Cesàro summation

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For the song "Cesaro Summability" by the band Tool, see Ænima.

In mathematical analysis, Cesàro summation assigns values to some infinite sums that are not convergent in the usual sense. 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).

The term summation can be misleading, as some statements and proofs regarding Cesàro summation can be said to implicate the Eilenberg–Mazur swindle. For example, it is commonly applied to Grandi's series with the conclusion that the sum of that series is 1/2, a result that can readily be disproven.

Definition[edit]

Let {an} be a sequence, and let

be the kth 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. If a series is convergent, then it is Cesàro summable and its Cesàro sum is zero. For any convergent sequence, the corresponding series is Cesàro summable and the limit of the sequence coincides with the Cesàro sum.

Examples

Let an = (−1)n+1 for n ≥ 1. That is, {an} is the sequence

and let G denote the series

Then the sequence of partial sums is

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

are

so that

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

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

and let G now denote the series

Then the sequence of partial sums {sn} is

and the evaluation of G diverges to infinity. The terms of the sequence of means of partial sums {tn } 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[edit]

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 Σan, define the quantities

(where the upper indices do not denote exponents) and define Enα to be Anα for the series 1 + 0 + 0 + 0 + · · ·. Then the (C, α) sum of Σan is denoted by (C, α)-Σan and has the value

if it exists (Shawyer & Watson 1994, pp.16-17). This description represents an -times iterated application of the initial summation method and can be restated as

Even more generally, for , let Anα be implicitly given by the coefficients of the series

and Enα as above. In particular, Enα are the binomial coefficients of power −1 − α. Then the (C, α) sum of Σ an is defined as above.

If Σan has a (C, α) sum, then it also has a (C, β) sum for every β>α, and the sums agree; furthermore we have an = o(nα) if α > −1 (see little-o notation).

Cesàro summability of an integral[edit]

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

exists and is finite (Titchmarsh 1948, §1.15). 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[edit]

References[edit]