Abstract
In this paper we discuss some convergence and divergence properties of subsequences of Cesàro means with varying parameters of Walsh–Fourier series. We give necessary and sufficient conditions for the convergence regarding the weighted variation of numbers.
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1 Walsh functions
We shall denote the set of all nonnegative integers by \(\mathbb {N}\), the set of all integers by \(\mathbb {Z}\) and the set of dyadic rational numbers in the unit interval \(\mathbb {I}:=[0,1)\) by \(\mathbb {Q}\). In particular, each element of \(\mathbb {Q}\) has the form \(\frac{p}{2^{n}}\) for some \(p,n\in \mathbb {N}\), \(0\le p\le 2^{n}\).
Denote the dyadic enpansion of \(n\in \mathbb {N}\) and \(x\in \mathbb {I}\) by
and
Denote by \(\dotplus \) the logical addition on \(\mathbb {I}\). That is, for any \(x,y\in \mathbb {I}\) and \(k,n\in \mathbb {N}\),
and by the definition of \(w_{n}\) we have
The sets \(I_{n}\left( x\right) :=\left\{ y\in \mathbb {I}:y_{0}=x_{0},\ldots ,y_{n-1}=x_{n-1}\right\} \) for \(x\in \mathbb {I},I_{n}:=I_{n}\left( 0\right) \) for \(0<n\in \mathbb {N}\) and \(I_{0}\left( x\right) :=\mathbb {I}\) are the dyadic intervals of \(\mathbb {I}\). For \(0<n\in \mathbb {N}\) let \(\left| n\right| :=\max \left\{ j\in \mathbb {N}:n_{j}\ne 0\right\} \), that is, \(2^{\left| n\right| }\le n<2^{\left| n\right| +1}.\)
The Rademacher system is defined by
The Walsh–Paley system is defined as the sequence of the Walsh–Paley functions:
The Walsh–Dirichlet kernel is defined by
Recall that (see [23])
As usual, denote by \(L_{1}(\mathbb {I})\) the set of measurable functions defined on \(\mathbb {I}\) for which
Let \(f\in L_{1}(\mathbb {I})\). The partial sums of the Walsh–Fourier series are defined as follows:
where the number
is said to be the ith Walsh–Fourier coefficient of the function f. Set
The maximal function is defined by
2 Cesàro means with varying parameters
The \(\left( C,\alpha _{n}\right) \) means of the Walsh–Fourier series of the function f is given by
where
for any \(n\in \textbf{N},\alpha _n \ne -1,-2,\ldots \). It is known that [29]
The \(\left( C,\alpha _{n}\right) \) kernel is defined by
The idea of Cesàro means with variable parameters of numerical sequences is due to Kaplan [17] and the introduction of these \(\left( C,\alpha _{n}\right) \) means of Fourier series is due to Akhobadze [2, 3] who investigated the behavior of the \(L_{1}\)-norm convergence of \(\sigma _{n}^{\alpha _{n}}\left( f\right) \rightarrow f\) for the trigonometric system.
The a.e. divergence of Cesàro means with varying parameters of Walsh–Fourier series was investigated by Tetunashvili [25]. Abu Joudeh and Gát in [1] proved the almost everywhere convergence (with some restrictions) of the Cesàro \(\left( C,\alpha _{n}\right) \) means of integrable functions.
The first result with respect to the a.e. convergence of the Walsh–Fejér means \(\sigma _{n}^{\alpha _{n}}f\) for all integrable function f with constant sequence \(\alpha _{n}=\alpha >0\) is due to Fine [6] (see also Weisz [27]). On the rate of convergence of Cesàro means in this constant case see the papers of Yano [28] and Fridli [8]. Approximation properties of Cesàro means of negative order with constant sequence was investigated by the second author in [14]. That is why we investigate only the case when \(\alpha _n\in (0,1)\) for every n. It is true that in order for the definition of Cesàro mean to make sense we should only suppose that the paremeter is not a negative integer, but for negative parameters there are available divergence results (see, e.g., [14]) and for parameters at least 1 we have the very well-known almost everywhere and norm convergence results of these means. In other words, the only interesting situation is when \(\alpha _n\in (0,1)\) for every n.
It is easy to see that
The Fejér kernel is defined by
The following estimate was proved by Akhobadze [2, 3]. If \(k,n\in \mathbb {N}\), then
3 \(L_{1}\)-estimation for the kernel \(K_{n}^{\alpha _{n}}\)
For \(n=\sum \limits _{j=0}^{\infty }\varepsilon _{j}\left( n\right) 2^{j},\varepsilon _{j}\left( n\right) =0,1\), we define
Theorem 3.1
If \(\alpha _{n}\in (0,1)\) and \(n\in \mathbb {N}\), then there exist positive constants \(c_{1}\) and \(c_{2}\) (independent of n) such that
Proof of Theorem 3.1
We can write
Since in the case of \(\varepsilon _s(n)=1\)
and
from (1.2) and (3.1) we obtain
Applying Abel’s transformation twice we get
Combining (3.2)–(3.3) we obtain
From the estimates (2.2)–(2.4) we get
Since \(\sup \limits _{n}\left\| K_{n}\right\| _1 <2\) (see [24]; it even holds \(\sup \limits _{n}\left\| K_{n}\right\| _1 = 17/15\), see [26]) from (3.4) we infer
Next, we find an upper estimate for \(\left\| Q_{3}(n) \right\| _{1}\). We have
From (1.2) (we can suppose \(\varepsilon _s(n)=1\) otherwise nothing is to be added), we get
Since \(n_{\left( s-1\right) \oplus \left( 2^{s}-1\right) }<2^{s}\), from (1.3) we obtain
Consequently,
From (2.2)–(2.4) and (1.3) we get
Combining (3.4)–(3.8) we conclude that
Hence the upper estimate is proved.
Now, we find a lower estimate for \(\left\| Q_{31}(n) \right\| _{1}\). Let \(a_{i}\) and \(b_{i},i=1,\ldots ,s\), be strictly increasing sequences, i.e.,
for which
Then it is evident that \(b_{j}+1\le a_{j+1}\). But we may suppose even more, namely
and then the intervals defined below are disjoint sets. That is, set
Let \(x\in A_{k}\). Then we can write
Since
we can write
Consequently, from (3.12) we obtain
Hence,
Let \(x\in B_{k}\). Since \(n_{\left( b_{j}+1\right) }=n_{\left( b_{j}\right) }\) and \(n_{\left( a_{j}\right) }\le n_{\left( b_{j}\right) }\), we have
Hence
Since \(A_{i},B_{i}\), \(i=1,\ldots ,s\), are pairwise disjoint from (3.13) and (3.14), we have
Combining (3.5)–(3.14) we complete the proof of Theorem 3.1. \(\square \)
4 Uniform and L-convergence of \(\left( C,\alpha _{n}\right) \) means
The Hardy space \(H_{1}(\mathbb {I})\) consists all functions f that satisfy
For a nonnegative integer n let
and
Let \(C_{w}(\mathbb {I})\) denote the space of uniformly continuous functions on \(\mathbb {I}\) with the supremum norm
Let \(X=X(\mathbb {I})\) be either the space \(L_{1}\left( \mathbb {I }\right) \), or the space of uniformly continuous functions, that is, \( C_{w}(\mathbb {I})\). The corresponding norm is denoted by \( \left\| \cdot \right\| _{X}\).
We remind the reader that \(C_{w}(\mathbb {I})\) is the collection of all functions \(f:\mathbb {I}\rightarrow \mathbb {R}\) that are uniformly continuous from the dyadic topology of \(\mathbb {I}\) to the usual topology \( \mathbb {R}\), or for short: uniformly w-continuous.
The modulus of continuity, when \(X=C_{w}(\mathbb {I})\), and the integrated modulus of continuity, where \(X=L_{1}(\mathbb {I})\) are defined by
For Walsh–Fourier series Fine [5] has obtained a sufficient condition for the uniform convergence which is in complete analogy with the Dini–Lipshitz condition (see also [23]). Similar results are true for the space of integrable functions \(L_{1}(\mathbb {I})\) [22]. Gulicev [16] has estimated the rate of uniform convergence of a Walsh–Fourier series using the Lebesgue constant and the modulus of continuity. Uniform convergence of Walsh–Fourier series of the functions of classes of generalized bounded variation was investigated by the second author [13]. This problem has been considered for the Vilenkin group by Fridli [7] and Gát [9]. Lukomskii [19] considered uniform and \(L_{1}\) -convergence of subsequences of partial sums of Walsh–Fourier series. In particular, he proved that the condition \(\sup \limits _{A}V( m_{A}) <\infty \) is necessary and sufficient for the uniform and \(L_{1}\)-convergence of subsequences of partial sums \(S_{m_{A}}(f) \) of Walsh–Fourier series. In [4, 10,11,12, 15, 18,19,20,21] the X -norm convergence of subsequences of Walsh–Fourier series is investigated.
In this section we discuss some convergence and divergence properties of subsequences of Cesàro means with varying parameters of Walsh–Fourier series. The following are true.
Theorem 4.1
If \(f\in X(\mathbb {I})\) and \(\alpha _{n}\in \left( 0,1\right) \), then
Theorem 4.1 implies
Theorem 4.2
If for a function \(f\in X(\mathbb {I})\) and a subsequence \(\left\{ m_{n}:n\in \mathbb {N}\right\} \) we have
then the subsequence \(\sigma _{m_{n}}^{\alpha _{n}}(f)\) converges in X-norm.
Theorem 4.3
Let \(\left\{ m_{n}:n\in \mathbb {N}\right\} \) be such that
Then there exists \(\left\{ p_{k}:k\in \mathbb {N}\right\} \ \)and a function \( g\in X(\mathbb {I})\) such that
and
Theorem 4.4
a) Let \(\left\{ m_{n}:n\in \mathbb {N}\right\} \) be such that
Then the operator \(\sigma _{m_{n}}^{\alpha _{n}}(f)\) is bounded from the space \(L_{1}(\mathbb {I})\) to the space \(L_{1}(\mathbb {I})\);b) Let \(\left\{ m_{n}:n\in \mathbb {N}\right\} \) be such that
Then there exists \(f\in H_{1}(\mathbb {I})\subset L_{1}( \mathbb {I}) \) such that
Proof of Theorem 4.1
We can write
For \(J_{3}\) we have
Now we can write
From Minkowski’s inequality and Theorem 3.1 we get
Analogously, we can prove
Combining (4.2)–(4.5) we obtain
We can write
Using Minkowski’s inequality we get
Since
and
from (2.2)–(2.4) we get \(\left( r<|n|-1\right) \)
Consequently,
Analogously, we can prove
Finally, we estimate \(J_{2}\). Since \( D_{2^{m}-j}=D_{2^{m}}-w_{2^{m}-1}D_{j},j=0,1,\ldots ,2^{m}-1\), we have
Hence,
Combining (4.1), (4.6), (4.10) and (4.11) we complete the proof of Theorem 4.1. \(\square \)
Proof of Theorem 4.3
Since \(\sup \limits _{n}V(m_{n},\alpha ) =\infty \), there exists \( \left\{ p_{k}:k\in \mathbb {N}\right\} \) such that
At first we consider the case \(X(\mathbb {I})=L_{1}\left( \mathbb {I}\right) \). We set
If \(y\in I_{|m_{p_{k}}|}\), then for \(l=1,2,\ldots ,k-1\) we obtain
Consequently,
Further,
Since for \(j\ge k\)
from Theorem 3.1 we obtain
Using Theorem 4.1, from (4.12) and (4.14) we obtain for \( j<k\)
Consequently,
Combining (4.15)–(4.18) we obtain
Now, we discuss the case \(X(\mathbb {I})=C_{w}\left( \mathbb {I} \right) \). Let the conditions (4.12) and (4.13) be satisfied. We set
where
It is evident that \(h_{0}\in C_{w}(\mathbb {I})\). If \(y\in I_{|m_{p_{k}}|}\), then for \(j=1,2,\ldots ,k-1\) we obtain
and from (4.13) we get
Hence
Clearly
Using Theorem 3.1, from (4.12) and (4.13) we obtain
Combining (4.19)–(4.23) we complete the proof of Theorem 4.3. \(\square \)
Proof of Theorem 4.4
The validity of part a) immediately follows from Theorem 3.1. Now, we prove part b). Since \(\sup \limits _{a}V\left( m_{a},\alpha \right) =\infty \), without loss of generality we can suppose
Set
where
and
We can write
Hence,
and
Applying (1.3) and (4.24) we conclude that
Hence \(f\in H_{1}(\mathbb {I})\).
We can write
Let \(j>a\). Then it is easy to see that
Let \(j<a\). Then from Theorem 4.1 we have
Finally, we estimate \(\sigma _{m_{a}}^{\alpha _{a}}\left( f_{a}\right) \). Let \(m_{a}=2^{|m_{a}|}+m_{a}^{\prime }, 0\le m_{a}^{\prime }<2^{|m_{a}|}\). Then we can write
and consequently, from Theorem 3.1 and by (2.2)–(2.4), we get
Combining (4.25)–(4.28) we conclude that
\(\square \)
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The authors are deeply indebted to the anonymous referee for his/her valuable help.
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György Gát is supported by the project EFOP-3.6.1-16-2016-00022 and EFOP-3.6.2-16-2017-00015 supported by the European Union, co-financed by the European Social Fund. Ushangi Goginava is very thankful to the United Arab Emirates University (UAEU) for the Start-up Grant 12S100.
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Gát, G., Goginava, U. Cesàro means with varying parameters of Walsh–Fourier series. Period Math Hung 87, 57–74 (2023). https://doi.org/10.1007/s10998-022-00499-x
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DOI: https://doi.org/10.1007/s10998-022-00499-x