Research Article | Open Access

Shakro Tetunashvili, "On Divergence of Fourier Series by Some Methods of Summability", *Journal of Function Spaces*, vol. 2012, Article ID 542607, 9 pages, 2012. https://doi.org/10.1155/2012/542607

# On Divergence of Fourier Series by Some Methods of Summability

**Academic Editor:**V. M. Kokilashvili

#### Abstract

A new summability method of series is introduced and studied. The particular cases of this method are, for example, variable-order Cesaro and Riesz methods. Applications to divergence problem of Fourier series are given. An extension of Kolmogorov, Schipp, and Boฤkarevโs well-known theorems on divergence of Fourier trigonometric, Walsh, and orthonormal series is established.

#### 1. A New Summability Method of Series

Let be such triangular matrix which satisfies the following conditions:

By we denote a partial sum of a series and by we denote a mean constructed by the matrix, that is,

Theorem 1.1. *Let matrix (1.1) satisfies an inequality
**
Then for any series (1.3) which satisfies the following condition:
**
an equality
**
holds.*

Below we prove a Lemma which is used to prove Theorem 1.1.

Lemma 1.2. *For every natural number an inequality
**
holds.*

*Proof of the Lemma. *Using Abel transformation and we get
Therefore,
Thus, taking into account (1.1) we immediately get
So the Lemma is proved.

*Proof of Theorem 1.1. *According to the condition of Theorem 1.1 we have
for some . Note that inequalities which hold for every natural imply , that is, .

So, holds.

According to (1.12) there exists a natural number such that for every natural number we have
So according to the Lemma, for every an inequality
holds true; that is, if , then
Thus for every an inequality
holds.

So for every we have

Note that for every natural there exists at least one natural number , such that the partial sums of the series (1.3) satisfy the following condition:
We define by a formula:
So is maximal number among the above-mentioned natural numbers. Consequently,
According to the condition of Theorem 1.1,
Therefore,
that is,
A consequence of (1.24) is that there exists such natural that if then and since (1.17) holds for every , then (1.17) remains true for every , where .

So
Since , therefore,
Note that the last one and (1.25) imply
So according to (1.21) we have
that is, for every an inequality
Also, (1.23) and (1.29) imply
So we have finished the proof of Theorem 1.1.

Below we consider some consequences of Theorem 1.1.

Let be a triangular matrix, where the sequence is from and for every number is defined by the formula:

If , for every and (1.31) holds true, then the method is Cesaro summability method, and if , then the method coincides with convergence.

We introduce Cesaro summability method with variable orders, denoted by a symbol , which coincides with summability method defined by (1.31). Means of this method for series (1.3) we denoted by .

For we have the following.

Theorem 1.3. *Let a sequences be such that for some positive number we have
**
Then for any series (1.3) which satisfies the following condition:
**
an equality
**
holds.*

*Proof of Theorem 1.3. *Note that every satisfies condition (1.1) and condition (1.3). Indeed,
and , when .

For every we have
that is,
Therefore,
Note that the last one and (1.32) imply that
and if , we have
that is,
Note that implies the existence of such and natural , that if , then
that is, if , then
A consequence of (1.43) is that if (1.32) holds, then the matrix satisfies conditions of Theorem 1.1. This completes the proof of Theorem 1.3.

Theorem 1.3 directly implies the following.

Theorem 1.4. *Let be such sequence that
**
Then for every series (1.3) which satisfies
**
we have
*

#### 2. On Divergence of Fourier Series

It is well known the following.

Theorem A (Kolmogorov [1]). *There exists such summable function that Fourier trigonometric series of **
unboundedly diverges everywhere.*

Let be the Walsh system. Below we formulate Theorem B which is analogous of Theorem A and holds for Fourier-Walsh series.

Theorem B (Schipp [2, 3]). *There exists such summable function that Fourier-Walsh series of **
unboundedly diverges everywhere. *

Let be orthonormal functions system defined on , such that

Then below-mentioned theorem holds.

Theorem C (Bokarev [4]). *For every orthonormal system which satisfies (2.3), there exists such summable function defined on that its Fourier series constructed by system
**
unboundedly diverges in any point of some set with positive measure.*

Denote by , , and means of series (2.1), (2.2), and (2.4), respectively.

Theorem 1.3 implies that if satisfies (1.32), then Theorems A, B, and C hold for summability method.

Namely, the following Theorems hold true.

Theorem 2.1. *Let a sequence satisfies (1.32). Then there exists such summable function , that sequence unboundedly diverges everywhere.*

Theorem 2.2. *Let a sequence satisfies (1.32). Then there exists such summable function that sequence unboundedly diverges everywhere.*

Theorem 2.3. *If orthonormal system satisfies (2.3) and a sequence satisfies (1.32), then there exists such summable function , defined on , that sequence unboundedly diverges at every point of some set with positive measure.*

It is obvious that a consequence of Theorem 1.4 is that Theorems 2.1, 2.2, and 2.3 hold true if

*Remark 2.4. ** If every number ** will be replaced by ** in (1.31), then we get a summability method defined by ** matrix, which we call Riesz summability method with variable orders and denote it by symbol **.*

It can be proved analogously that Theorems 2.1, 2.2, and 2.3 remain true for Riesz summability method with variable orders, that is, for method, where satisfies (1.32).

#### Acknowledgment

This paper is supported by the Grant GNSF/STO9_23_3โ100.

#### References

- A. N. Kolmogorov, โUne serie de Fourier-Lebesgue divergente partout,โ
*Comptes Rendus de l'Acadรฉmie des Sciences*, vol. 183, pp. 1327โ1329, (1926). View at: Google Scholar - F. Schipp, โรber die Grรถssenordnung der Partialsummen der Entwicklung integrierbarer Funktionen nach
*W*-Systemen,โ*Acta Scientiarum Mathematicarum*, vol. 28, no. 1-2, pp. 123โ134, 1967. View at: Google Scholar - F. Schipp, โรber die divergenz der Walsh-Fourierreihen,โ
*Annales Universitatis Scientiarum Budapestinensis de Rolando Eรถtvรถs Nominatae. Sectio Mathematica*, vol. 12, pp. 49โ62, 1969. View at: Google Scholar | Zentralblatt MATH - S. V. Boฤkarev, โA Fourier series that diverges on a set of positive measure for an arbitrary bounded orthonormal system,โ
*Matematicheskii Sbornik*, vol. 98(140), no. 3(11), pp. 436โ449, 1975 (Russian). View at: Google Scholar

#### Copyright

Copyright ยฉ 2012 Shakro Tetunashvili. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.