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APPLICATIONS Contact geometry has — as does symplectic geometry — broad applications in Physics , e.g. Geometrical Optics , Classical Mechanics , Thermodynamics , Geometric Quantization , and applied mathematics such as Control Theory . One can prove amusing things, like 'You can always Parallel-park your car, provided the space is big enough'. Contact geometry has many applications to Low-dimensional Topology ; one such connection is the fact that every Three-manifold has a contact structure. CONTACT FORMS AND STRUCTURES A ''contact form'' α on a 2n+1 dimensional manifold M is a (local) 1-form with the property that : A ''contact structure'' ξ on a manifold is the kernel of a contact form α, i.e. a completely nonintegrable hyperplane field. Roughly this means that you cannot find a piece of a hypersurface tangent to ξ on an open set. It also follows from this definition that dα, when restricted to ξ, is nondegenerate. This means that ξ is a Symplectic Bundle on the manifold. Since symplectic spaces are even-dimensional, contact manifolds need to be odd dimensional. As a prime example, consider on R3, endowed with coordinates :(x, y, z), the 1-form :dz -ydx. The contact plane ξ at a point :(x,y,z) is spanned by vectors :X1 = ∂y and :X2 = ∂x+y∂z. (Draw a picture of this!). Actually one can generalize this example to any R2n+1. By a Theorem Of Darboux , every contact structure on a manifold looks locally like this.
:α=''dt''+λ. The last example showed how to obtain contact manifolds from symplectic ones. Vice versa one gets a symplectic manifold out of a contact manifold by crossing with R: If α is a contact form for a manifold M, then :ω=d(etα) is a symplectic form on M×R, where t denotes the variable in the R-direction. LEGENDRIAN SUBMANIFOLDS AND KNOTS The most interesting subspaces of a contact manifold are its Legendrian Submanifold s. The non-integrability of the contact hyperplane field on a (2n+1)-dimensional manifold means that no 2n-dimensional submanifold has it as its tangent bundle, not even locally. However, it is in general possible to find n-dimensional (embedded or immersed) submanifolds whose tangent spaces lie inside the contact field. Legendrian submanifolds are analogous to Lagrangian submanifolds of symplectic manifolds. There is a precise relation: the lift of a Legendrian submanifold in a symplectization of a contact manifold is a Lagrangian submanifold. The simplest example of Legendrian submanifolds are Legendrian knots inside a contact three-manifold. Inequivalent Legendrian knots may be equivalent as smooth knots. Legendrian submanifolds are very rigid objects; in some situations, being Legendrian forces submanifolds to be unknotted. Symplectic Field Theory provides invariants of Legendrian submanifolds called Relative Contact Homology that can sometimes distinguish distinct Legendrian submanifolds that are topologically identical. REEB VECTOR FIELD If α is a contact form for a given contact structure, the Reeb vector field R can be defined as the unique element of the kernel of dα such that α(R)=1. Its dynamics can be used to study the structure of the contact manifold or even the underlying manifold using techniques of Floer Homology such as Symplectic Field Theory and Embedded Contact Homology . SOME HISTORICAL REMARKS The roots of contact geometry appear in work of . The theory of contact transformations (i.e. transformations preserving a contact structure) was developed by Sophus Lie , with the dual aims of studying differential equations (e.g. the Legendre Transformation ) and describing the 'change of space element', familiar from Projective Duality . REFERENCES Introductions to contact geometry:
Contact three-manifolds and Legendrian knots:
Information on the history of contact geometry:
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