 Analytic continuation

In complex analysis, a branch of mathematics, analytic continuation is a technique to extend the domain of a given analytic function. Analytic continuation often succeeds in defining further values of a function, for example in a new region where an infinite series representation in terms of which it is initially defined becomes divergent.
The stepwise continuation technique may, however, come up against difficulties. These may have an essentially topological nature, leading to inconsistencies (defining more than one value). They may alternatively have to do with the presence of mathematical singularities. The case of several complex variables is rather different, since singularities then cannot be isolated points, and its investigation was a major reason for the development of sheaf cohomology.
Contents
Initial discussion
Suppose f is an analytic function defined on an open subset U of the complex plane . If V is a larger open subset of , containing U, and F is an analytic function defined on V such that
then F is called an analytic continuation of f. In other words, the restriction of F to U is the function f we started with.
Analytic continuations are unique in the following sense: if V is the connected domain of two analytic functions F_{1} and F_{2} such that U is contained in V and for all z in U
 F_{1}(z) = F_{2}(z) = f(z),
then
 F_{1} = F_{2}
on all of V. This is because F_{1} − F_{2} is an analytic function which vanishes on the open, connected domain U of f and hence must vanish on its entire domain. This follows directly from the identity theorem for holomorphic functions.
Applications
A common way to define functions in complex analysis proceeds by first specifying the function on a small domain only, and then extending it by analytic continuation. In practice, this continuation is often done by first establishing some functional equation on the small domain and then using this equation to extend the domain. Examples are the Riemann zeta function and the gamma function.
The concept of a universal cover was first developed to define a natural domain for the analytic continuation of an analytic function. The idea of finding the maximal analytic continuation of a function in turn led to the development of the idea of Riemann surfaces.
The power series defined above is generalized by the idea of a germ. The general theory of analytic continuation and its generalizations are known as sheaf theory.
Formal definition of a germ
Let
be a power series converging in the disc D_{r}(z_{0}) defined by
 .
Note that without loss of generality, here and below, we will always assume that a maximal such r was chosen, even if that r is ∞. Also note that it would be equivalent to begin with an analytic function defined on some small open set. We say that the vector
 g = (z_{0}, α_{0}, α_{1}, α_{2}, ...)
is a germ of f. The base g_{0} of g is z_{0}, the stem of g is (α_{0}, α_{1}, α_{2}, ...) and the top g_{1} of g is α_{0}. The top of g is the value of f at z_{0}, the bottom of g.
Any vector g = (z_{0}, α_{0}, α_{1}, ...) is a germ if it represents a power series of an analytic function around z_{0} with some radius of convergence r > 0. Therefore, we can safely speak of the set of germs .
The topology of the set of germs
Let g and h be germs. If h_{0} − g_{0} < r where r is the radius of convergence of g and if the power series that g and h define identical functions on the intersection of the two domains, then we say that h is generated by (or compatible with) g, and we write g ≥ h. This compatibility condition is neither transitive, symmetric nor antisymmetric. If we extend the relation by transitivity, we obtain a symmetric relation, which is therefore also an equivalence relation on germs (but not an ordering). This extension by transitivity is one definition of analytic continuation. The equivalence relation will be denoted .
We can define a topology on . Let r > 0, and let
The sets U_{r}(g), for all r > 0 and g ∈ define a basis of open sets for the topology on .
A connected component of (i.e., an equivalence class) is called a sheaf. We also note that the map φ_{g}(h) = h_{0} from U_{r}(g) to C where r is the radius of convergence of g, is a chart. The set of such charts forms an atlas for , hence is a Riemann surface. is sometimes called the universal analytic function.
Examples of analytic continuation
is a power series corresponding to the natural logarithm near z = 1. This power series can be turned into a germ
This germ has a radius of convergence of 1, and so there is a sheaf S corresponding to it. This is the sheaf of the logarithm function.
The uniqueness theorem for analytic functions also extends to sheaves of analytic functions: if the sheaf of an analytic function contains the zero germ (i.e., the sheaf is uniformly zero in some neighborhood) then the entire sheaf is zero. Armed with this result, we can see that if we take any germ g of the sheaf S of the logarithm function, as described above, and turn it into a power series f(z) then this function will have the property that exp(f(z))=z. If we had decided to use a version of the inverse function theorem for analytic functions, we could construct a wide variety of inverses for the exponential map, but we would discover that they are all represented by some germ in S. In that sense, S is the "one true inverse" of the exponential map.
In older literature, sheaves of analytic functions were called multivalued functions. See sheaf for the general concept.
Monodromy theorem
Main article: Monodromy theoremThe monodromy theorem gives a sufficient condition for the existence of a direct analytic continuation (i.e., an extension of an analytic function to an analytic function on a bigger set).
Suppose D is an open set in , and f an analytic function on D. If G is a simply connected domain containing D, such that f has an analytic continuation along every path in G, starting from some fixed point a in D, then f has a direct analytic continuation to G.
In the above language this means that if G is a simply connected domain, and S is a sheaf whose set of base points contains G, then there exists an analytic function f on G whose germs belong to S.
Hadamard's gap theorem
Main article: OstrowskiHadamard gap theoremFor a power series
with coefficients mostly zero in the precise sense that they vanish outside a sequence of exponents k(i) with
for some fixed δ > 0, the circle centre z_{0} and with radius the radius of convergence is a natural boundary. Such a power series defines a lacunary function.
Polya's theorem
Let be a power series, then there exist such that
has the convergence disc of f around z_{0} as a natural boundary.
The proof of this theorem makes use of Hadamard's gap theorem.
See also
References
 Lars Ahlfors (1979). Complex Analysis (3 ed.). McGrawHill. pp. 172, 284.
 Weisstein, Eric W., "Analytic Continuation" from MathWorld.
External links
 Analytic Continuation at MathPages
Categories: Complex analysis
 Riemann surfaces
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