The properties of the bounded linear operators $T$ on a Hilbert space which satisfy the condition $T{T}^{*}=U^{*}{T}^{*}TU$ where $U$ is unitary, are studied in relation to those of normal, hyponormal, quasinormal and subnormal operators.

We show that there are linear operators on Hilbert space that have n-dimensional subspaces with dense orbit, but no (n-1)-dimensional subspaces with dense orbit. This leads to a new class of operators, called the n-supercyclic operators. We show that many cohyponormal operators are n-supercyclic. Furthermore, we prove that for an n-supercyclic operator, there are n circles centered at the origin such that every component of the spectrum must intersect one of these circles.

We prove that for normal operators $N_1,N_2\in ℒ\left(\mathscr{H}\right),$ the generalized commutator $[N_1,N_2;X]$ approaches zero when $[N_1,N_2;[N_1,N_2;X]]$ tends to zero in the norm of the Schatten-von Neumann class ${𝒞}_p$ with $p>1$ and $X$ varies in a bounded set of such a class.

We show that every class A operator has a scalar extension. In particular, such operators with rich spectra have nontrivial invariant subspaces. Also we give some spectral properties of the scalar extension of a class A operator. Finally, we show that every class A operator is nonhypertransitive.

A review of known decompositions of pairs of isometries is given. A new, finer decomposition and its properties are presented.

The purpose of the paper is to introduce and study a new class of operators on semi-Hilbertian spaces, i.e. spaces generated by positive semi-definite sesquilinear forms. Let $\mathscr{H}$ be a Hilbert space and let $A$ be a positive bounded operator on $\mathscr{H}$. The semi-inner product ${\langle h\mid k\rangle }_A:=\langle Ah\mid k\rangle$, $h,k\in \mathscr{H}$, induces a semi-norm ${\parallel ·\parallel }_A$. This makes $\mathscr{H}$ into a semi-Hilbertian space. An operator $T\in {ℬ}_A\left(\mathscr{H}\right)$ is said to be $(n,m)$-$A$-normal if $[T^n,{\left(T^{{♯}_A}\right)}^m]:=T^n{\left(T^{{♯}_A}\right)}^m-{\left(T^{{♯}_A}\right)}^m{T}^n=0$ for some positive integers $n$ and $m$.

Let T be a bounded linear operator on a complex Hilbert space . For positive integers n and k, an operator T is called (n,k)-quasiparanormal if $\left|\right|T^{1+n}\left(T^{k}x\right){\left|\right|}^{1/(1+n)}\left|\right|T^k{x\left|\right|}^{n/(1+n)}\ge \left|\right|T\left(T^{k}x\right)\left|\right|$ for x ∈ . The class of (n,k)-quasiparanormal operators contains the classes of n-paranormal and k-quasiparanormal operators. We consider some properties of (n,k)-quasiparanormal operators: (1) inclusion relations and examples; (2) a matrix representation and SVEP (single valued extension property); (3) ascent and Bishop’s property (β); (4) quasinilpotent...