# M. B. Bekker

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### Scale-invariant self-adjoint extensions of scale-invariant symmetric operators: continuous versus discrete

Miron B. Bekker, Martin J. Bohner, Mark A. Nudel'man, Hristo Voulov

Methods Funct. Anal. Topology **21** (2015), no. 1, 41-55

We continue our study of a $q$-difference version of a second-order differential operator which depends on a real parameter. This version was introduced in our previous three articles on the subject. First we study general symmetric and scale-invariant operators on a Hilbert space. We show that if the index of defect of the operator under consideration is $(1,1)$, then the operator either does not admit any scale-invariant self-adjoint extension, or it admits exactly one scale-invariant self-adjoint extension, or it admits exactly two scale-invariant self-adjoint extensions, or all self-adjoint extensions are scale invariant. We then apply these results to the differential operator and the corresponding difference operator under consideration. For the continuous case, we show that the interval of the parameter, for which the differential operator is not semi-bounded, contains an infinite sequence of values for which all self-adjoint extensions are scale-invariant, while for the remaining values of the parameter from that interval, there are no scale-invariant self-adjoint extensions. For the corresponding difference operator, we show that if it is not semi-bounded, then it does not admit any scale-invariant self-adjoint extension. We also show that both differential and difference operators, at value(s) of the parameter that cor espond to the endpoint(s) of the interval(s) of semi-boundedness, have exactly one scale-invariant self-adjoint extension.

### A q-difference operator with discrete and simple spectrum

Miron B. Bekker, Martin J. Bohner, Hristo Voulov

Methods Funct. Anal. Topology **17** (2011), no. 4, 281-294

We continue our study of a $q$-difference version of a second-order differential operator which depends on a real parameter. This version was introduced in our previous article. For values of the parameter for which the difference operator is self adjoint, we show that the spectrum of the operator is discrete and simple. When $q$ approaches $1$, the spectrum fills the whole positive or negative semiaxis.

### On non-densely defined invariant Hermitian contractions

Methods Funct. Anal. Topology **13** (2007), no. 3, 223-235

We consider a non-densely defined Hermitian contractive operator which is unitarily equivalent to its linear-fractional transformation. We show that such an operator always admits self-adjoint extensions which are also unitarily equivalent to their linear-fractional transformation.

### The Helson-Szegö theorem for operator-valued weight

Miron Bekker, A. P. Ugolʹnikov

Methods Funct. Anal. Topology **10** (2004), no. 3, 11-16

### On a two-dimensional trigonometric moment problem

Methods Funct. Anal. Topology **9** (2003), no. 3, 185-188