: 978-1-4684-9332-0
Online ISBN: 978-1-4684-9330-6
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