# Abel's theorem

In mathematics, Abel's theorem for power series relates a limit of a power series to the sum of its coefficients. It is named after Norwegian mathematician Niels Henrik Abel.

# Abel's theorem

Jump to navigation

Jump to search

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please help to improve this article by introducing more precise citations. (February 2013) (Learn how and when to remove this template message) |

In mathematics, **Abel's theorem** for power series relates a limit of a power series to the sum of its coefficients.

It is named after Norwegian mathematician Niels Henrik Abel.

## Theorem[]

Let the Taylor series

be a power series with real coefficients

$$$$Suppose that the series

converges.

Then

is continuous from the left at

${\scriptscriptstyle x=1,}$that is,

The same theorem holds for complex power series

provided that

$$ entirely within a single *Stolz sector*, that is, a region of the open unit disk where

for some fixed finite

$1\}"{\scriptscriptstyle M1}$1}">.

Without this restriction, the limit may fail to exist: for example, the power series

0}{frac {z^{3^{n}}-z^{2cdot 3^{n}}}{n}}}">

converges to

$$at

${\scriptscriptstyle z=1,}$but is unbounded near any point of the form

$$so the value at

${\scriptscriptstyle z=1}$is not the limit as

$$tends to 1 in the whole open disk.

Note that

$$is continuous on the real closed interval

$$for

$$by virtue of the uniform convergence of the series on compact subsets of the disk of convergence. Abel's theorem allows us to say more, namely that

$$is continuous on

$$## Remarks[]

As an immediate consequence of this theorem, if

$$is any nonzero complex number for which the series

converges, then it follows that

in which the limit is taken from below.

The theorem can also be generalized to account for sums which diverge to infinity.^{[citation needed]} If

then

However, if the series is only known to be divergent, but for reasons other than diverging to infinity, then the claim of the theorem may fail: take, for example, the power series for

At

${\scriptscriptstyle z=1}$the series is equal to

$$but

$$We also remark the theorem holds for radii of convergence other than

${\scriptscriptstyle R=1}$: let

be a power series with radius of convergence

$$and suppose the series converges at

${\scriptscriptstyle x=R.}$Then

$$is continuous from the left at

${\scriptscriptstyle x=R,}$that is,

## Applications[]

The utility of Abel's theorem is that it allows us to find the limit of a power series as its argument (that is,

$$) approaches

$$from below, even in cases where the radius of convergence,

$$of the power series is equal to

$$and we cannot be sure whether the limit should be finite or not. See for example, the binomial series. Abel's theorem allows us to evaluate many series in closed form. For example, when

we obtain

by integrating the uniformly convergent geometric power series term by term on

$$converges to

$$by Abel's theorem. Similarly,

converges to

$$$$

is called the generating function of the sequence

$$Abel's theorem is frequently useful in dealing with generating functions of real-valued and non-negative sequences, such as probability-generating functions. In particular, it is useful in the theory of Galton–Watson processes.

## Outline of proof[]

After subtracting a constant from

$$we may assume that

$$Let

$$Then substituting

$$and performing a simple manipulation of the series (summation by parts) results in

Given

$0,\}"{\scriptscriptstyle \epsilon 0,}$0,"> pick

$$large enough so that

$$for all

$$and note that

when

$$lies within the given Stolz angle. Whenever

$$is sufficiently close to

$$we have

so that

$$when

$$is both sufficiently close to

$$and within the Stolz angle.

## Related concepts[]

Converses to a theorem like Abel's are called Tauberian theorems: There is no exact converse, but results conditional on some hypothesis. The field of divergent series, and their summation methods, contains many theorems *of abelian type* and *of tauberian type*.

## See also[]

- Abel's summation formula – Integration by parts version of Abel's method for summation by parts
- Nachbin resummation
- Summation by parts – Theorem to simplify sums of products of sequences

## Further reading[]

Ahlfors, Lars Valerian (September 1, 1980).

*Complex Analysis*(Third ed.). McGraw Hill Higher Education. pp. 41–42. ISBN 0-07-085008-9. - Ahlfors called it*Abel's limit theorem*.

## External links[]

- Abel summability at PlanetMath.
*(a more general look at Abelian theorems of this type)* - A.A. Zakharov (2001) [1994], "Abel summation method",
*Encyclopedia of Mathematics*, EMS Press - Weisstein, Eric W. "Abel's Convergence Theorem".
*MathWorld*.

Si quieres conocer otros artículos parecidos a **Abel's theorem** puedes visitar la categoría **Mathematical series**.

Deja una respuesta