A note on linear approximately orthogonality preserving mappings
  In this paper, linear $\varepsilon$-orthogonality preserving mappings are studied. We define $\hat{\varepsilon}\left(T\right) $ as the smallest $\varepsilon$ for which $T$ is $\varepsilon$-orthogonality preserving, and then derive an exact formula for $\hat{\varepsilon}\left( T\right) $ in terms of $\left \Vert T\right \Vert $ and the minimum modulus $m\left( T\right) $ of $T$. We see that $\varepsilon$-orthogonality preserving mappings (for some $\varepsilon<1$) are exactly the operators that are bounded from below. We improve an upper bounded in the stability equation given in [7, Theorem 2.3], which was thought to be sharp. 
Ye Zhang, Yanni Chen, Don Hadwin and Liang Kong
math.FA math.OA