Dehn Twist
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In
geometric topology In mathematics, geometric topology is the study of manifolds and Map (mathematics)#Maps as functions, maps between them, particularly embeddings of one manifold into another. History Geometric topology as an area distinct from algebraic topo ...
, a branch of
mathematics Mathematics is a field of study that discovers and organizes methods, Mathematical theory, theories and theorems that are developed and Mathematical proof, proved for the needs of empirical sciences and mathematics itself. There are many ar ...
, a Dehn twist is a certain type of
self-homeomorphism In mathematics, particularly topology, the homeomorphism group of a topological space is the group (mathematics), group consisting of all homeomorphisms from the space to itself with function composition as the group binary operation, operation. The ...
of a
surface A surface, as the term is most generally used, is the outermost or uppermost layer of a physical object or space. It is the portion or region of the object that can first be perceived by an observer using the senses of sight and touch, and is ...
(two-dimensional
manifold In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a N ...
).


Definition

Suppose that ''c'' is a
simple closed curve In mathematics, a curve (also called a curved line in older texts) is an object similar to a line, but that does not have to be straight. Intuitively, a curve may be thought of as the trace left by a moving point. This is the definition that ...
in a closed,
orientable In mathematics, orientability is a property of some topological spaces such as real vector spaces, Euclidean spaces, surfaces, and more generally manifolds that allows a consistent definition of "clockwise" and "anticlockwise". A space is o ...
surface ''S''. Let ''A'' be a
tubular neighborhood In mathematics, a tubular neighborhood of a submanifold of a smooth manifold is an open set around it resembling the normal bundle. The idea behind a tubular neighborhood can be explained in a simple example. Consider a smooth curve in the p ...
of ''c''. Then ''A'' is an
annulus Annulus (or anulus) or annular indicates a ring- or donut-shaped area or structure. It may refer to: Human anatomy * ''Anulus fibrosus disci intervertebralis'', spinal structure * Annulus of Zinn, a.k.a. annular tendon or ''anulus tendineus comm ...
,
homeomorphic In mathematics and more specifically in topology, a homeomorphism ( from Greek roots meaning "similar shape", named by Henri Poincaré), also called topological isomorphism, or bicontinuous function, is a bijective and continuous function betw ...
to the
Cartesian product In mathematics, specifically set theory, the Cartesian product of two sets and , denoted , is the set of all ordered pairs where is an element of and is an element of . In terms of set-builder notation, that is A\times B = \. A table c ...
of a circle and a
unit interval In mathematics, the unit interval is the closed interval , that is, the set of all real numbers that are greater than or equal to 0 and less than or equal to 1. It is often denoted ' (capital letter ). In addition to its role in real analysi ...
''I'': :c \subset A \cong S^1 \times I. Give ''A'' coordinates (''s'', ''t'') where ''s'' is a
complex number In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
of the form e^ with \theta \in , 2\pi and . Let ''f'' be the map from ''S'' to itself which is the identity outside of ''A'' and inside ''A'' we have :f(s, t) = \left(se^, t\right). Then ''f'' is a Dehn twist about the curve ''c''. Dehn twists can also be defined on a non-orientable surface ''S'', provided one starts with a
2-sided In mathematics, specifically in topology of manifolds, a compact codimension-one submanifold F of a manifold M is said to be 2-sided in M when there is an embedding ::h\colon F\times 1,1to M with h(x,0)=x for each x\in F and ::h(F\times 1,1\c ...
simple closed curve ''c'' on ''S''.


Example

Consider the
torus In geometry, a torus (: tori or toruses) is a surface of revolution generated by revolving a circle in three-dimensional space one full revolution about an axis that is coplanarity, coplanar with the circle. The main types of toruses inclu ...
represented by a
fundamental polygon In mathematics, a fundamental polygon can be defined for every compact Riemann surface of genus greater than 0. It encodes not only information about the topology of the surface through its fundamental group but also determines the Riemann surfa ...
with edges ''a'' and ''b'' :\mathbb^2 \cong \mathbb^2/\mathbb^2. Let a closed curve be the line along the edge ''a'' called \gamma_a. Given the choice of gluing homeomorphism in the figure, a tubular neighborhood of the curve \gamma_a will look like a band linked around a doughnut. This neighborhood is homeomorphic to an
annulus Annulus (or anulus) or annular indicates a ring- or donut-shaped area or structure. It may refer to: Human anatomy * ''Anulus fibrosus disci intervertebralis'', spinal structure * Annulus of Zinn, a.k.a. annular tendon or ''anulus tendineus comm ...
, say :a(0; 0, 1) = \ in the complex plane. By extending to the torus the twisting map \left(e^, t\right) \mapsto \left(e^, t\right) of the annulus, through the homeomorphisms of the annulus to an open cylinder to the neighborhood of \gamma_a, yields a Dehn twist of the torus by ''a''. :T_a: \mathbb^2 \to \mathbb^2 This self homeomorphism acts on the closed curve along ''b''. In the tubular neighborhood it takes the curve of ''b'' once along the curve of ''a''. A homeomorphism between topological spaces induces a natural isomorphism between their
fundamental groups In the mathematical field of algebraic topology, the fundamental group of a topological space is the group of the equivalence classes under homotopy of the loops contained in the space. It records information about the basic shape, or holes, of ...
. Therefore one has an automorphism :_\ast: \pi_1\left(\mathbb^2\right) \to \pi_1\left(\mathbb^2\right): \mapsto \left _a(x)\right/math> where 'x''are the homotopy classes of the closed curve ''x'' in the torus. Notice _\ast( = /math> and _\ast( = *a/math>, where b*a is the path travelled around ''b'' then ''a''.


Mapping class group

It is a theorem of
Max Dehn Max Wilhelm Dehn (November 13, 1878 – June 27, 1952) was a German mathematician most famous for his work in geometry, topology and geometric group theory. Dehn's early life and career took place in Germany. However, he was forced to retire in 1 ...
that maps of this form generate the
mapping class group In mathematics, in the subfield of geometric topology, the mapping class group is an important algebraic invariant of a topological space. Briefly, the mapping class group is a certain discrete group corresponding to symmetries of the space. Mo ...
of isotopy classes of orientation-preserving homeomorphisms of any closed, oriented
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
-g surface. W. B. R. Lickorish later rediscovered this result with a simpler proof and in addition showed that Dehn twists along 3g - 1 explicit curves generate the mapping class group (this is called by the punning name "Lickorish twist theorem"); this number was later improved by Stephen P. Humphries to 2g + 1, for g > 1, which he showed was the minimal number. Lickorish also obtained an analogous result for non-orientable surfaces, which require not only Dehn twists, but also "
Y-homeomorphism In mathematics, the y-homeomorphism, or crosscap slide, is a special type of auto-homeomorphism in non-orientable surfaces. It can be constructed by sliding a Möbius strip included on the surface around an essential 1-sided closed curve until t ...
s."


See also

*
Fenchel–Nielsen coordinates In mathematics, Fenchel–Nielsen coordinates are coordinates for Teichmüller space introduced by Werner Fenchel and Jakob Nielsen. Definition Suppose that ''S'' is a compact Riemann surface of genus ''g'' > 1. The Fenchel–Nielsen c ...
*
Lantern relation In geometric topology, a branch of mathematics, the lantern relation is a relation that appears between certain Dehn twists in the mapping class group of a surface. The most general version of the relation involves seven Dehn twists. The relation ...


References

* Andrew J. Casson, Steven A Bleiler, ''Automorphisms of Surfaces After Nielsen and Thurston'',
Cambridge University Press Cambridge University Press was the university press of the University of Cambridge. Granted a letters patent by King Henry VIII in 1534, it was the oldest university press in the world. Cambridge University Press merged with Cambridge Assessme ...
, 1988. . * Stephen P. Humphries, "Generators for the mapping class group," in: ''Topology of low-dimensional manifolds'' (''Proc. Second Sussex Conf.'', Chelwood Gate, 1977), pp. 44–47, Lecture Notes in Math., 722,
Springer Springer or springers may refer to: Publishers * Springer Science+Business Media, aka Springer International Publishing, a worldwide publishing group founded in 1842 in Germany formerly known as Springer-Verlag. ** Springer Nature, a multinationa ...
, Berlin, 1979. * W. B. R. Lickorish, "A representation of orientable combinatorial 3-manifolds." ''Ann. of Math.'' (2) 76 1962 531—540. * W. B. R. Lickorish, "A finite set of generators for the homotopy group of a 2-manifold", ''Proc. Cambridge Philos. Soc.'' 60 (1964), 769–778. {{MR, 0171269 Geometric topology Homeomorphisms