On the sum between a closable operator $T$ and a $T$-bounded operator (1409.5711v1)

Abstract: We provide several perturbation theorems regarding closable operators on a real or complex Hilbert space. In particular we extend some classical results due to Hess--Kato, Kato--Rellich and W\"ust. Our approach involves ranges of matrix operators of the form $\left(\begin{array}{cc}!! I & A!!\ !! -B& I!!\end{array}\right)$.