Unitary invariance and spectral variation |
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Institution: | Indian Statistical Institute New Delhi-110016, India;Department of Mathematics University of Guelph Guelph, Ontario N1G 2W1, Canada |
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Abstract: | We call a norm on operators or matrices weakly unitarily invariant if its value at operator A is not changed by replacing A by U1AU, provided only that U is unitary. This class includes such norms as the numerical radius. We extend to all such norms an inequality that bounds the spectral variation when a normal operator A is replaced by another normal B in terms of the arclength of any normal path from A to B, computed using the norm in question. Related results treat the local metric geometry of the “manifold” of normal operators. We introduce a representation for weakly unitarily invariant matrix norms in terms of function norms over the unit ball, and identify this correspondence explicitly in certain cases. |
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