49 research outputs found
Metric operators, generalized hermiticity and lattices of Hilbert lpaces
A quasi-Hermitian operator is an operator that is similar to its adjoint in
some sense, via a metric operator, i.e., a strictly positive self-adjoint
operator. Whereas those metric operators are in general assumed to be bounded,
we analyze the structure generated by unbounded metric operators in a Hilbert
space. It turns out that such operators generate a canonical lattice of Hilbert
spaces, that is, the simplest case of a partial inner product space
(PIP-space). We introduce several generalizations of the notion of similarity
between operators, in particular, the notion of quasi-similarity, and we
explore to what extend they preserve spectral properties. Then we apply some of
the previous results to operators on a particular PIP-space, namely, a scale of
Hilbert spaces generated by a metric operator. Finally, motivated by the recent
developments of pseudo-Hermitian quantum mechanics, we reformulate the notion
of pseudo-Hermitian operators in the preceding formalism.Comment: 51pages; will appear as a chapter in \textit{Non-Selfadjoint
Operators in Quantum Physics: Mathematical Aspects}; F. Bagarello, J-P.
Gazeau, F. H. Szafraniec and M. Znojil, eds., J. Wiley, 201
Subnormal operators regarded as generalized observables and compound-system-type normal extension related to su(1,1)
In this paper, subnormal operators, not necessarily bounded, are discussed as
generalized observables. In order to describe not only the information about
the probability distribution of the output data of their measurement but also a
framework of their implementations, we introduce a new concept
compound-system-type normal extension, and we derive the compound-system-type
normal extension of a subnormal operator, which is defined from an irreducible
unitary representation of the algebra su(1,1). The squeezed states are
characterized as the eigenvectors of an operator from this viewpoint, and the
squeezed states in multi-particle systems are shown to be the eigenvectors of
the adjoints of these subnormal operators under a representation. The affine
coherent states are discussed in the same context, as well.Comment: LaTeX with iopart.cls, iopart12.clo, iopams.sty, The previous version
has some mistake
On Unbounded Composition Operators in -Spaces
Fundamental properties of unbounded composition operators in -spaces are
studied. Characterizations of normal and quasinormal composition operators are
provided. Formally normal composition operators are shown to be normal.
Composition operators generating Stieltjes moment sequences are completely
characterized. The unbounded counterparts of the celebrated Lambert's
characterizations of subnormality of bounded composition operators are shown to
be false. Various illustrative examples are supplied
A quasi-affine transform of an unbounded operator
Some results on quasi-affinity for bounded operators are extended to unbounded ones and normal extensions of an unbounded operator are discussed in connection with quasi-affinity