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, monodromy's fundamental meaning 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 codes what does happen as we 'run round'.

1 Definition
2 Example
3 Differential equations in the complex domain
4 Topological and geometric aspects
5 Definition via Galois theory
6 See also
7 References

[edit] Definition

Let $X$ be a connected and locally connected based topological space with base point x, and let $p:\tilde{X}\to X$ be a covering with fiber $F = p - 1 (x)$ . For a loop $\gamma :[0,1]\longrightarrow X$ based at $x$ , denote a lift under the covering map (starting at a point $\tilde{x}\in F$ ) by $\tilde{\gamma}$ . Finally, we denote by $\tilde{x}\cdot\gamma$ the endpoint $\tilde{\gamma}(1)$ , which is generally different from $\tilde{x}$ . There are theorems which state that this construction gives a well-defined right group action of the fundamental group $π 1 (X, x)$ on F, and that the stabilizer of $\tilde{x}$ is exactly $p_{*}(\pi_1(\tilde{X},\tilde{x}))$ , that is, an element $[γ]$ fixes a point in F if and only if it is represented by the image of a loop in $\tilde{X}$ based at $\tilde{x}$ . This action is called the monodromy action and the corresponding homomorphism $\pi_1(X,x)\longrightarrow Sym(F)$ 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 $ρ > 0$ . 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] Definition via Galois theory

Let $\mathbb{F}(x)$ denote the field of fractions of the ring $\mathbb{F}[x]$ where $\mathbb{F}$ is also a field. An element $f(y) \in \mathbb{F}(y)$ determines a finite field extension

$\mathbb{F}(x) \hookrightarrow \mathbb{F}(y)$

by setting

f (y) = x

which is generally not Galois but which has Galois closure

$L_{f}. \, \!$

The associated Galois group of the extension $L_f/\mathbb{F}(x)$ is called the monodromy group of the extension.

In the case of $\mathbb{F} = \mathbb{C}$ Riemann surface theory enters and allows for the geometric interpretation given above. In the case that the extension $\mathbb{C}(y)$ is already Galois, the associated monodromy group is sometimes called a group of deck transformations.