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On strong law of large numbers and growth rate for a class of random variables
Journal of Inequalities and Applications volume 2013, Article number: 563 (2013)
Abstract
In this paper, we study the strong law of large numbers for a class of random variables satisfying the maximal moment inequality with exponent 2. Our results embrace the Kolmogorov strong law of large numbers and the Marcinkiewicz strong law of large numbers for this class of random variables. In addition, strong growth rate for weighted sums of this class of random variables is presented.
MSC:60F15.
1 Introduction
Let be a sequence of random variables defined on a fixed probability space . , , . Let and be sequences of constant with . Then is said to obey the general strong law of large numbers (SLLN) with norming constant if the normed weighted sums
holds. Note that the SLLN of the form (1.1) embraces the Kolmogorov SLLN (, ) and the Marcinkiewicz SLLN (, , ). When , fundamental results for the SLLN were obtained.
Under an independent assumption, many SLLNs for the weighted sums are obtained. One can refer to Adler and Rosalsky [1], Chow and Teicher [2], Fernholz and Teicher [3], Jamison et al. [4] and Teicher [5].
Under a pairwise independent assumption, Rosalsky [6] obtained some SLLNs for weighted sums of pairwise independent and identically distributed random variables. Sung [7] obtained sufficient conditions for (1.1) if is a sequence of pairwise independent random variables satisfying . Sung [8] presented the following result: a.s., where is a sequence of positive constants with and is a sequence of pairwise independent and identically distributed random variables.
For more details about strong limit theorems for dependent case, one can refer to Wu [9], Wu and Jiang [10], Hu et al. [11], Shen et al. [12], Zhou et al. [13] and Zhou [14], and so forth.
Recently Sung [15] gave the following definition.
Definition 1.1 (Sung [15])
A random variable sequence is said to satisfy the maximal moment inequality with exponent 2 if for all , there exists a constant C independent of n and m such that
We can see that a wide class of mean zero random variables satisfies (1.2). Inspired by Sung [7, 15], we establish SLLN of the form (1.1) for a class of random variables satisfying the maximal moment inequality with exponent 2.
The rest of the paper is organized as follows. In Section 2, some preliminary definition and lemmas are presented. In Section 3, main results and their proofs are provided.
Throughout the paper, let be the indicator function of the set A. C denotes a positive constant not depending on n, which may be different in various places. Let and be sequences of positive numbers, represents that there exists a constant such that for all n.
2 Preliminaries
The following lemmas and definition will be needed in this paper.
Lemma 2.1 (Sung [7])
Let be a sequence of random variables and put for . Assume that for some . Then
-
(i)
.
-
(ii)
.
-
(iii)
for any sequence satisfying .
Lemma 2.2 (Sung [15])
Let be a sequence of random variables satisfying the maximal moment inequality with exponent 2. If , then converges almost surely.
Definition 2.3 A random variable sequence is said to be stochastically dominated by a random variable X if there exists a constant C such that
for all and .
Lemma 2.4 Let be a sequence of random variables which is stochastically dominated by a random variable X. For any and , the following statement holds:
where C is a positive constant.
Lemma 2.5 (Hu [16])
Let be a nondecreasing unbounded sequence of positive numbers. Let be nonnegative numbers, and for . Let r be a fixed positive number. Assume that for each ,
If
then
and with the growth rate
where
And
If we further assume that for infinitely many n, then
Proof It follows from Corollary 2.1.1 of Hu [16] that (2.6)-(2.8) hold. By (2.3) and Theorem 1.1 of Fazekas and Klesov [17], we have
Therefore
following from the monotone convergence theorem of Rao [18]. Equation (2.11) follows from the proof of Lemma 1.2 of Hu and Hu [19]. □
3 Main results
Theorem 3.1 Let be a sequence of random variables and put for . Denote , , where p is a positive constant. Let and be sequences of positive numbers with . Suppose that satisfies the maximal moment inequality with exponent 2. Assume that the following two conditions hold:
If , then
Proof By Lemma 2.1(i),
Therefore follows from the Borel-Cantelli lemma and (3.4). Thus (3.3) is equivalent to the following:
So, in order to prove (3.3), we need only to prove
and
Firstly, we prove (3.6). In view of Lemma 2.1(i), (ii) and (3.2), we have
Thus it follows by Lemma 2.2 that
By Kronecker’s lemma, we can obtain (3.6) immediately.
Secondly, we prove (3.7). By (3.1) and Lemma 2.1(iii), we can get
By (3.2) and Lemma 2.1(i), we have
and
Thus it follows by Kronecker’s lemma that
and
Therefore,
follows from (3.9)-(3.11). Hence the result is proved. □
Theorem 3.2 Let be a sequence of mean zero random variables, which is stochastically dominated by a random variable X. Let and be sequences of positive numbers with . Put for , . Denote , , and suppose that satisfies the maximal moment inequality with exponent 2. Assume that the following two conditions hold:
Then
Proof Let . By (3.13), we can see that as . By (3.12) and (3.13),
which implies from the Borel-Cantelli lemma. So, in order to prove (3.14), we need only to prove
By (3.12), (3.13), Lemma 2.4, and the proof of (3.15), we have
Combining Lemma 2.2, (3.17) and Kronecker’s lemma, we can get
To complete the proof of (3.16), it suffices to show that
By (3.12), (3.13) and , it follows that
Observe that
So, we can get
from (3.15), (3.20) and (3.21). Consequently,
which implies (3.19) from Kronecker’s lemma. We complete the proof of theorem. □
Theorem 3.3 Let be a sequence of mean zero random variables satisfying the maximal moment inequality with exponent 2. Denote , and . For , assume that
Then
and with the growth rate
where
And
If we further assume that for infinitely many n, then
In addition, for any ,
Proof Since is a sequence of mean zero random variables satisfying the maximal moment inequality with exponent 2, we have
And we can obtain for all from its definition. Denote and , . By (3.23), we can get
Thus (2.4) holds. It follows from Remark 2.1 in [16] that (2.4) implies (2.5). By Lemma 2.5, we can get (3.24)-(3.29) immediately. It follows from (3.28) that
The proof is completed. □
Remark 3.4 It is easy to see that a wide class of mean zero random variables satisfies the maximal moment inequality with exponent 2. Examples include independent random variables, negatively associated random variables (see Matula [20]), negatively superadditive dependent random variables (see Shen et al. [12]), φ-mixing random variables and AANA random variables (see Wang et al. [21, 22]), and -mixing random variables (see Utev et al. [23]). So Theorems 3.1-3.3 hold for this wide class of random variables.
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Acknowledgements
The authors are most grateful to the editor and the anonymous referee for their careful reading and insightful comments. This work is supported by the National Natural Science Foundation of China (11171001, 11201001), Natural Science Foundation of Anhui Province (1208085QA03), Humanities and Social Sciences Project from Ministry of Education of China (12YJC91007), Key Program of Research and Development Foundation of Hefei University (13KY05ZD) and Doctoral Research Start-up Funds Projects of Anhui University.
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Shen, Y., Yang, J. & Hu, S. On strong law of large numbers and growth rate for a class of random variables. J Inequal Appl 2013, 563 (2013). https://doi.org/10.1186/1029-242X-2013-563
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DOI: https://doi.org/10.1186/1029-242X-2013-563