On point spectrum of critical almost Mathieu operators

2021 ◽  
Vol 392 ◽  
pp. 107997
Author(s):  
S. Jitomirskaya
Keyword(s):  
Point Spectrum

scholarly journals The boundary of the numerical range

1981 ◽  
Vol 22 (1) ◽  
pp. 69-72 ◽  
Author(s):  
G. de Barra
Keyword(s):  
Hilbert Space ◽  
Convex Hull ◽  
Similar Condition ◽  
Point Spectrum ◽  
Numerical Range ◽  
Normal Operator ◽  
Density Condition ◽  
Compact Normal Operator

In [1] it was shown that for a compact normal operator on a Hilbert space the numerical range was the convex hull of the point spectrum. Here it is shown that the same holds for a semi-normal operator whose point spectrum satisfies a density condition (Theorem 1). In Theorem 2 a similar condition is shown to imply that the numerical range of a semi-normal operator is closed. Some examples are given to indicate that the condition in Theorem 1 cannot be relaxed too much.

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