The characterizations of the stable perturbation of a closed operator by   a linear operator in Banach spaces
  In this paper, we investigate the invertibility of $I_Y+\delta TT^+$ when $T$ is a closed operator from $X$ to $Y$ with a generalized inverse $T^+$ and $\delta T$ is a linear operator whose domain contains $D(T)$ and range is contained in $D(T^+)$. The characterizations of the stable perturbation $T+\delta T$ of $T$ by $\delta T$ in Banach spaces are obtained. The results extend the recent main results of Huang's in Linear Algebra and its Applications. 
Fapeng Du and Yifeng Xue
math.NA math.FA