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In mathematics, the **Thurston norm** is a function on the second homology group of an oriented 3-manifold introduced by William Thurston, which measures in a natural way the topological complexity of homology classes represented by surfaces.

In mathematics, a **3-manifold** is a space that locally looks like Euclidean 3-dimensional space. A 3-manifold can be thought of as a possible shape of the universe. Just as a sphere looks like a plane to a small enough observer, all 3-manifolds look like our universe does to a small enough observer. This is made more precise in the definition below.

**William Paul Thurston** was an American mathematician. He was a pioneer in the field of low-dimensional topology. In 1982, he was awarded the Fields Medal for his contributions to the study of 3-manifolds. From 2003 until his death he was a professor of mathematics and computer science at Cornell University.

Let be a differential manifold and . Then can be represented by a smooth embedding where is a (generally not connected) surface which is compact and without boundary. The Thurston norm of is then defined to be^{ [1] }

In mathematics, an **embedding** is one instance of some mathematical structure contained within another instance, such as a group that is a subgroup.

In topology, a **surface** is a two-dimensional manifold. Some surfaces arise as the boundaries of three-dimensional solids; for example, the sphere is the boundary of the solid ball. Other surfaces arise as graphs of functions of two variables; see the figure at right. However, surfaces can also be defined abstractly, without reference to any ambient space. For example, the Klein bottle is a surface that cannot be embedded in three-dimensional Euclidean space.

where the minimum is taken over all embedded surfaces (the being the connected components) representing as above and is the absolute value of the Euler characteristic for surfaces which are not spheres (and 0 for spheres).

In mathematics, and more specifically in algebraic topology and polyhedral combinatorics, the **Euler characteristic** is a topological invariant, a number that describes a topological space's shape or structure regardless of the way it is bent. It is commonly denoted by .

This function satisfies the following properties:

- for ;
- for .

These properties imply that extends to a function on which can then be extended by continuity to a seminorm on .^{ [2] } By Poincaré duality one can define the Thurston norm on .

In linear algebra, functional analysis, and related areas of mathematics, a **norm** is a function that assigns a strictly positive *length* or *size* to each vector in a vector space—except for the zero vector, which is assigned a length of zero. A **seminorm**, on the other hand, is allowed to assign zero length to some non-zero vectors.

In mathematics, the **Poincaré duality** theorem, named after Henri Poincaré, is a basic result on the structure of the homology and cohomology groups of manifolds. It states that if *M* is an *n*-dimensional oriented closed manifold, then the *k*th cohomology group of *M* is isomorphic to the (*n* − *k*)th homology group of *M*, for all integers *k*

When is compact with boundary the Thurston norm is defined in a similar manner on the relative homology group and its Poincaré dual .

In algebraic topology, a branch of mathematics, the **(singular) homology** of a topological space **relative to** a subspace is a construction in singular homology, for pairs of spaces. The relative homology is useful and important in several ways. Intuitively, it helps determine what part of an absolute homology group comes from which subspace.

It follows from further work of Gabai ^{ [3] } that one can also define the Thurston norm using only immersed surfaces. This implies that the Thurston norm is also equal to half the Gromov norm on homology.

**David Gabai**, a mathematician, is the Hughes-Rogers Professor of Mathematics at Princeton University. Focused on low-dimensional topology and hyperbolic geometry, he is a leading researcher in those subjects.

In mathematics, an **immersion** is a differentiable function between differentiable manifolds whose derivative is everywhere injective. Explicitly, *f* : *M* → *N* is an immersion if

In mathematics, the **Gromov norm** of a compact oriented *n*-manifold is a norm on the homology given by minimizing the sum of the absolute values of the coefficients over all singular chains representing a cycle. The Gromov norm of the manifold is the Gromov norm of the fundamental class.

The Thurston norm was introduced in view of its applications to fiberings and foliations of 3-manifolds.

The unit ball of the Thurston norm of a 3-manifold is a polytope with integer vertices. It can be used to describe the structure of the set of fiberings of over the circle: if can be written as the mapping torus of a diffeomorphism of a surface then the embedding represents a class in a top-dimensional (or open) face of : moreover all other integer points on the same face are also fibers in such a fibration.^{ [4] }

Embedded surfaces which minimise the Thurston norm in their homology class are exactly the closed leaves of foliations of .^{ [3] }

- ↑ Thurston 1986.
- ↑ Thurston 1986, Theorem 1.
- 1 2 Gabai 1983.
- ↑ Thurston 1986, Theorem 5.

In mathematics, mathematical physics and the theory of stochastic processes, a **harmonic function** is a twice continuously differentiable function *f* : *U* → **R** where *U* is an open subset of **R**^{n} that satisfies Laplace's equation, i.e.

In mathematics, **homology** is a general way of associating a sequence of algebraic objects such as abelian groups or modules to other mathematical objects such as topological spaces. Homology groups were originally defined in algebraic topology. Similar constructions are available in a wide variety of other contexts, such as abstract algebra, groups, Lie algebras, Galois theory, and algebraic geometry.

In topology, two continuous functions from one topological space to another are called **homotopic** if one can be "continuously deformed" into the other, such a deformation being called a **homotopy** between the two functions. A notable use of homotopy is the definition of homotopy groups and cohomotopy groups, important invariants in algebraic topology.

In mathematics, a **foliation** is an equivalence relation on an *n*-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension *p*, modeled on the decomposition of the real coordinate space **R**^{n} into the cosets *x* + **R**^{p} of the standardly embedded subspace **R**^{p}. The equivalence classes are called the **leaves** of the foliation. If the manifold and/or the submanifolds are required to have a piecewise-linear, differentiable, or analytic structure then one defines piecewise-linear, differentiable, or analytic foliations, respectively. In the most important case of differentiable foliation of class *C ^{r}* it is usually understood that

In mathematics, **cobordism** is a fundamental equivalence relation on the class of compact manifolds of the same dimension, set up using the concept of the boundary of a manifold. Two manifolds of the same dimension are *cobordant* if their disjoint union is the *boundary* of a compact manifold one dimension higher.

In the mathematical field of geometric topology, a **Heegaard splitting** is a decomposition of a compact oriented 3-manifold that results from dividing it into two handlebodies.

The representation theory of groups is a part of mathematics which examines how groups act on given structures.

In mathematics, an incompressible surface is a surface properly embedded in a 3-manifold, which, in intuitive terms, is a "nontrivial" surface that has been simplified as much as possible by compressing disks. They are useful for decomposition of Haken manifolds and for normal surface theory.

In algebraic geometry, a **very ample line bundle** is one with enough global sections to set up an embedding of its base variety or manifold into projective space. An **ample line bundle** is one such that some positive power is very ample. **Globally generated sheaves** are those with enough sections to define a morphism to projective space.

In mathematics, a **taut foliation** is a codimension 1 foliation of a 3-manifold with the property that there is a single transverse circle intersecting every leaf. By transverse circle, is meant a closed loop that is always transverse to the tangent field of the foliation. Equivalently, by a result of Dennis Sullivan, a codimension 1 foliation is taut if there exists a Riemannian metric that makes each leaf a minimal surface.

In mathematics, the **Teichmüller space** of a (real) topological surface , is a space that parametrizes complex structures on up to the action of homeomorphisms that are isotopic to the identity homeomorphism. Each point in may be regarded as an isomorphism class of "marked" Riemann surfaces, where a "marking" is an isotopy class of homeomorphisms from to itself.

In mathematics, a **manifold** is a topological space that locally resembles Euclidean space near each point. More precisely, each point of an *n*-dimensional manifold has a neighbourhood that is homeomorphic to the Euclidean space of dimension *n*. In this more precise terminology, a manifold is referred to as an ** n-manifold**.

In mathematics, an algebraic variety *V* in projective space is a **complete intersection** if the ideal of V is generated by exactly *codim V* elements. That is, if *V* has dimension *m* and lies in projective space *P*^{n}, there should exist *n* − *m* homogeneous polynomials

In topology, **Borel−Moore homology** or **homology with closed support** is a homology theory for locally compact spaces, introduced by Borel and Moore (1960).

In mathematics, more precisely in group theory and hyperbolic geometry, **Arithmetic Kleinian groups** are a special class of Kleinian groups constructed using orders in quaternion algebras. They are particular instances of arithmetic groups. An **arithmetic hyperbolic three-manifold** is the quotient of hyperbolic space by an arithmetic Kleinian group. These manifolds include some particularly beautiful or remarkable examples.

In differential topology, a branch of mathematics, a **stratifold** is a generalization of a differentiable manifold where certain kinds of singularities are allowed. More specifically a stratifold is stratified into differentiable manifolds of (possibly) different dimensions. Stratifolds can be used to construct new homology theories. For example, they provide a new geometric model for ordinary homology. The concept of stratifolds was invented by Matthias Kreck. The basic idea is similar to that of a topologically stratified space, but adapted to differential topology.

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. It can be interpreted as the space of projective measured foliations on the surface.

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