Monodromy theorem

Monodromy theorem Illustration of analytic continuation along a curve (only a finite number of the disks {displaystyle U_{t}} are shown). Analytic continuation along a curve of the natural logarithm (the imaginary part of the logarithm is shown only).

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 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.

Contents 1 Analytic continuation along a curve 2 Properties of analytic continuation along a curve 3 Monodromy theorem 4 See also 5 References 6 External links 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) {displaystyle gamma :[0,1]to mathbb {C} .} Let {displaystyle f} be an analytic function defined on an open disk {displaystyle U} centered at {displaystyle gamma (0).} An analytic continuation of the pair {displaystyle (f,U)} along {displaystyle gamma } is a collection of pairs {displaystyle (f_{t},U_{t})} for {displaystyle 0leq tleq 1} such that {displaystyle f_{0}=f} and {displaystyle U_{0}=U.} For each {displaystyle tin [0,1],U_{t}} is an open disk centered at {displaystyle gamma (t)} and {displaystyle f_{t}:U_{t}to mathbb {C} } is an analytic function. For each {displaystyle tin [0,1]} there exists {displaystyle varepsilon >0} such that for all {displaystyle t'in [0,1]} with {displaystyle |t-t'|0} and the complex logarithm defined in a neighborhood of this point, and one lets {displaystyle gamma } be the circle of radius {displaystyle a} centered at the origin (traveled counterclockwise from {displaystyle (a,0)} ), then by doing an analytic continuation along this curve one will end up with a value of the logarithm at {displaystyle (a,0)} which is {displaystyle 2pi i} plus the original value (see the second illustration on the right).

Monodromy theorem Homotopy with fixed endopoints is necessary for the monodromy theorem to hold.

As noted earlier, two analytic continuations along the same curve yield the same result at the curve's endpoint. However, given two different curves branching out from the same point around which an analytic function is defined, with the curves reconnecting at the end, it is not true in general that the analytic continuations of that function along the two curves will yield the same value at their common endpoint.

Indeed, one can consider, as in the previous section, the complex logarithm defined in a neighborhood of a point {displaystyle (a,0)} and the circle centered at the origin and radius {displaystyle a.} Then, it is possible to travel from {displaystyle (a,0)} to {displaystyle (-a,0)} in two ways, counterclockwise, on the upper half-plane arc of this circle, and clockwise, on the lower half-plane arc. The values of the logarithm at {displaystyle (-a,0)} obtained by analytic continuation along these two arcs will differ by {displaystyle 2pi i.} If, however, one can continuously deform one of the curves into another while keeping the starting points and ending points fixed, and analytic continuation is possible on each of the intermediate curves, then the analytic continuations along the two curves will yield the same results at their common endpoint. This is called the monodromy theorem and its statement is made precise below.

Let {displaystyle U} be an open disk in the complex plane centered at a point {displaystyle P} and {displaystyle f:Uto mathbb {C} } be a complex-analytic function. Let {displaystyle Q} be another point in the complex plane. If there exists a family of curves {displaystyle gamma _{s}:[0,1]to mathbb {C} } with {displaystyle sin [0,1]} such that {displaystyle gamma _{s}(0)=P} and {displaystyle gamma _{s}(1)=Q} for all {displaystyle sin [0,1],} the function {displaystyle (s,t)in [0,1]times [0,1]to gamma _{s}(t)in mathbb {C} } is continuous, and for each {displaystyle sin [0,1]} it is possible to do an analytic continuation of {displaystyle f} along {displaystyle gamma _{s},} then the analytic continuations of {displaystyle f} along {displaystyle gamma _{0}} and {displaystyle gamma _{1}} will yield the same values at {displaystyle Q.} The monodromy theorem makes it possible to extend an analytic function to a larger set via curves connecting a point in the original domain of the function to points in the larger set. The theorem below which states that is also called the monodromy theorem.

Let {displaystyle U} be an open disk in the complex plane centered at a point {displaystyle P} and {displaystyle f:Uto mathbb {C} } be a complex-analytic function. If {displaystyle W} is an open simply-connected set containing {displaystyle U,} and it is possible to perform an analytic continuation of {displaystyle f} on any curve contained in {displaystyle W} which starts at {displaystyle P,} then {displaystyle f} admits a direct analytic continuation to {displaystyle W,} meaning that there exists a complex-analytic function {displaystyle g:Wto mathbb {C} } whose restriction to {displaystyle U} is {displaystyle f.} See also Analytic continuation Monodromy References Krantz, Steven G. (1999). Handbook of complex variables. Birkhäuser. ISBN 0-8176-4011-8. Jones, Gareth A.; Singerman, David (1987). Complex functions: an algebraic and geometric viewpoint. Cambridge University Press. ISBN 0-521-31366-X. Triebel, Hans (1986). Analysis and mathematical physics, English ed. D. Reidel Pub. Co. ISBN 90-277-2077-0. External links Monodromy theorem at MathWorld Monodromy theorem at PlanetMath. Monodromy theorem at the Encyclopaedia of Mathematics Categories: Theorems in complex analysis