Vector bundle

In mathematics, a vector bundle is a geometrical construct where to every point of a topological space (or manifold, or algebraic variety) we attach a vector space in a compatible way, so that all those vectorspaces, "glued together", form another topological space (or manifold or variety). A typical example is the tangent bundle of a differentiable manifold: to every point of the manifold we attach the tangent space of the manifold at that point. Or consider a smooth curve in R², and attach to every point of the curve the line normal to the curve at that point; this yields the "normal bundle" of the curve.

This article deals mostly with real vector bundles, with finite-dimensional fibers. Complex vector bundles are important in many cases, too; they are a special case, meaning that they can be seen as extra structure on an underlying real bundle.

Table of contents

1 Definition and first consequences 2 Morphisms 3 Sections and locally free sheaves 4 Operations on vector bundles 5 Variants and generalizations

Definition and first consequences

A real vector bundle is given by the following data:

• topological spaces X (the "base space") and E (the "total space")
• a continuous map π : E → X (the "projection")
• for every x in X, the structure of a real vector space on the fiber π⁻¹({x})

satisfying the following compatibility condition: for every point in X there is an open neighborhood U, a natural number n, and a homeomorphism φ : U × 'R\'ⁿ → π⁻¹(U) such that for every point x in U:

• πφ(x,v) = x for all vectors v in Rⁿ
• the map v |-> φ(x,v) yields an isomorphism between the vector spaces Rⁿ and π⁻¹({x}).

The open neighborhood U together with the homeomorphism φ is called a local trivialization of the bundle. The local trivialization shows that "locally" the map π looks like the projection of U × Rⁿ on U.

A vector bundle is called tivial if there is a "global trivialization", i.e. if it looks like the projection X × Rⁿ → X.

Every vector bundle π : E → X is surjective, since vector spaces cannot be empty.

Every fiber π⁻¹({x}) is a finite-dimensional real vector space and hence has a dimension d_x. The function x |-> d_x is locally constant, i.e. it is constant on all connected components of X. If it is constant globally on X, we call this dimension the rank of the vector bundle. Vector bundles of rank 1 are called line bundles.

Morphisms

A morphism from the vector bundle π₁ : E₁ → X₁ to the vector bundle π₁ : E₂ → X₂ is given by a pair of continuous maps f : E₁ → E₂ and g : X₁ → X₂ such that

• gπ₁ = π₂f
• for every x in X₁, the map π₁⁻¹({x}) → π₂⁻¹({g(x)}) induced by f is a linear transformation between vector spaces.

The composition of two morphisms is again a morphism, and we obtain the category of vector bundles.

We can also consider the category of all vector bundles on a fixed base space X. As morphisms in this category we take those morphisms of vector bundles whose map on the base space is the identity map on X. (Note that this category is not abelian; the kernel of a morphism of vector bundles is in general not a vector bundle in any natural way.)

Sections and locally free sheaves

Given a vector bundle π : E → X and an open subset U of X, we can consider sections of π on U, i.e. continuous functions s : U → E with πs = id_U. Essentially, a section assigns to every point of U a vector from the attached vector space, in a continuous manner. As an example, sections of the tangent bundle of a differential manifold are nothing but vector fields on that manifold.

Let F(U) be the set of all sections on U. F(U) always contains at least one element: the function s that maps every elememnt x of U to the zero element of the vector space π⁻¹({x}). With the pointwise addition and scalar multiplication of sections, F(U) becomes itself a real vector space. The collection of these vectorspace is a sheaf of vector spaces on X.

If s is an element of F(U) and α : U → R is a continuous map, then αs is in F(U). We see that F(U) is a module over the ring of continuous real-valued functions on U. Furthermore, if O_X denotes the structure sheaf of continuous real-valued functions on X, then F becomes a sheaf of O_X-modules.

Not every sheaf of O_X-modules arises in this fashion from a vector bundle: only the locally free ones do. (The reason: locally we are looking for sections of a projection U × Rⁿ → U; these are precisely the continuous functions U → Rⁿ, and such a function is an n-tuple of continuous functions U → R.)

Even more: the category of real vector bundles on X is equivalent to the category of locally free and finitely generated sheaves of O_X-modules. So we can think of the vector bundles as sitting inside the category of sheaves of O_X-modules; this latter category is abelian, so this is where we can compute kernels of morphisms of vector bundles.

Operations on vector bundles

Mention

• Direct sums
• Tensor products
• Duals

Variants and generalizations

Vector bundles are special fiber bundles, loosely speaking those where the fibers are vector spaces.

Smooth vector bundles are defined by requiring that E and X be smooth manifolds, π : E → X be a smooth map, and the local trivialization maps φ be diffeomorphisms.

Replacing real vector spaces with complex ones, we obtain complex vector bundles. This is a special case of reduction of the structure group of a bundle. Vector spaces over other topological fields may also be used, but that is comparatively rare.

If we allow arbitrary Banach spaces in the local trivialization (rather than only Rⁿ), we obtain Banach bundles.