Cotangent space

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In differential geometry, one can attach to every point p of a differentiable manifold a vector space called the cotangent space at p. Typically, the cotangent space is defined as the dual of the tangent space at p, although there are more direct definitions (see below). The elements of the cotangent space are called tangent covectors. All cotangent spaces have the same dimension, equal to the dimension of the manifold.

Note that since the tangent space and the cotangent space at a point are both real vector spaces of the same dimension, they are isomorphic to each other. However, it is important to point out that they are not naturally isomorphic. That is, given a tangent covector there is no canonical tangent vector associated with it. The situation changes with the introduction of a Riemannian metric or a symplectic form in which case the added structure gives rise to a natural isomorphism.

For this reason it is important to maintain the distinction between the tangent space and the cotangent space. Many definitions are more natural on one space than on the other.

All the cotangent spaces of a manifold can be "glued together" to form a new differentiable manifold of twice the dimension, the cotangent bundle of the manifold.

Contents

Formal definitions

Definition as linear functionals

Let M be a smooth manifold and let p be a point in M. Let TpM be the tangent space at p. Then the cotangent space at p is defined as the dual space of TpM:

Tp*M = (TpM)*

Concretely, elements of the cotangent space are linear functionals on TpM. That is, every element φ ∈ Tp*M is a linear map

φ : TpMR

The elements of Tp*M are called tangent covectors.

Alternate definition

In some cases, one might like to have a direct definition of the cotangent space without reference to the tangent space. Such a definition can be formulated in terms of equivalence classes of smooth functions on M.

Let M be a smooth manifold and let p be a point in M. Let Ip be the ideal of all functions in C(M) vanishing at p, and let Ip2 be the set of functions of the form fg for f,gIp. Then Ip and Ip2 are real vector spaces and the cotangent space is defined as the quotient space Tp*M = Ip / Ip2.

The differential of a function

Let M be a smooth manifold and let f ∈ C(M) be a smooth function. The differential of f at a point p is the map

dfp(Xp) = Xp(f)

where Xp is a tangent vector at p, thought of as a derivation. That is <math>X(f)=\mathcal{L}_Xf<math> is the Lie derivative of f in the direction X, and one has <math>df(X)=X(f)<math>. Equivalently, we can think of tangent vectors as tangents to curves, and write

dfp(γ′(0)) = (f o γ)′(0)

In either case, dfp is a linear map on TpM and hence it is a tangent covector at p.

We can then define the differential map d : C(M) → Tp*M at a point p as the map which sends f to dfp. Properties of the differential map include:

  1. d is a linear map: d(af + bg) = a df + b dg for constants a and b,
  2. d(fg)p = f(p)dg + g(p)df,

The differential map provides the link between the two alternate definitions of the cotangent bundle given above. Given a function fIp (a smooth function vanishing at p) we can form the linear functional dfp as above. Since the map d restricts to 0 on Ip2 (the reader should verify this), d descends to a map from Ip / Ip2 to the dual of the tangent space, (TpM)*. One can show that this map is an isomorphism, establishing the equivalence of the two definitions.

The pullback of a smooth map

Just as every differentiable map f : MN between manifolds induces a linear map (called the pushforward or derivative) between the tangent spaces

<math>f_{*}^{}\colon T_p M \to T_{f(p)} N<math>

every such map induces a linear map (called the pullback) between the cotangent spaces, only this time in the reverse direction:

<math>f^{*}\colon T_{f(p)}^{*} N \to T_{p}^{*} M<math>

The pullback is naturally defined as the dual (or transpose) of the pushforward. Unraveling the definition, this means the following:

<math>(f^{*}\theta)(X_p) = \theta(f_{*}^{}X_p)<math>

where θ ∈ Tf(p)*N and XpTpM. Note carefully where everything lives.

If we define tangent covectors in terms of equivalence classes of smooth maps vanishing at a point then the definition of the pullback is even more straightforward. Let g be a smooth function on N vanishing at f(p). Then the pullback of the covector determined by g (denoted dg) is given by

<math>f^{*}dg = d(g \circ f)<math>

That is, it is the equivalence class of functions on M vanishing at p determined by g o f.

Exterior powers

The kth exterior power of the cotangent space, denoted Λk(Tp*M), is another important object in differential geometry. Vectors in the kth exterior power are called differential k-forms. They can be thought of as alternating, multilinear maps on k tangent vectors. For this reason, tangent covectors are frequently called one-forms.

Reference

  • John M.Lee, Introduction to Smooth Manifolds, (2003) Springer Graduate Texts in Mathematics 218.
  • Jurgen Jost, Riemannian Geometry and Geometric Analysis, (2002) Springer-Verlag, Berlin ISBN 3-540-4267-2.
  • Ralph Abraham and Jerrold E. Marsden, Foundations of Mechanics, (1978) Benjamin-Cummings, London ISBN 0-8053-0102-X.
  • Charles W. Misner, Kip S. Thorne, John Archibald Wheeler, Gravitation, (1970) W.H. Freeman, New York; ISBN 0-7167-0344-0.
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