Product of nonnegative selfadjoint operators in unbounded settings

In this paper, necessary and sufficient conditions are established for the factorization of a closed, in general, unbounded operator $T=AB$ into a product of two nonnegative selfadjoint operators $A$ and $B.$ Already the special case, where $A$ or $B$ is bounded, leads to new results and is of wider interest, since the problem is connected to the notion of similarity of the operator $T$ to a selfadjoint one, but, in fact, goes beyond this case. It is proved that this subclass of operators can be characterized not only by means of quasi-affinity of $T^*$ to an operator $S=S^* \geq 0$, but also via Sebestyén inequality, a result known in the setting of bounded operators $T.$ Another subclass of operators $T,$ where $A$ or $B$ has a bounded inverse, leads to a similar analysis. This gives rise to a reversed version of Sebestyén inequality which is introduced in the present paper. It is shown that this second subclass, where $A^{-1}$ or $B^{-1}$ is bounded, can be characterized in a similar way by means of quasi-affinity of $T,$ rather that $T^*,$ to an operator $S=S^*\geq 0$. Furthermore, the connection between these two classes and weak-similarity as well as quasi-similarity to some $S=S^*\geq 0$ is investigated. Finally, the special case where $ S$ is bounded is considered.