An extension and refinement of the theorems of Douglas and Sebesty\'en for unbounded operators

Abstract

For a closed densely defined operator $T$ from a Hilbert space $\mathfrak{H}$ to a Hilbert space $\mathfrak{K}$, necessary and sufficient conditions are established for the factorization of $T$ with a bounded nonnegative operator $X$ on $\mathfrak{K}$. This result yields a new extension and a refinement of a well-known theorem of R.G. Douglas, which shows that the operator inequality $A^*A\leq \lambda^2 B^*B, \lambda \geq 0$, is equivalent to the factorization $A=CB$ with $\|C\|\leq \lambda$. The main results give necessary and sufficient conditions for the existence of an intermediate selfadjoint operator $H\geq 0$, such that $A^*A \leq \lambda H \leq \lambda^2 B^*B$. The key results are proved by first extending a theorem of Z. Sebesty\'en to the setting of unbounded operators.