A contact form on a smooth -dimensional manifold is a -form such that is everywhere non-zero. The pair is called a contact manifold. See also Contact structure.
A contact manifold carries a distinguished vector field , called the characteristic vector field or Reeb field, which is uniquely determined by the following equations: and for all vector fields . The flow generated by (when it is complete) is called the contact flow. Sometimes the name "contact flow" is used for the -dimensional foliation consisting of the unparametrized orbits of , [a5].
If the flow is a Riemannian foliation in the sense of Reinhart–Molino [a7], i.e., if there is a holonomy-invariant transverse metric for , then is called a -contact flow, and the pair is called a -contact manifold. This definition is equivalent to requiring that the flow of is a -parameter group of isometries for some contact metric (a Riemannian metric such that there exists an endomorphism of the tangent bundle such that , , , and for all vector fields and on ). If one has in addition , where is the Levi-Civita connection of , then one says that is a Sasakian manifold, [a4], [a12].
As a consequence of the Meyer–Steenrod theorem [a6], a -contact flow on a compact -dimensional manifold is almost periodic: the closure of in the isometry group of (of the associated contact metric) is a torus , of dimension in between and , which acts on while preserving the contact form , [a3]. The "completely integrable" case has been studied in [a2]: these structures are determined by the image of their contact moment mapping.
The existence of -contact flows poses restrictions on the topology of the manifold. For instance, since a -contact flow can be approximated by a periodic -contact flow, only Seifert fibred compact manifolds can carry a -contact flow. Another example of a restriction is the Tachibana theorem, asserting that the first Betti number of a compact Sasakian manifold is either zero or even, [a9]. This shows that no torus can carry a Sasakian structure. Actually, P. Rukimbira [a8] showed that no torus can carry a -contact flow.
A. Weinstein [a11] has conjectured that the contact flow of a compact contact manifold has at least one periodic orbit. Despite important breakthroughs (including [a10]), this conjecture is not quite settled at present (1996). However, it is known that -contact flows on compact manifolds have at least two periodic orbits [a3].
Examples of -contact manifolds include the contact manifolds with a periodic contact flow (these include the regular contact manifolds), such as the sphere equipped with the contact form that is the restriction to of the -form
on . More generally, compact contact hypersurfaces (in the sense of M. Okumura) [a1] in Kähler manifolds of constant positive holomorphic sectional curvature carry -contact flows. A large set of examples is provided by the Brieskorn manifolds: In [a12] it is shown that every Brieskorn manifold admits many Sasakian structures, hence carries many -contact flows.
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K-contact-flow. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=K-contact-flow&oldid=24485