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Complete convergence for weighted sums of arrays of rowwise -mixing random variables
Journal of Inequalities and Applications volume 2013, Article number: 356 (2013)
Abstract
Let be an array of rowwise -mixing random variables. Some sufficient conditions for complete convergence for weighted sums of arrays of rowwise -mixing random variables are presented without assumptions of identical distribution. As applications, the Baum and Katz type result and the Marcinkiewicz-Zygmund type strong law of large numbers for sequences of -mixing random variables are obtained.
MSC:60F15.
1 Introduction
The concept of complete convergence was introduced by Hsu and Robbins [1] as follows. A sequence of random variables is said to converge completely to a constant C if for all . In view of the Borel-Cantelli lemma, this implies that almost surely (a.s.). The converse is true if the are independent. Hsu and Robbins [1] proved that the sequence of arithmetic means of independent and identically distributed (i.i.d.) random variables converges completely to the expected value if the variance of the summands is finite. Erdös [2] proved the converse. The result of Hsu-Robbins-Erdös is a fundamental theorem in probability theory and has been generalized and extended in several directions by many authors. See, for example, Spitzer [3], Baum and Katz [4], Gut [5], Zarei [6], and so forth. The main purpose of the paper is to provide complete convergence for weighted sums of arrays of rowwise -mixing random variables.
Firstly, let us recall the definitions of sequences of -mixing random variables and arrays of rowwise -mixing random variables.
Let be a sequence of random variables defined on a fixed probability space . Write . Given two σ-algebras ℬ, ℛ in ℱ, let
Define the -mixing coefficients by
Obviously, , and .
Definition 1.1 A sequence of random variables is said to be a -mixing sequence if there exists such that .
An array of random variables is called rowwise -mixing random variables if for every , is a sequence of -mixing random variables.
-mixing random variables were introduced by Bradley [7] and many applications have been found. -mixing is similar to ρ-mixing, but both are quite different. Many authors have studied this concept and provided interesting results and applications. See, for example, Bryc and Smolenski [8], Peligrad [9, 10], Peligrad and Gut [11], Utev and Peligrad [12], Gan [13], Cai [14], Zhu [15], Wu and Jiang [16, 17], An and Yuan [18], Kuczmaszewska [19], Sung [20], Wang et al. [21–23], and so on.
Recently, An and Yuan [18] obtained a complete convergence result for weighted sums of identically distributed -mixing random variables as follows.
Theorem 1.1 Let and . Let be a sequence of identically distributed -mixing random variables with . Assume that is an array of real numbers satisfying
Then the following statements are equivalent:
-
(i)
;
-
(ii)
for all .
Sung [20] pointed out that the array satisfying both (1.1) and (1.2) does not exist and obtained a new complete convergence result for weighted sums of identically distributed -mixing random variables as follows.
Theorem 1.2 Let and . Let be a sequence of identically distributed -mixing random variables with . Assume that is an array of real numbers satisfying
If , then
Conversely, if (1.4) holds for any array satisfying (1.3), then .
For more details about the complete convergence result for weighted sums of dependent sequences, one can refer to Wu [24, 25], Wang et al. [26, 27], and so forth. The main purpose of this paper is to further study the complete convergence for weighted sums of arrays of rowwise -mixing random variables under mild conditions. The main idea is inspired by Baek et al. [28] and Wu [25]. As applications, the results of Baum and Katz [4] from the i.i.d. case to the arrays of rowwise -mixing setting are obtained. The Marcinkiewicz-Zygmund type strong law of large numbers for sequences of -mixing random variables is provided. We give some sufficient conditions for complete convergence for weighted sums of arrays of rowwise -mixing random variables without assumption of identical distribution. The techniques used in the paper are the Rosenthal type inequality and the truncation method.
Throughout this paper, the symbol C denotes a positive constant which is not necessarily the same one in each appearance and denotes the integer part of x. For a finite set A, the symbol ♯A denotes the number of elements in the set A. Let be the indicator function of the set A. Denote , and .
The paper is organized as follows. Two important lemmas are provided in Section 2. The main results and their proofs are presented in Section 3. We get complete convergence for arrays of rowwise -mixing random variables which are stochastically dominated by a random variable X.
2 Preliminaries
Firstly, we give the definition of stochastic domination.
Definition 2.1 A sequence of random variables is said to be stochastically dominated by a random variable X if there exists a positive constant C such that
for all and .
An array of rowwise random variables is said to be stochastically dominated by a random variable X if there exists a positive constant C such that
for all , and .
The proofs of the main results of the paper are based on the following two lemmas. One is the classic Rosenthal type inequality for -mixing random variables obtained by Utev and Peligrad [12], the other is the fundamental inequalities for stochastic domination.
Lemma 2.1 (cf. Utev and Peligrad [[12], Theorem 2.1])
Let be a sequence of -mixing random variables, , for some and for every . Then there exists a positive constant C depending only on p such that
Lemma 2.2 Let be an array of rowwise random variables which is stochastically dominated by a random variable X. For any and , the following two statements hold:
where and are positive constants.
Proof The proof of this lemma can be found in Wu [29] or Wang et al. [30]. □
3 Main results and their applications
In this section, we provide complete convergence for weighted sums of arrays of rowwise -mixing random variables. As applications, the Baum and Katz type result and the Marcinkiewicz-Zygmund type strong law of large numbers for sequences of -mixing random variables are obtained. Let be an array of rowwise -mixing random variables. We assume that the mixing coefficients in each row are the same.
Theorem 3.1 Let be an array of rowwise -mixing random variables which is stochastically dominated by a random variable X and for all , , . Let be an array of constants such that
and
Assume further that and there exists some such that and . If , then for all ,
If and , then (3.3) still holds for all .
Proof Without loss of generality, we assume that for all and (otherwise, we use and instead of respectively, and note that ). From the conditions (3.1) and (3.2), we assume that
If and , then the result can be easily proved by the following:
In the following, we consider the case of . Denote
It is easy to check that for any ,
which implies that
Firstly, we show that
Actually, by the conditions , Lemma 2.2, (3.4) and (since ), we have that
which implies (3.6). It follows from (3.5) and (3.6) that for n large enough,
Hence, to prove (3.3), we only need to show that
and
By (3.4) and , we can get that
which implies (3.7).
By Markov’s inequality, Lemma 2.1, ’s inequality and Jensen’s inequality, we have for that
Take
which implies that and . Since , we have by Lemma 2.2, Markov’s inequality and (3.4) that
By Lemma 2.2 again, we can see that
has been proved by (3.7). In the following, we show that . Denote
It is easily seen that for and for all . Hence,
It is easily seen that for all , we have that
which implies that for all ,
Therefore,
and
Thus, the inequality (3.8) follows from (3.9)-(3.11), (3.13), (3.15) and (3.16). This completes the proof of the theorem. □
Theorem 3.2 Let be an array of rowwise -mixing random variables which is stochastically dominated by a random variable X and for all , . Let be an array of constants such that (3.1) holds and
If , then for all ,
Proof We use the same notations as those in Theorem 3.1. According to the proof of Theorem 3.1, we only need to show that (3.7) and (3.8) hold, where and .
The fact yields that
which implies (3.7) for .
By Markov’s inequality, Lemmas 2.1 and 2.2, we can get that
Here, and are and when , respectively. Similar to the proof of , we can get that
Similar to the proof of , we have
This completes the proof of the theorem from the statements above. □
By Theorems 3.1 and 3.2, we can extend the results of Baum and Katz [4] for independent and identically distributed random variables to the case of arrays of rowwise -mixing random variables as follows.
Corollary 3.1 Let be an array of rowwise -mixing random variables which is stochastically dominated by a random variable X and for all , .
-
(i)
Let and . If , then for all ,
(3.22) -
(ii)
If , then for all ,
(3.23)
Proof (i) Let if and if . Hence, conditions (3.1) and (3.2) hold for and . . It is easy to check that
Therefore, the desired result (3.22) follows from Theorem 3.1 immediately.
-
(ii)
Let if and if . Hence, conditions (3.1) and (3.17) hold for . Therefore, the desired result (3.23) follows from Theorem 3.2 immediately. This completes the proof of the corollary. □
Similar to the proofs of Theorems 3.1-3.2 and Corollary 3.1, we can get the following Baum and Katz type result for sequences of -mixing random variables.
Theorem 3.3 Let be a sequence of -mixing random variables which is stochastically dominated by a random variable X and for .
-
(i)
Let and . If , then for all ,
(3.24) -
(ii)
If , then for all ,
(3.25)
By Theorem 3.3, we can get the Marcinkiewicz-Zygmund type strong law of large numbers for -mixing random variables as follows.
Corollary 3.2 Let be a sequence of -mixing random variables which is stochastically dominated by a random variable X and for .
-
(i)
Let and . If , then
(3.26) -
(ii)
If , then
(3.27)
Proof (i) By (3.24), we can get that for all ,
By Borel-Cantelli lemma, we obtain that
For all positive integers n, there exists a positive integer such that . We have by (3.28) that
which implies (3.26).
-
(ii)
Similar to the proof of (i), we can get (ii) immediately. The details are omitted. This completes the proof of the corollary. □
Remark 3.1 We point out that the cases and are considered in Theorem 3.1 and the case is considered in Theorem 3.2, respectively. Theorem 3.1 and Theorem 3.2 consider the complete convergence for weighted sums of arrays of rowwise -mixing random variables, while Theorem 3.3 considers the complete convergence for weighted sums of sequences of -mixing random variables. In addition, Theorem 3.1 and Theorem 3.2 could be applied to obtain the Baum and Katz type result for arrays of rowwise -mixing random variables, while Theorem 3.3 could be applied to establish the Marcinkiewicz-Zygmund type strong law of large numbers for sequences of -mixing random variables.
References
Hsu PL, Robbins H: Complete convergence and the law of large numbers. Proc. Natl. Acad. Sci. USA 1947, 33(2):25–31. 10.1073/pnas.33.2.25
Erdös P: On a theorem of Hsu and Robbins. Ann. Math. Stat. 1949, 20(2):286–291. 10.1214/aoms/1177730037
Spitzer FL: A combinatorial lemma and its application to probability theory. Trans. Am. Math. Soc. 1956, 82(2):323–339. 10.1090/S0002-9947-1956-0079851-X
Baum LE, Katz M: Convergence rates in the law of large numbers. Trans. Am. Math. Soc. 1965, 120(1):108–123. 10.1090/S0002-9947-1965-0198524-1
Gut A: Complete convergence for arrays. Period. Math. Hung. 1992, 25(1):51–75. 10.1007/BF02454383
Zarei H, Jabbari H: Complete convergence of weighted sums under negative dependence. Stat. Pap. 2011, 52(2):413–418. 10.1007/s00362-009-0238-4
Bradley RC: On the spectral density and asymptotic normality of weakly dependent random fields. J. Theor. Probab. 1992, 5: 355–374. 10.1007/BF01046741
Bryc W, Smolenski W: Moment conditions for almost sure convergence of weakly correlated random variables. Proc. Am. Math. Soc. 1993, 119(2):629–635. 10.1090/S0002-9939-1993-1149969-7
Peligrad M: On the asymptotic normality of sequences of weak dependent random variables. J. Theor. Probab. 1996, 9(3):703–715. 10.1007/BF02214083
Peligrad M: Maximum of partial sums and an invariance principle for a class of weak dependent random variables. Proc. Am. Math. Soc. 1998, 126(4):1181–1189. 10.1090/S0002-9939-98-04177-X
Peligrad M, Gut A: Almost sure results for a class of dependent random variables. J. Theor. Probab. 1999, 12: 87–104. 10.1023/A:1021744626773
Utev S, Peligrad M: Maximal inequalities and an invariance principle for a class of weakly dependent random variables. J. Theor. Probab. 2003, 16(1):101–115. 10.1023/A:1022278404634
Gan SX:Almost sure convergence for -mixing random variable sequences. Stat. Probab. Lett. 2004, 67: 289–298. 10.1016/j.spl.2003.12.011
Cai GH:Strong law of large numbers for -mixing sequences with different distributions. Discrete Dyn. Nat. Soc. 2006., 2006: Article ID 27648
Zhu MH:Strong laws of large numbers for arrays of rowwise -mixing random variables. Discrete Dyn. Nat. Soc. 2007., 2007: Article ID 74296
Wu QY, Jiang YY:Some strong limit theorems for -mixing sequences of random variables. Stat. Probab. Lett. 2008, 78(8):1017–1023. 10.1016/j.spl.2007.09.061
Wu QY, Jiang YY:Strong limit theorems for weighted product sums of -mixing sequences of random variables. J. Inequal. Appl. 2009., 2009: Article ID 174768
An J, Yuan DM:Complete convergence of weighted sums for -mixing sequence of random variables. Stat. Probab. Lett. 2008, 78(12):1466–1472. 10.1016/j.spl.2007.12.020
Kuczmaszewska A:On Chung-Teicher type strong law of large numbers for -mixing random variables. Discrete Dyn. Nat. Soc. 2008., 2008: Article ID 140548
Sung SH:Complete convergence for weighted sums of -mixing random variables. Discrete Dyn. Nat. Soc. 2010., 2010: Article ID 630608
Wang XJ, Hu SH, Shen Y, Yang WZ: Some new results for weakly dependent random variable sequences. Chinese J. Appl. Probab. Statist. 2010, 26(6):637–648.
Wang XJ, Xia FX, Ge MM, Hu SH, Yang WZ:Complete consistency of the estimator of nonparametric regression models based on -mixing sequences. Abstr. Appl. Anal. 2012., 2012: Article ID 907286
Wang XJ, Li XQ, Yang WZ, Hu SH: On complete convergence for arrays of rowwise weakly dependent random variables. Appl. Math. Lett. 2012, 25: 1916–1920. 10.1016/j.aml.2012.02.069
Wu QY: Sufficient and necessary conditions of complete convergence for weighted sums of PNQD random variables. J. Appl. Math. 2012., 2012: Article ID 104390
Wu QY: A complete convergence theorem for weighted sums of arrays of rowwise negatively dependent random variables. J. Inequal. Appl. 2012., 2012: Article ID 50 10.1186/1029-242X-2012-50
Wang XJ, Hu SH, Yang WZ: Convergence properties for asymptotically almost negatively associated sequence. Discrete Dyn. Nat. Soc. 2010., 2010: Article ID 218380
Wang XJ, Hu SH, Yang WZ: Complete convergence for arrays of rowwise asymptotically almost negatively associated random variables. Discrete Dyn. Nat. Soc. 2011., 2011: Article ID 717126
Baek JI, Choi IB, Niu SL: On the complete convergence of weighted sums for arrays of negatively associated variables. J. Korean Stat. Soc. 2008, 37: 73–80. 10.1016/j.jkss.2007.08.001
Wu QY: Probability Limit Theory for Mixing Sequences. Science Press of China, Beijing; 2006.
Wang XJ, Hu SH, Yang WZ, Wang XH: On complete convergence of weighted sums for arrays of rowwise asymptotically almost negatively associated random variables. Abstr. Appl. Anal. 2012., 2012: Article ID 315138
Acknowledgements
The authors are most grateful to the editor Jewgeni Dshalalow and anonymous referees for careful reading of the manuscript and valuable suggestions which helped in improving an earlier version of this paper. This work was supported by the National Natural Science Foundation of China (11201001, 11171001), the Natural Science Foundation of Anhui Province (1308085QA03, 11040606M12, 1208085QA03), the 211 project of Anhui University, the Youth Science Research Fund of Anhui University, Applied Teaching Model Curriculum of Anhui University (XJYYXKC04) and the Students Science Research Training Program of Anhui University (KYXL2012007, kyxl2013003).
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Shen, A., Wu, R., Wang, X. et al. Complete convergence for weighted sums of arrays of rowwise -mixing random variables. J Inequal Appl 2013, 356 (2013). https://doi.org/10.1186/1029-242X-2013-356
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DOI: https://doi.org/10.1186/1029-242X-2013-356