# Cesàro summation

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In mathematical analysis, **Cesàro summation** assigns values to some infinite sums that are not convergent in the usual sense, while coinciding with the standard sum if they are convergent. The Cesàro sum is defined as the limit of the arithmetic mean of the partial sums of the series.

Cesàro summation is named for the Italian analyst Ernesto Cesàro (1859–1906).

## Definition

Let {*a*_{n}} be a sequence, and let

be the *k*th partial sum of the series

The series is called **Cesàro summable**, with Cesàro sum , if the average value of its partial sums tends to :

In other words, the Cesàro sum of an infinite series is the limit of the arithmetic mean (average) of the first *n* partial sums of the series, as *n* goes to infinity. It is easy to show that any convergent series is Cesàro summable, and the sum of the series agrees with its Cesàro sum. However, as the first example below demonstrates, there are series that diverge but are nonetheless Cesàro summable.

## Examples

Let *a*_{n} = (−1)^{n+1} for *n* ≥ 1. That is, {*a*_{n}} is the sequence

Then the sequence of partial sums {*s*_{n}} is

so that the series *G*, known as Grandi's series, clearly does not converge. On the other hand, the terms of the sequence {*t*_{n}} of the (partial) means of the {*s*_{n}} where

are

so that

Therefore the Cesàro sum of the series *G* is 1/2.

On the other hand, now let *a*_{n} = *n* for *n* ≥ 1. That is, {*a*_{n}} is the sequence

and let *G* now denote the series

Then the sequence of partial sums {*s*_{n}} is

and the evaluation of *G* diverges to infinity.
The terms of the sequence of means of partial sums {*t*_{n} } are here

Thus, this sequence diverges to infinity as well as *G*, and *G* is now **not** Cesàro summable. In fact, any series which diverges to (positive or negative) infinity the Cesàro method also leads to a sequence that diverges likewise, and hence such a series is not Cesàro summable.

## (C, α) summation

In 1890, Ernesto Cesàro stated a broader family of summation methods which have since been called (C, α) for non-negative integers α. The (C, 0) method is just ordinary summation, and (C, 1) is Cesàro summation as described above.

The higher-order methods can be described as follows: given a series Σ*a*_{n}, define the quantities

(where the upper indices do not denote exponents) and define *E*_{n}^{α} to be *A*_{n}^{α} for the series 1 + 0 + 0 + 0 + · · ·. Then the (C, α) sum of Σ*a*_{n} is denoted by (C, α)-Σ*a*_{n} and has the value

if it exists Template:Harv. This description represents an -times iterated application of the initial summation method and can be restated as

Even more generally, for , let *A*_{n}^{α} be implicitly given by the coefficients of the series

and *E*_{n}^{α} as above. In particular, *E*_{n}^{α} are the binomial coefficients of power −1 − α. Then the (C, α) sum of Σ *a*_{n} is defined as above.

If Σ*a*_{n} has a (C, α) sum, then it also has a (C, β) sum for every β>α, and the sums agree; furthermore we have *a*_{n} = *o*(*n*^{α}) if α > −1 (see little-o notation).

## Cesàro summability of an integral

Let α ≥ 0. The integral is Cesàro summable (C, α) if

exists and is finite Template:Harv. The value of this limit, should it exist, is the (C, α) sum of the integral. Analogously to the case of the sum of a series, if α=0, the result is convergence of the improper integral. In the case α=1, (C, 1) convergence is equivalent to the existence of the limit

which is the limit of means of the partial integrals.

As is the case with series, if an integral is (C,α) summable for some value of α ≥ 0, then it is also (C,β) summable for all β > α, and the value of the resulting limit is the same.

## See also

- Abel summation
- Abel's summation formula
- Abel–Plana formula
- Abelian and tauberian theorems
- Borel summation
- Cesàro mean
- Divergent series
- Euler summation
- Fejér's theorem
- Lambert summation
- Perron's formula
- Ramanujan summation
- Riesz mean
- Silverman–Toeplitz theorem
- Summation by parts

## References

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