The compatible Grassmannian

Let A   be a positive injective operator in a Hilbert space (H,〈⋅,⋅〉), and denote by [⋅,⋅] the inner product defined by A  : [f,g]=〈Af,g〉[f,g]=〈Af,g〉. A closed subspace S⊂HS⊂H is called A  -compatible if there exists a closed complement for SS, which is orthogonal to SS with respect to the inner product [⋅,⋅]. Equivalently, if there exists a necessarily unique bounded idempotent operator QSQS such that R(QS)=SR(QS)=S, which is symmetric for this inner product. The compatible Grassmannian GrAGrA is the set of all A  -compatible subspaces of HH. By parametrizing it via the one to one correspondence S↔QSS↔QS, this set is shown to be a differentiable submanifold of the Banach space of all bounded operators in HH which are symmetric with respect to the form [⋅,⋅]. A Banach–Lie group acts naturally on the compatible Grassmannian, the group of all invertible operators in HH which preserve the form [⋅,⋅]. Each connected component in GrAGrA of a compatible subspace SS of finite dimension, turns out to be a symplectic leaf in a Banach Lie–Poisson space. For 1⩽p⩽∞1⩽p⩽∞, in the presence of a fixed [⋅,⋅]-orthogonal (direct sum) decomposition of HH, H=S0+N0H=S0+N0, we study the restricted compatible Grassmannian (an analogue of the restricted, or Sato Grassmannian). This restricted compatible Grassmannian is shown to be a submanifold of the Banach space of p  -Schatten operators which are symmetric for the form [⋅,⋅]. It carries the locally transitive action of the Banach–Lie group of invertible operators which preserve [⋅,⋅], and are of the form G=1+KG=1+K, with K in the p-Schatten class. The connected components of this restricted Grassmannian are characterized by means of the Fredholm index of pairs of projections. Finsler metrics which are isometric for the group actions are introduced for both compatible Grassmannians, and minimality results for curves are proved.