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# Cotangent bundle

In differential geometry, the cotangent bundle of a manifold is the vector bundle of all the cotangent spaces at every point in the manifold.

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## One-forms

Smooth sections of the cotangent bundle are differential one-forms.

## The cotangent bundle as phase space

### Symplectic form

The cotangent bundle has a canonical symplectic 2-form on it, as an exterior derivative of a one-form. The one-form assigns to a vector in the tangent bundle of the cotangent bundle the application of the element in the cotangent bundle (a linear functional) to the projection of the vector into the tangent bundle (the differential of the projection of the cotangent bundle to the original manifold). Proving this form is, indeed, symplectic can be done by noting that being symplectic is a local property: since the cotangent bundle is locally trivial, this definition need only be checked on $\mathbb{R}^n \times \mathbb{R}^n$. But there the one form defined is the sum of yidxi, and the differential is the canonical symplectic form, the sum of $dy_i{\and}dx_i$.

### Phase space

If the manifold M represents the set of possible positions in a dynamical system, then the cotangent bundle $\!\,T^{*}\!M$ can be thought of as the set of possible positions and momentums. For example, this is an easy way to describe the (non-trivial) phase space of a three-dimensional spherical pendulum: a weighted ball constrained to move along a 2-sphere. The above symplectic construction, along with an appropriate energy function, gives a complete determination of the physics of system. See Hamiltonian mechanics for more information, and the article on geodesic flow for an explicit construction of the Hamiltonian equations of motion.