monodromy representation
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In mathematics, monodromy is the study of how objects from
mathematical analysis Analysis is the branch of mathematics dealing with continuous functions, limit (mathematics), limits, and related theories, such as Derivative, differentiation, Integral, integration, measure (mathematics), measure, infinite sequences, series (m ...
,
algebraic topology Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...
, algebraic geometry and differential geometry behave as they "run round" a singularity. As the name implies, the fundamental meaning of ''monodromy'' comes from "running round singly". It is closely associated with
covering map A covering of a topological space X is a continuous map \pi : E \rightarrow X with special properties. Definition Let X be a topological space. A covering of X is a continuous map : \pi : E \rightarrow X such that there exists a discrete spa ...
s and their degeneration into ramification; the aspect giving rise to monodromy phenomena is that certain
function Function or functionality may refer to: Computing * Function key, a type of key on computer keyboards * Function model, a structured representation of processes in a system * Function object or functor or functionoid, a concept of object-oriente ...
s we may wish to define fail to be ''single-valued'' as we "run round" a path encircling a singularity. The failure of monodromy can be measured by defining a monodromy group: a
group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...
of transformations acting on the data that encodes what happens as we "run round" in one dimension. Lack of monodromy is sometimes called ''polydromy''.


Definition

Let be a connected and
locally connected In topology and other branches of mathematics, a topological space ''X'' is locally connected if every point admits a neighbourhood basis consisting entirely of open, connected sets. Background Throughout the history of topology, connectedness ...
based
topological space In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...
with base point , and let p: \tilde \to X be a covering with
fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorpora ...
F = p^(x). For a loop based at , denote a
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...
under the covering map, starting at a point \tilde \in F, by \tilde. Finally, we denote by \tilde \cdot \tilde the endpoint \tilde(1), which is generally different from \tilde. There are theorems which state that this construction gives a well-defined
group action In mathematics, a group action on a space is a group homomorphism of a given group into the group of transformations of the space. Similarly, a group action on a mathematical structure is a group homomorphism of a group into the automorphism ...
of the fundamental group on , and that the stabilizer of \tilde is exactly p_*\left(\pi_1\left(\tilde, \tilde\right)\right), that is, an element fixes a point in if and only if it is represented by the image of a loop in \tilde based at \tilde. This action is called the monodromy action and the corresponding
homomorphism In algebra, a homomorphism is a structure-preserving map between two algebraic structures of the same type (such as two groups, two rings, or two vector spaces). The word ''homomorphism'' comes from the Ancient Greek language: () meaning "same" ...
into the
automorphism group In mathematics, the automorphism group of an object ''X'' is the group consisting of automorphisms of ''X'' under composition of morphisms. For example, if ''X'' is a finite-dimensional vector space, then the automorphism group of ''X'' is the g ...
on is the algebraic monodromy. The image of this homomorphism is the monodromy group. There is another map whose image is called the topological monodromy group.


Example

These ideas were first made explicit in complex analysis. In the process of
analytic continuation In complex analysis, a branch of mathematics, analytic continuation is a technique to extend the domain of definition of a given analytic function. Analytic continuation often succeeds in defining further values of a function, for example in a n ...
, a function that is an
analytic function In mathematics, an analytic function is a function that is locally given by a convergent power series. There exist both real analytic functions and complex analytic functions. Functions of each type are infinitely differentiable, but complex ...
in some open subset of the punctured complex plane may be continued back into , but with different values. For example, take : \begin F(z) &= \log(z) \\ E &= \ \end then analytic continuation anti-clockwise round the circle : , z, = 1 will result in the return, not to but : F(z) + 2\pi i In this case the monodromy group is
infinite cyclic In group theory, a branch of abstract algebra in pure mathematics, a cyclic group or monogenous group is a group, denoted C''n'', that is generated by a single element. That is, it is a set of invertible elements with a single associative binary ...
and the covering space is the universal cover of the punctured complex plane. This cover can be visualized as the
helicoid The helicoid, also known as helical surface, after the plane and the catenoid, is the third minimal surface to be known. Description It was described by Euler in 1774 and by Jean Baptiste Meusnier in 1776. Its name derives from its similarity ...
(as defined in the helicoid article) restricted to . The covering map is a vertical projection, in a sense collapsing the spiral in the obvious way to get a punctured plane.


Differential equations in the complex domain

One important application is to
differential equation In mathematics, a differential equation is an equation that relates one or more unknown functions and their derivatives. In applications, the functions generally represent physical quantities, the derivatives represent their rates of change, an ...
s, where a single solution may give further linearly independent solutions by
analytic continuation In complex analysis, a branch of mathematics, analytic continuation is a technique to extend the domain of definition of a given analytic function. Analytic continuation often succeeds in defining further values of a function, for example in a n ...
. Linear differential equations defined in an open, connected set ''S'' in the complex plane have a monodromy group, which (more precisely) is a
linear representation Representation theory is a branch of mathematics that studies abstract algebraic structures by ''representing'' their elements as linear transformations of vector spaces, and studies modules over these abstract algebraic structures. In essenc ...
of the fundamental group of ''S'', summarising all the analytic continuations round loops within ''S''. The inverse problem, of constructing the equation (with regular singularities), given a representation, is called the Riemann–Hilbert problem. For a regular (and in particular Fuchsian) linear system one usually chooses as generators of the monodromy group the operators ''Mj'' corresponding to loops each of which circumvents just one of the poles of the system counterclockwise. If the indices ''j'' are chosen in such a way that they increase from 1 to ''p'' + 1 when one circumvents the base point clockwise, then the only relation between the generators is the equality M_1\cdots M_=\operatorname. The Deligne–Simpson problem is the following realisation problem: For which tuples of conjugacy classes in GL(''n'', C) do there exist irreducible tuples of matrices ''Mj'' from these classes satisfying the above relation? The problem has been formulated by
Pierre Deligne Pierre René, Viscount Deligne (; born 3 October 1944) is a Belgian mathematician. He is best known for work on the Weil conjectures, leading to a complete proof in 1973. He is the winner of the 2013 Abel Prize, 2008 Wolf Prize, 1988 Crafoord P ...
and
Carlos Simpson Carlos Tschudi Simpson (born 30 June 1962) is an American mathematician, specializing in algebraic geometry. Simpson received his Ph.D. in 1987 from Harvard University, where he was supervised by Wilfried Schmid; his thesis was titled ''Systems of ...
was the first to obtain results towards its resolution. An additive version of the problem about residua of Fuchsian systems has been formulated and explored by Vladimir Kostov. The problem has been considered by other authors for matrix groups other than GL(''n'', C) as well. and the references therein.


Topological and geometric aspects

In the case of a covering map, we look at it as a special case of a fibration, and use the
homotopy lifting property In mathematics, in particular in homotopy theory within algebraic topology, the homotopy lifting property (also known as an instance of the right lifting property or the covering homotopy axiom) is a technical condition on a continuous function from ...
to "follow" paths on the base space ''X'' (we assume it path-connected for simplicity) as they are lifted up into the cover ''C''. If we follow round a loop based at ''x'' in ''X'', which we lift to start at ''c'' above ''x'', we'll end at some ''c*'' again above ''x''; it is quite possible that ''c'' ≠ ''c*'', and to code this one considers the action of the fundamental group 1(''X'', ''x'') as a permutation group on the set of all ''c'', as a monodromy group in this context. In differential geometry, an analogous role is played by
parallel transport In geometry, parallel transport (or parallel translation) is a way of transporting geometrical data along smooth curves in a manifold. If the manifold is equipped with an affine connection (a covariant derivative or connection on the tangent b ...
. In a
principal bundle In mathematics, a principal bundle is a mathematical object that formalizes some of the essential features of the Cartesian product X \times G of a space X with a group G. In the same way as with the Cartesian product, a principal bundle P is equi ...
''B'' over a
smooth manifold In mathematics, a differentiable manifold (also differential manifold) is a type of manifold that is locally similar enough to a vector space to allow one to apply calculus. Any manifold can be described by a collection of charts (atlas). One ma ...
''M'', a connection allows "horizontal" movement from fibers above ''m'' in ''M'' to adjacent ones. The effect when applied to loops based at ''m'' is to define a
holonomy In differential geometry, the holonomy of a connection on a smooth manifold is a general geometrical consequence of the curvature of the connection measuring the extent to which parallel transport around closed loops fails to preserve the geomet ...
group of translations of the fiber at ''m''; if the structure group of ''B'' is ''G'', it is a subgroup of ''G'' that measures the deviation of ''B'' from the product bundle ''M'' × ''G''.


Monodromy groupoid and foliations

Analogous to the fundamental groupoid it is possible to get rid of the choice of a base point and to define a monodromy groupoid. Here we consider (homotopy classes of) lifts of paths in the base space ''X'' of a fibration p:\tilde X\to X. The result has the structure of a
groupoid In mathematics, especially in category theory and homotopy theory, a groupoid (less often Brandt groupoid or virtual group) generalises the notion of group in several equivalent ways. A groupoid can be seen as a: *'' Group'' with a partial func ...
over the base space ''X''. The advantage is that we can drop the condition of connectedness of ''X''. Moreover the construction can also be generalized to
foliation In mathematics (differential geometry), 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 ...
s: Consider (M,\mathcal) a (possibly singular) foliation of ''M''. Then for every path in a leaf of \mathcal we can consider its induced diffeomorphism on local transversal sections through the endpoints. Within a simply connected chart this diffeomorphism becomes unique and especially canonical between different transversal sections if we go over to the
germ Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embryo ...
of the diffeomorphism around the endpoints. In this way it also becomes independent of the path (between fixed endpoints) within a simply connected chart and is therefore invariant under homotopy.


Definition via Galois theory

Let F(''x'') denote the field of the rational functions in the variable ''x'' over the
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grass ...
F, which is the
field of fractions In abstract algebra, the field of fractions of an integral domain is the smallest field in which it can be embedded. The construction of the field of fractions is modeled on the relationship between the integral domain of integers and the field ...
of the
polynomial ring In mathematics, especially in the field of algebra, a polynomial ring or polynomial algebra is a ring (which is also a commutative algebra) formed from the set of polynomials in one or more indeterminates (traditionally also called variables ...
F 'x'' An element ''y'' = ''f''(''x'') of F(''x'') determines a finite field extension ''F(''x'') : F(''y'') This extension is generally not Galois but has Galois closure ''L''(''f''). The associated
Galois group In mathematics, in the area of abstract algebra known as Galois theory, the Galois group of a certain type of field extension is a specific group associated with the field extension. The study of field extensions and their relationship to the po ...
of the extension 'L''(''f'') : F(''y'')is called the monodromy group of ''f''. In the case of F = C
Riemann surface In mathematics, particularly in complex analysis, a Riemann surface is a connected one-dimensional complex manifold. These surfaces were first studied by and are named after Bernhard Riemann. Riemann surfaces can be thought of as deformed ver ...
theory enters and allows for the geometric interpretation given above. In the case that the extension ''C(''x'') : C(''y'')is already Galois, the associated monodromy group is sometimes called a group of deck transformations. This has connections with the Galois theory of covering spaces leading to the Riemann existence theorem.


See also

*
Braid group A braid (also referred to as a plait) is a complex structure or pattern formed by interlacing two or more strands of flexible material such as textile yarns, wire, or hair. The simplest and most common version is a flat, solid, three-strande ...
*
Monodromy theorem In complex analysis, the monodromy theorem is an important result about analytic continuation of a complex-analytic function to a larger set. The idea is that one can extend a complex-analytic function (from here on called simply ''analytic fun ...
*
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 a punctured disk)


Notes


References

*{{springer, author=V. I. Danilov, title=Monodromy, id=M/m064700 * "Group-groupoids and monodromy groupoids", O. Mucuk, B. Kılıçarslan, T. ¸Sahan, N. Alemdar, Topology and its Applications 158 (2011) 2034–2042 doi:10.1016/j.topol.2011.06.048 * R. Brow
Topology and Groupoids
(2006). * P.J. Higgins, "Categories and groupoids", van Nostrand (1971

* H. Żołądek, "The Monodromy Group", Birkhäuser Basel 2006; doi: 10.1007/3-7643-7536-1 Mathematical analysis Complex analysis Differential geometry Algebraic topology Homotopy theory