# 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 new region where an infinite series representation in terms of which it is initially defined becomes divergent.

The step-wise 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 singularities. The case of several complex variables is rather different, since singularities then need not be isolated points, and its investigation was a major reason for the development of sheaf cohomology.

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*

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.

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 below is generalized by the idea of a *germ*. The general theory of analytic continuation and its generalizations is known as sheaf theory. Let

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

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}.

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 *multi-valued functions*. See sheaf for the general concept.

Suppose that a power series has radius of convergence *r* and defines an analytic function *f* inside that disc. Consider points on the circle of convergence. A point for which there is a neighbourhood on which *f* has an analytic extension is *regular*, otherwise *singular*. The circle is a **natural boundary** if all its points are singular.

More generally, we may apply the definition to any open connected domain on which *f* is analytic, and classify the points of the boundary of the domain as regular or singular: the domain boundary is then a natural boundary if all points are singular, in which case the domain is a *domain of holomorphy*.

The 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).

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*.

the circle of convergence is a natural boundary. Such a power series is called lacunary. This theorem has been substantially generalized by Eugen Fabry (see Fabry's gap theorem) and George Pólya.

A useful theorem: A sufficient condition for analytic continuation to the non-positive integersExample I: The connection of the Riemann zeta function to the Bernoulli numbersSuppose that *F* is a smooth, sufficiently decreasing function on the positive reals satisfying the additional condition that

In application to number theoretic contexts, we consider such *F* to be the summatory function of the arithmetic function *f*,

We are interested in the analytic continuation of the DGF of *f*, or equivalently of the Dirichlet series over *f* at *s*,

We can form the DGF, or Dirichlet generating function, of any prescribed *f* given our smooth target function *F* by performing summation by parts as