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Invertible completions of upper triangular operator matrices
Authors:Jin Kyu Han   Hong Youl Lee   Woo Young Lee
Affiliation:Department of Mathematics Education, Mokwon University, Daejon 301-719, Korea

Hong Youl Lee ; Department of Mathematics, Woosuk University, Wanju-gun, Cheonbuk 565-800, Korea

Woo Young Lee ; Department of Mathematics, Sung Kyun Kwan University, Suwon 440-746, Korea

Abstract:In this note we prove that if

begin{equation*}M_{C}=left (begin{smallmatrix}A&C 0&Bend{smallmatrix} right) end{equation*}

is a $2times 2$ upper triangular operator matrix acting on the Banach space $Xoplus Y$, then $M_{C}$ is invertible for some $Cin mathcal{L}(Y,X)$ if and only if $Ain mathcal{L}(X)$ and $Bin mathcal{L}(Y)$ satisfy the following conditions:

(i)
$A$ is left invertible;
(ii)
$B$ is right invertible;
(iii)
$X/A(X)cong B^{-1}(0)$.
Furthermore we show that $sigma (A)cup sigma (B)=sigma (M_{C})cup W$, where $W$ is the union of certain of the holes in $sigma (M_{C})$ which happen to be subsets of $sigma (A)cap sigma (B)$.

Keywords:Spectrum   regular   $2times 2$ upper triangular operator matrices
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