Abstract
In this paper, we introduce the concepts of lacunary statistical τ-convergence, lacunary statistically τ-bounded and lacunary statistically τ-Cauchy in the framework of locally solid Riesz spaces. We also define a new type of convergence, that is, -convergence in this setup and prove some interesting results related to these notions.
MSC:40A35, 40G15, 46A40.
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1 Introduction and preliminaries
In 1951 Fast [1] presented the following definition of statistical convergence for sequences of real numbers. We shall denote by ℕ the set of all natural numbers. Let and . Then the natural density of K is defined by if the limit exists, where the vertical bars indicate the number of elements in the enclosed set. The sequence is said to be statistically convergent to L if for every , the set has natural density zero (cf. Fast [1]), i.e., for each ,
In this case, we write . Note that every convergent sequence is statistically convergent, but not conversely. For example, suppose that the sequence is defined as
It is clear that the sequence is statistically convergent to 0, but it is not convergent.
In 1985 Fridy [2] presented the notion of a statistically Cauchy sequence and proved that it is equivalent to statistical convergence. Active research on this topic was started after the papers of Fridy. Mursaleen and Edely [3] extended these concepts from single sequences to double sequences by using two dimensional analogue of natural density. In the recent past, Mursaleen and Mohiuddine [4, 5] defined these notions for double sequences in locally solid Riesz spaces as well as in intuitionistic fuzzy normed spaces and proved some interesting results. Subsequently, the statistical convergence for sequences of real numbers in several spaces has been extensively investigated by a number of authors, and there are many interesting results concerning this concept. For more details related to this concept, we refer to [6–24] and references therein.
Now, we recall some basic definitions and notions related to the concept of locally solid Riesz spaces. Let X be a real vector space and ≤ be a partial order on this space. Then X is said to be an ordered vector space if it satisfies the following properties:
-
(i)
if and , then for each .
-
(ii)
if and , then for each .
If in addition X is a lattice with respect to the partial order ≤, then X is said to be a Riesz space (or a vector lattice) [25].
For an element x of a Riesz space X, the positive part of x is defined by , the negative part of x by and the absolute value of x by , where θ is the zero element of X.
A subset S of a Riesz space X is said to be solid if and implies .
A topological vector space is a vector space X, which has a (linear) topology τ, such that the algebraic operations of addition and scalar multiplication in X are continuous. Continuity of addition means that the function defined by is continuous on , and continuity of scalar multiplication means that the function defined by is continuous on .
Every linear topology τ on a vector space X has a base for the neighborhoods of θ satisfying the following properties:
() Each is a balanced set, that is, holds for all and every with .
() Each is an absorbing set, that is, for every , there exists such that .
() For each , there exists some with .
A linear topology τ on a Riesz space X is said to be locally solid [26] if τ has a base at zero consisting of solid sets. A locally solid Riesz space is a Riesz space equipped with a locally solid topology τ.
The rest of the paper is organized as follows. In Section 2, first we recall the notion of lacunary sequences and define the concepts of lacunary statistically τ-convergent and lacunary statistically τ-bounded and prove some interesting results. Section 3 is devoted to introduce concept of lacunary statistically τ-Cauchy and to proving that a lacunary statistically τ-convergent sequence is lacunary statistically τ-Cauchy. Also, we define -convergent and prove that it is equivalent to lacunary statistically τ-convergent for a first countable space.
2 Lacunary statistical τ-convergence
By a lacunary sequence, we mean an increasing integer sequence such that and as . Throughout this paper, the intervals determined by θ will be denoted by , and the ratio will be abbreviated by .
Let . The number
is said to be the θ-density of K, provided the limit exists.
In 1993 Fridy and Orhan [27] defined the concept of lacunary statistical convergence as follows.
Let θ be a lacunary sequence. Then a sequence is said to be -convergent to the number L if for every , the set has θ-density zero, where
In this case, we write or .
Remarks 2.1 It is well known that every convergent sequence is lacunary statistically convergent, but the converse is not true. For example, let the sequence be defined by
Then x is lacunary statistically convergent to 0, but it is not convergent.
We shall assume throughout this paper that the symbol will denote any base at zero consisting of solid sets and satisfying the conditions (), () and () in a locally solid topology. For our convenience, here and in what follows, we shall write an LSR-space instead of a locally solid Riesz space.
Definition 2.2 Let be an LSR-space and θ be a lacunary sequence. Then a sequence in X is said to be lacunary statistically τ-convergent (or -convergent) to the element if for every τ-neighborhood U of zero, , where , i.e.,
In this case, we write or .
Definition 2.3 Let be an LSR-space and θ be a lacunary sequence. We say that a sequence in X is lacunary statistically τ-bounded (or -bounded) if for every τ-neighborhood U of zero there exists some such that the set has θ-density zero (shortly, ), i.e.,
Theorem 2.4 Let be a Hausdorff LSR-space and θ be a lacunary sequence. Suppose that and are two sequences in X. Then the following hold:
-
(i)
If and , then .
-
(ii)
If , then , .
-
(iii)
If and , then .
Proof (i) Suppose that and . Let U be any τ-neighborhood of zero. Then there exists such that . Choose any such that . We define the following sets:
Since and , we have . Thus , and in particular . Now, let . Then
Hence, for every τ-neighborhood U of zero, we have . Since is Hausdorff, the intersection of all τ-neighborhoods U of zero is the singleton set . Thus, we get , i.e. .
-
(ii)
Let U be an arbitrary τ-neighborhood of zero and . Then there exists such that and also
Since Y is balanced, implies for every with . Hence,
Thus, we obtain
for each τ-neighborhood U of zero. Now, let and be the smallest integer greater than or equal to . There exists such that . Since , the set
has θ-density zero. Therefore,
Since the set Y is solid, we have . This implies that . Thus,
for each τ-neighborhood U of zero. Hence, .
-
(iii)
Let U be an arbitrary τ-neighborhood of zero. Then there exists such that . Choose E in such that . Since and , we have , where
Let . Hence, we have and
Therefore,
Since U is arbitrary, we have . □
Theorem 2.5 Let be an LSR-space and θ be a lacunary sequence. If a sequence is lacunary statistically τ-convergent, then it is lacunary statistically τ-bounded.
Proof Suppose is lacunary statistically τ-convergent to the point and let U be an arbitrary τ-neighborhood of zero. Then there exists such that . Let us choose such that . Since , the set
has θ-density zero. Since E is absorbing, there exists such that . Let α be such that and . Since E is solid and , we have . Since E is balanced, implies . Then we have
for each . Thus,
Hence, is lacunary statistically τ-bounded. □
Theorem 2.6 Let be an LSR-space and θ be a lacunary sequence. If , and are three sequences such that
-
(i)
, for all ,
-
(ii)
,
then .
Proof Let U be an arbitrary τ-neighborhood of zero, there exists such that . Choose such that . From the condition (ii), we have , where
Also, we get , and from (i), we have
for all . This implies that for all , we get
Since Y is solid, we have . Thus,
for each τ-neighborhood U of zero. Hence, . □
3 Lacunary statistically τ-Cauchy and -convergence
In the present section, first we define the concept of lacunary statistically τ-Cauchy in locally solid Riesz spaces as follows.
Definition 3.1 Let be an LSR-space and θ be a lacunary sequence. A sequence in X is lacunary statistically τ-Cauchy if for every τ-neighborhood U of zero there exists such that
Theorem 3.2 Let be an LSR-space and θ be a lacunary sequence. If a sequence is lacunary statistically τ-convergent, then it is lacunary statistically τ-Cauchy.
Proof Suppose that . Let U be an arbitrary τ-neighborhood of zero, there exists such that . Choose such that . Then
Also, we have
for all , where
Therefore, the set
For every τ-neighborhood U of zero, there exists such that for all ,
Hence, is lacunary statistically τ-Cauchy. □
Now, we define another type of convergence in locally solid Riesz spaces.
Definition 3.3 Let θ be a lacunary sequence. A sequence in an LSR-space is said to be -convergent to if there exists an index set , , with such that . In this case, we write .
Theorem 3.4 Let θ be a lacunary sequence. A sequence is lacunary statistically τ-convergent to a number ξ if it is -convergent to ξ in a locally solid Riesz space .
Proof Let U be an arbitrary τ-neighborhood of ξ. Since is -convergent to ξ, there is an index set , , with and , such that and imply . Then
Therefore,
Hence, x is lacunary statistically τ-convergent to ξ. □
Note that the converse holds for a first countable space.
Recall that a first countable space is a topological space satisfying the ‘first axiom of countability’. Specifically, a space X is said to be first countable if each point has a countable neighborhood basis (local base). That is, for each point x in X there exists a sequence of open neighborhoods of x such that for any open neighborhood V of x there exists an integer i with contained in V.
Theorem 3.5 Let be a first countable LSR-space and θ be a lacunary sequence. If a sequence is lacunary statistically τ-convergent to a number ξ, then it is -convergent to ξ.
Proof Let x be lacunary statistically τ-convergent to a number ξ. Fix a countable local base at ξ. For every , put
and
Then and
-
(1)
and
-
(2)
, .
Now, we have to show that for , is convergent to ξ. Suppose that is not convergent to ξ. Therefore, for infinitely many terms. Let
Then
-
(3)
,
and by (1), . Hence, , which contradicts (2). Therefore, is convergent to ξ. Hence, by Definition 3.3, x is -convergent to ξ. □
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Mohiuddine, S.A., Alghamdi, M.A. Statistical summability through a lacunary sequence in locally solid Riesz spaces. J Inequal Appl 2012, 225 (2012). https://doi.org/10.1186/1029-242X-2012-225
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DOI: https://doi.org/10.1186/1029-242X-2012-225