Monodromy

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In mathematics, monodromy is the study of how objects from mathematical analysis, algebraic topology 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 maps and their degeneration into ramification; the aspect giving rise to monodromy phenomena is that certain functions we may wish to define fail to be single-valued as we 'run round' a path encircling a singularity. The failure of monodromy is best measured by defining a monodromy group: a group of transformations acting on the data that encodes what does happen as we 'run round'.

Contents

[edit] Definition

Let Failed to parse (Missing texvc executable; please see math/README to configure.): X

be a connected and locally connected based topological space with base point x, and let Failed to parse (Missing texvc executable; please see math/README to configure.): p:\tilde{X}\to X
be a covering with fiber Failed to parse (Missing texvc executable; please see math/README to configure.): F = p^{-1}(x)

. For a loop Failed to parse (Missing texvc executable; please see math/README to configure.): \gamma :[0,1]\longrightarrow X

based at Failed to parse (Missing texvc executable; please see math/README to configure.): x

, denote a lift under the covering map (starting at a point Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{x}\in F ) by Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{\gamma} . Finally, we denote by Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{x}\cdot\gamma

the endpoint Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{\gamma}(1)

, which is generally different from Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{x} . There are theorems which state that this construction gives a well-defined group action of the fundamental group Failed to parse (Missing texvc executable; please see math/README to configure.): \pi_1(X,x)

on F, and that the stabilizer of Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{x}
is exactly Failed to parse (Missing texvc executable; please see math/README to configure.): p_{*}(\pi_1(\tilde{X},\tilde{x}))

, that is, an element Failed to parse (Missing texvc executable; please see math/README to configure.): [\gamma]

fixes a point in F if and only if it is represented by the image of a loop in Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{X}
based at Failed to parse (Missing texvc executable; please see math/README to configure.): \tilde{x}

. This action is called the monodromy action and the corresponding homomorphism Failed to parse (Missing texvc executable; please see math/README to configure.): \pi_1(X,x)\longrightarrow Sym(F)

is the monodromy.

[edit] Example

These ideas were first made explicit in complex analysis. In the process of analytic continuation, a function that is an analytic function F(z) in some open subset E of the punctured disk D given

0 < |z| < 1

may be continued back into E, but with different values. For example if we take

F(z) = log z

and E to be defined by

Re(z) > 0

then analytic continuation anti-clockwise round the circle

|z| = 0.5

will result in the return, not to F(z) but

F(z)+2πi.

In this case the monodromy group is infinite cyclic and the covering space is the universal cover of the punctured complex plane. This cover can be visualized as the helicoid (as defined in the helicoid article) restricted to Failed to parse (Missing texvc executable; please see math/README to configure.): \rho>0 . The covering map is vertical projection, in a sense collapsing the spiral in the obvious way to get a punctured plane.

[edit] Differential equations in the complex domain

One important application is to differential equations, where a single solution may give further linearly independent solutions by analytic continuation. 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 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.

[edit] 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 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 π1(X,x) as a permutation group on the set of all c, as monodromy group in this context.

In differential geometry, an analogous role is played by parallel transport. In a principal bundle B over a smooth manifold 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 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 MxG.

[edit] Monodromy groupoid and Foliations

Analog 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 Failed to parse (Missing texvc executable; please see math/README to configure.): p:\tilde X\to X . The result has the structure of a groupoid 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 foliations: Consider Failed to parse (Missing texvc executable; please see math/README to configure.): (M,\mathcal{F})

a (possibly singular) foliation of M.  Then for every path in a leaf of Failed to parse (Missing texvc executable; please see math/README to configure.): \mathcal{F}
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 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.

[edit] Definition via Galois theory

Let Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{F}(x)

denote the field of fractions of the ring Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{F}[x]
where Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{F}
is also a field.  An element Failed to parse (Missing texvc executable; please see math/README to configure.): f(y) \in \mathbb{F}(y)
determines a finite field extension
Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{F}(x) \hookrightarrow \mathbb{F}(y)


by setting

Failed to parse (Missing texvc executable; please see math/README to configure.): f(y) = x


which is generally not Galois but which has Galois closure

Failed to parse (Missing texvc executable; please see math/README to configure.): L_{f}. \, \!


The associated Galois group of the extension Failed to parse (Missing texvc executable; please see math/README to configure.): L_f/\mathbb{F}(x)

is called the monodromy group of the extension.

In the case of Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{F} = \mathbb{C}

Riemann surface theory enters and allows for the geometric interpretation given above.  In the case that the extension Failed to parse (Missing texvc executable; please see math/README to configure.): \mathbb{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.

[edit] See also

[edit] References

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