Invariant subspaces of under the action of biconjugates
We study conditions on an infinite dimensional separable Banach space implying that is the only non-trivial invariant subspace of under the action of the algebra of biconjugates of bounded operators on : . Such a space is called simple. We characterize simple spaces among spaces which contain an isomorphic copy of , and show in particular that any space which does not contain and has property (u) of Pelczynski is simple.