It is known (Lindenstrauss, Tzafriri, On the complemented subspaces problem) that a real Banach space all of whose closed subspaces are complemented (i.e. have a closed supplement) is isomorphic (as a tvs) to a Hilbert space. But I am interested in complementing a special kind of subspaces: subspaces F of a Banach space E satisfying either of the following two equivalent conditions:
(i) $E/F \simeq F$
(ii) $\exists f \in \mathcal{L}(E) \quad F = \ker f = im f$
Are such "half-dimensional" subspaces always complemented? Is there a class of spaces (larger than that of Hilbert spaces) where this is true?
Actually, (i)=>(ii) is true for any tvs, and (ii)=>(i) is true for complete metrizable spaces (over nondiscrete valued fields). Under that same condition, two closed algebraic supplements are topological supplements (from the open mapping theorem). So the question is still interesting in that setting. By the way, is the Lindenstrauss--Tzafriri theorem true for such spaces (or any class larger than that of real Banach spaces --- or real Fréchet spaces, as the mathscinet review says)?
