An extension and refinement of the theorems of Douglas and Sebestyén 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 $A^{\ast }A\le {\lambda }^2{B}^{\ast }B,\lambda \ge 0$, is equivalent to the factorization $A=CB$ with $\Vert C\Vert \le \lambda$. The main results give necessary and sufficient conditions for the existence of an intermediate selfadjoint operator $H\ge 0$, such that $A^{\ast }A\le \lambda H\le {\lambda }^2{B}^{\ast }B$. The key results are proved by first extending a theorem of Z. Sebestyén to the setting of unbounded operators.

9 pages