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In
complex analysis, the monodromy theorem is an important result about
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 ...
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 function'') along curves starting in the original domain of the function and ending in the larger set. A potential problem of this analytic continuation along a curve strategy is there are usually many curves which end up at the same point in the larger set. The monodromy theorem gives sufficient conditions for analytic continuation to give the same value at a given point regardless of the curve used to get there, so that the resulting extended analytic function is well-defined and single-valued.
Before stating this theorem it is necessary to define analytic continuation along a curve and study its properties.
Analytic continuation along a curve
The definition of analytic continuation along a curve is a bit technical, but the basic idea is that one starts with an analytic function defined around a point, and one extends that function along a curve via analytic functions defined on small overlapping disks covering that curve.
Formally, consider a curve (a
continuous function)
Let
be an analytic function defined on an
open disk
In geometry, a disk (also spelled disc). is the region in a plane bounded by a circle. A disk is said to be ''closed'' if it contains the circle that constitutes its boundary, and ''open'' if it does not.
For a radius, r, an open disk is usu ...
centered at
An ''analytic continuation'' of the pair
along
is a collection of pairs
for
such that
*
and
* For each
is an open disk centered at
and
is an analytic function.
* For each