On quasinilpotent operators and the invariant subspace problem

We show that a bounded quasinilpotent operator $T$ acting on an infinite dimensional Banach space has an invariant subspace if and only if there exists a rank one operator $F$ and a scalar $alphainmathbb{C}$, $alpha eq 0$, $alpha eq 1$, such that $T+F$ and $T+alpha F$ are also quasinilpotent. We also prove that for any fixed rank-one operator $F$, almost all perturbations $T+alpha F$ have invariant subspaces of infinite dimension and codimension.