In mathematics, in the area of complex analysis, Nachbin's theorem (named after Leopoldo Nachbin) is commonly used to establish a bound on the growth rates for an analytic function. This article will provide a brief review of growth rates, including the idea of a function of exponential type. Classification of growth rates based on type help provide a finer tool than big O or Landau notation, since a number of theorems about the analytic structure of the bounded function and its integral transforms can be stated. In particular, Nachbin's theorem may be used to give the domain of convergence of the generalized Borel transform, given below.
A function f(z) defined on the complex plane is said to be of exponential type if there exist constants M and τ such that
in the limit of . Here, the complex variable z was written as to emphasize that the limit must hold in all directions θ. Letting τ stand for the infimum of all such τ, one then says that the function f is of exponential type τ.
For example, let . Then one says that is of exponential type π, since π is the smallest number that bounds the growth of along the imaginary axis. So, for this example, Carlson's theorem cannot apply, as it requires functions of exponential type less than π.
Bounding may be defined for other functions besides the exponential function. In general, a function is a comparison function if it has a series
with for all n, and
as . If τ is the infimum of all such τ one says that f is of Ψ-type τ.
Nachbin's theorem states that a function f(z) with the series
is of Ψ-type τ if and only if
If f is of Ψ-type τ, then the exterior of the domain of convergence of , and all of its singular points, are contained within the disk
Furthermore, one has
where the contour of integration γ encircles the disk . This generalizes the usual Borel transform for exponential type, where . The integral form for the generalized Borel transform follows as well. Let be a function whose first derivative is bounded on the interval , so that
where . Then the integral form of the generalized Borel transform is
The ordinary Borel transform is regained by setting . Note that the integral form of the Borel transform is just the Laplace transform.
Nachbin resummation (generalized Borel transform) can be used to sum divergent series that escape to the usual Borel summation or even to solve (asymptotically) integral equations of the form:
where f(t) may or may not be of exponential growth and the kernel K(u) has a Mellin transform. The solution, pointed out by L. Nachbin himself, can be obtained as with and M(n) is the Mellin transform of K(u). an example of this is the Gram series
- Divergent series
- Euler summation
- Cesàro summation
- Lambert summation
- Phragmén–Lindelöf principle
- Abelian and tauberian theorems
- Van Wijngaarden transformation
- L. Nachbin, "An extension of the notion of integral functions of the finite exponential type", Anais Acad. Brasil. Ciencias. 16 (1944) 143–147.
- Ralph P. Boas, Jr. and R. Creighton Buck, Polynomial Expansions of Analytic Functions (Second Printing Corrected), (1964) Academic Press Inc., Publishers New York, Springer-Verlag, Berlin. Library of Congress Card Number 63-23263. (Provides a statement and proof of Nachbin's theorem, as well as a general review of this topic.)
- A.F. Leont'ev (2001), "Function of exponential type", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- A.F. Leont'ev (2001), "Borel transform", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Garcia J. Borel Resummation & the Solution of Integral Equations Prespacetime Journal nº 4 Vol 4. 2013 http://prespacetime.com/index.php/pst/issue/view/42/showToc