In mathematics, the **Thurston boundary** of Teichmüller space of a surface is obtained as the boundary of its closure in the projective space of functionals on simple closed curves on the surface. The Thurston boundary can be interpreted as the space of projective measured foliations on the surface.

- Measured foliations on surfaces
- Compactification of Teichmüller space
- Embedding in the space of functionals
- The Thurston boundary
- Applications
- Pseudo-Anosov diffeomorphisms
- Applications to the mapping class group
- Applications to 3–manifolds
- Actions on real trees
- Notes
- References

The Thurston boundary of the Teichmüller space of a closed surface of genus is homeomorphic to a sphere of dimension . The action of the mapping class group on the Teichmüller space extends continuously over the union with the boundary.

Let be a closed surface. A *measured foliation* on is a foliation on which may admit isolated singularities, together with a *transverse measure*, i.e. a function which to each arc transverse to the foliation associates a positive real number . The foliation and the measure must be compatible in the sense that the measure is invariant if the arc is deformed with endpoints staying in the same leaf.^{ [1] }

Let be the space of isotopy classes of closed simple curves on . A measured foliation can be used to define a function as follows: if is any curve let

where the supremum is taken over all collections of disjoint arcs which are transverse to (in particular if is a closed leaf of ). Then if the intersection number is defined by:

- .

Two measured foliations are said to be *equivalent* if they define the same function on (there is a topological criterion for this equivalence via *Whitehead moves*). The space of *projective measured laminations* is the image of the set of measured laminations in the projective space via the embedding . If the genus of is at least 2, the space is homeomorphic to the -dimensional sphere (in the case of the torus it is the 2-sphere; there are no measured foliations on the sphere).

Let be a closed surface. Recall that a point in the Teichmüller space is a pair where is a hyperbolic surface (a Riemannian manifold with sectional curvatures all equal to ) and a homeomorphism, up to a natural equivalence relation. The Teichmüller space can be realised as a space of functionals on the set of isotopy classes of simple closed curves on as follows. If and then is defined to be the length of the unique closed geodesic on in the isotopy class . The map is an embedding of into , which can be used to give the Teichmüller space a topology (the right-hand side being given the product topology).

In fact, the map to the projective space is still an embedding: let denote the image of there. Since this space is compact, the closure is compact: it is called the *Thurston compactification* of the Teichmüller space.

The boundary is equal to the subset of . The proof also implies that the Thurston compactfification is homeomorphic to the -dimensional closed ball.^{ [2] }

A diffeomorphism is called pseudo-Anosov if there exists two transverse measured foliations, such that under its action the underlying foliations are preserved, and the measures are multiplied by a factor respectively for some (called the stretch factor). Using his compactification Thurston proved the following characterisation of pseudo-Anosov mapping classes (i.e. mapping classes which contain a pseudo-Anosov element), which was in essence known to Nielsen and is usually called the Nielsen-Thurston classification. A mapping class is pseudo-Anosov if and only if:

- it is not reducible (i.e. there is no and such that );
- it is not of finite order (i.e. there is no such that is the isotopy class of the identity).

The proof relies on the Brouwer fixed point theorem applied to the action of on the Thurston compactification . If the fixed point is in the interior then the class is of finite order; if it is on the boundary and the underlying foliation has a closed leaf then it is reducible; in the remaining case it is possible to show that there is another fixed point corresponding to a transverse measured foliation, and to deduce the pseudo-Anosov property.

The action of the mapping class group of the surface on the Teichmüller space extends continuously to the Thurston compactification. This provides a powerful tool to study the structure of this group; for example it is used in the proof of the Tits alternative for the mapping class group. It can also be used to prove various results about the subgroup structure of the mapping class group.^{ [3] }

The compactification of Teichmüller space by adding measured foliations is essential in the definition of the ending laminations of a hyperbolic 3-manifold.

A point in Teichmüller space can alternatively be seen as a faithful representation of the fundamental group into the isometry group of the hyperbolic plane , up to conjugation. Such an isometric action gives rise (via the choice of a principal ultrafilter) to an action on the asymptotic cone of , which is a real tree. Two such actions are equivariantly isometric if and only if they come from the same point in Teichmüller space. The space of such actions (endowed with a natural topology) is compact, and hence we get another compactification of Teichmüller space. A theorem of R. Skora states that this compactification is equivariantly homeomorphic the Thurston compactification.^{ [4] }

- ↑ Fathi, Laudenbach & Poénaru 2012, Exposé 5.
- ↑ Fathi, Laudenbach & Poénaru 2012, Exposé 8.
- ↑ Ivanov 1992.
- ↑ Bestvina, Mladen. "-trees in topology, geometry and group theory".
*Handbook of geometric topology*. North-Holland. pp. 55–91.

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- Fathi, Albert; Laudenbach, François; Poénaru, Valentin (2012).
*Thurston's work on surfaces Translated from the 1979 French original by Djun M. Kim and Dan Margalit*. Mathematical Notes. Vol. 48. Princeton University Press. pp. xvi+254. ISBN 978-0-691-14735-2. - Ivanov, Nikolai (1992).
*Subgroups of Teichmüller Modular Groups*. American Math. Soc.

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