Left invertible completions of upper triangular operator matrices with unbounded entries

Abstract

Given two closed, in general unbounded, operators A and C, we investigate the left invertible completion of the partial operator matrix $$\left( {\begin{array}{*{20}{c}} A&? \\ 0&C \end{array}} \right)$$ . Based on the space decomposition technique, the alternative sufficient and necessary conditions are given according to whether the dimension of $$\mathcal{R}(A)^ \bot$$ is finite or infinite. As a direct consequence, the perturbation of left spectra is further presented.