In mathematics, more particularly in the fields of dynamical systems and geometric topology, an Anosov map on a manifold M is a certain type of mapping, from M to itself, with rather clearly marked local directions of "expansion" and "contraction". Anosov systems are a special case of Axiom A systems.
Anosov diffeomorphisms were introduced by Dmitri Victorovich Anosov, who proved that their behaviour was in an appropriate sense generic (when they exist at all).^{[1]}
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Transcription
Contents
Overview
Three closely related definitions must be distinguished:
 If a differentiable map f on M has a hyperbolic structure on the tangent bundle, then it is called an Anosov map. Examples include the Bernoulli map, and Arnold's cat map.
 If the map is a diffeomorphism, then it is called an Anosov diffeomorphism.
 If a flow on a manifold splits the tangent bundle into three invariant subbundles, with one subbundle that is exponentially contracting, and one that is exponentially expanding, and a third, nonexpanding, noncontracting onedimensional subbundle (spanned by the flow direction), then the flow is called an Anosov flow.
A classical example of Anosov diffeomorphism is the Arnold's cat map.
Anosov proved that Anosov diffeomorphisms are structurally stable and form an open subset of mappings (flows) with the C^{1} topology.
Not every manifold admits an Anosov diffeomorphism; for example, there are no such diffeomorphisms on the sphere . The simplest examples of compact manifolds admitting them are the tori: they admit the socalled linear Anosov diffeomorphisms, which are isomorphisms having no eigenvalue of modulus 1. It was proved that any other Anosov diffeomorphism on a torus is topologically conjugate to one of this kind.
The problem of classifying manifolds that admit Anosov diffeomorphisms turned out to be very difficult, and still as of 2012^{[update]} has no answer. The only known examples are infranil manifolds, and it is conjectured that they are the only ones.
A sufficient condition for transitivity is that all points are nonwandering: .
Also, it is unknown if every volumepreserving Anosov diffeomorphism is ergodic. Anosov proved it under a assumption. It is also true for volumepreserving Anosov diffeomorphisms.
For transitive Anosov diffeomorphism there exists a unique SRB measure (the acronym stands for Sinai, Ruelle and Bowen) supported on such that its basin is of full volume, where
Anosov flow on (tangent bundles of) Riemann surfaces
As an example, this section develops the case of the Anosov flow on the tangent bundle of a Riemann surface of negative curvature. This flow can be understood in terms of the flow on the tangent bundle of the Poincaré halfplane model of hyperbolic geometry. Riemann surfaces of negative curvature may be defined as Fuchsian models, that is, as the quotients of the upper halfplane and a Fuchsian group. For the following, let H be the upper halfplane; let Γ be a Fuchsian group; let M = H/Γ be a Riemann surface of negative curvature as the quotient of "M" by the action of the group Γ, and let be the tangent bundle of unitlength vectors on the manifold M, and let be the tangent bundle of unitlength vectors on H. Note that a bundle of unitlength vectors on a surface is the principal bundle of a complex line bundle.
Lie vector fields
One starts by noting that is isomorphic to the Lie group PSL(2,R). This group is the group of orientationpreserving isometries of the upper halfplane. The Lie algebra of PSL(2,R) is sl(2,R), and is represented by the matrices
which have the algebra
The exponential maps
define rightinvariant flows on the manifold of , and likewise on . Defining and , these flows define vector fields on P and Q, whose vectors lie in TP and TQ. These are just the standard, ordinary Lie vector fields on the manifold of a Lie group, and the presentation above is a standard exposition of a Lie vector field.
Anosov flow
The connection to the Anosov flow comes from the realization that is the geodesic flow on P and Q. Lie vector fields being (by definition) left invariant under the action of a group element, one has that these fields are left invariant under the specific elements of the geodesic flow. In other words, the spaces TP and TQ are split into three onedimensional spaces, or subbundles, each of which are invariant under the geodesic flow. The final step is to notice that vector fields in one subbundle expand (and expand exponentially), those in another are unchanged, and those in a third shrink (and do so exponentially).
More precisely, the tangent bundle TQ may be written as the direct sum
or, at a point , the direct sum
corresponding to the Lie algebra generators Y, J and X, respectively, carried, by the left action of group element g, from the origin e to the point q. That is, one has and . These spaces are each subbundles, and are preserved (are invariant) under the action of the geodesic flow; that is, under the action of group elements .
To compare the lengths of vectors in at different points q, one needs a metric. Any inner product at extends to a leftinvariant Riemannian metric on P, and thus to a Riemannian metric on Q. The length of a vector expands exponentially as exp(t) under the action of . The length of a vector shrinks exponentially as exp(t) under the action of . Vectors in are unchanged. This may be seen by examining how the group elements commute. The geodesic flow is invariant,
but the other two shrink and expand:
and
where we recall that a tangent vector in is given by the derivative, with respect to t, of the curve , the setting .
Geometric interpretation of the Anosov flow
When acting on the point of the upper halfplane, corresponds to a geodesic on the upper half plane, passing through the point . The action is the standard Möbius transformation action of SL(2,R) on the upper halfplane, so that
A general geodesic is given by
with a, b, c and d real, with . The curves and are called horocycles. Horocycles correspond to the motion of the normal vectors of a horosphere on the upper halfplane.
See also
Notes
 ^ Dmitri V. Anosov, Geodesic flows on closed Riemannian manifolds with negative curvature, (1967) Proc. Steklov Inst. Mathematics. 90.
References
 Hazewinkel, Michiel, ed. (2001) [1994], "Ysystem,Usystem, Csystem", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 9781556080104
 Anthony Manning, Dynamics of geodesic and horocycle flows on surfaces of constant negative curvature, (1991), appearing as Chapter 3 in Ergodic Theory, Symbolic Dynamics and Hyperbolic Spaces, Tim Bedford, Michael Keane and Caroline Series, Eds. Oxford University Press, Oxford (1991). ISBN 019853390X (Provides an expository introduction to the Anosov flow on SL(2,R).)
 This article incorporates material from Anosov diffeomorphism on PlanetMath, which is licensed under the Creative Commons Attribution/ShareAlike License.
 Toshikazu Sunada, Magnetic flows on a Riemann surface, Proc. KAIST Math. Workshop (1993), 93–108.