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Manifold modelled on Hilbert spaces
In mathematics, a Hilbert manifold is a manifold modeled on Hilbert spaces. Thus it is a separable Hausdorff space in which each point has a neighbourhood homeomorphic to an infinite dimensional Hilbert space. The concept of a Hilbert manifold provides a possibility of extending the theory of manifolds to infinite dimensional setting. Analogous to the finite dimensional situation, one can define a differentiable Hilbert manifold by considering a maximal atlas in which the transition maps are differentiable.

Many basic constructions of the manifold theory, such as the tangent space of a manifold and a tubular neighbourhood of a submanifold (of finite codimension) carry over from the finite dimensional situation to the Hilbert setting with little change. However, in statements involving maps between manifolds, one often has to restrict consideration to Fredholm maps, that is, maps whose differential at every point is Fredholm. The reason for this is that Sard's lemma holds for Fredholm maps, but not in general. Notwithstanding this difference, Hilbert manifolds have several very nice properties. Kuiper's theorem: If is a compact topological space or has the homotopy type of a CW complex then every (real or complex) Hilbert space bundle over is trivial. In particular, every Hilbert manifold is parallelizable. Every smooth Hilbert manifold can be smoothly embedded onto an open subset of the model Hilbert space. Every homotopy equivalence between two Hilbert manifolds is homotopic to a diffeomorphism. In particular every two homotopy equivalent Hilbert manifolds are already diffeomorphic. This stands in contrast to lens spaces and exotic spheres, which demonstrate that in the finite dimensional situation, homotopy equivalence, homeomorphism, and diffeomorphism of manifolds are distinct properties. Although Sard's Theorem does not hold in general, every continuous map from a Hilbert manifold can be arbitrary closely approximated by a smooth map which has no critical points.