Definitions

# Nachbin's theorem

In mathematics, in the area of complex analysis, Nachbin's theorem 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. The theorem is named in honour of the Brazilian mathematician Leopoldo Nachbin.

## Exponential type

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

$|f\left(re^\left\{itheta\right\}\right)|le Me^\left\{tau r\right\}$

in the limit of $rtoinfty$. Here, the complex variable z was written as $z=re^\left\{itheta\right\}$ 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 $f\left(z\right)=sin\left(pi z\right)$. Then one says that $sin\left(pi z\right)$ is of exponential type π, since π is the smallest number that bounds the growth of $sin\left(pi z\right)$ along the imaginary axis. So, for this example, Carlson's theorem cannot apply, as it requires functions of exponential type less than π.

## Ψ type

Bounding may be defined for other functions besides the exponential function. In general, a function $Psi\left(t\right)$ is a comparison function if it has a series

$Psi\left(t\right)=sum_\left\{n=0\right\}^infty Psi_n t^n$

with $Psi_n>0$ for all n, and

$lim_\left\{ntoinfty\right\} frac\left\{Psi_\left\{n+1\right\}\right\}\left\{Psi_n\right\} = 0.$

Note that comparison functions are necessarily entire, which follows from the ratio test. If $Psi\left(t\right)$ is such a comparison function, one then says that f is of Ψ-type if there exist constants M and τ such that

$left|fleft\left(re^\left\{itheta\right\}right\right)right| le MPsi\left(tau r\right)$

as $rto infty$. If τ is the infimum of all such τ one says that f is of Ψ-type τ.

## Nachbin's theorem

Nachbin's theorem states that a function f(z) with the series

$f\left(z\right)=sum_\left\{n=0\right\}^infty f_n z^n$

is of Ψ-type τ if an only if

$limsup left| frac\left\{f_n\right\}\left\{Psi_n\right\} right|^\left\{1/n\right\} = tau.$

## Borel transform

Nachbin's theorem has immediate applications in Cauchy theorem-like situations, and for integral transforms. For example, the generalized Borel transform is given by

$F\left(w\right)=sum_\left\{n=0\right\}^infty frac\left\{f_n\right\}\left\{Psi_n w^\left\{n+1\right\}\right\}.$

If f is of Ψ-type τ, then the exterior of the domain of convergence of $F\left(w\right)$, and all of its singular points, are contained within the disk

$|w| le tau.$

Furthermore, one has

$f\left(z\right)=frac\left\{1\right\}\left\{2pi i\right\} oint_gamma Psi \left(zw\right) F\left(w\right), dw$

where the contour of integration γ encircles the disk $|w| le tau$. This generalizes the usual Borel transform for exponential type, where $Psi\left(t\right)=e^t$. The integral form for the generalized Borel transform follows as well. Let $alpha\left(t\right)$ be a function whose first derivative is bounded on the interval $\left[0,infty\right)$, so that

$frac\left\{1\right\}\left\{Psi_n\right\} = int_0^infty t^n, dalpha\left(t\right)$

where $dalpha\left(t\right)=alpha^\left\{prime\right\}\left(t\right),dt$. Then the integral form of the generalized Borel transform is

$F\left(w\right)=frac\left\{1\right\}\left\{w\right\} int_0^infty f left\left(frac\left\{t\right\}\left\{w\right\}right\right) , dalpha\left(t\right).$

The ordinary Borel transform is regained by setting $alpha\left(t\right)=e^\left\{-t\right\}$. 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 resummation'or even to solve (asymptotically) integral equation of the form:

$g\left(s\right)=sint_\left\{0\right\}^\left\{infty\right\} K\left(st\right)f\left(t\right),dt$

where f(t) may or may not be of exponential growth and the kernel K(u) has a Mellin transform.

## References

• 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.)
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