Abstract
In this paper we show that for a bounded linear operatorA on a complex Hilbert spaceH, the points on the boundary of the numerical range ofA with infinite curvature and unique tangent are in the essential spectrum ofA, thus positively answering a conjecture raised by Hubner in [3].
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Farid, F.O. On a conjecture of Hubner. Proc. Indian Acad. Sci. (Math. Sci.) 109, 373–378 (1999). https://doi.org/10.1007/BF02837995
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DOI: https://doi.org/10.1007/BF02837995