A. Azzouz

The upper semi-Fredholm radius, extensions of restrictions, and subprojectivity

For a bounded operator $T$ between Banach spaces, let $d_+(T)$ be its distance to the operators that are not upper semi-Fredholm, and let $\inn(T)$ be Schechter's quantity $\inf\|T|_M\|$ over infinite-dimensional subspaces $M$. One always has $\inn(T)\le d_+(T)$, and Gonz\'alez and Martin\'on showed that the two quanti...