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R E S E A R C H

Open Access

On the complete convergence for

weighted sums of a class of random variables

Jinghuan Zhao

1*

, Yan Sun

2

and Yu Miao

1

*Correspondence:

jinghuanzhao@163.com 1College of Mathematics and

Information Science, Henan Normal University, Xinxiang, Henan 453007, China

Full list of author information is available at the end of the article

Abstract

In this article, some new results as regards complete convergence for weighted sums n

i=1aniXiof random variables satisfying the Rosenthal type inequality are established

under some mild conditions. These results extend the corresponding theorems of Denget al.(Filomat 28(3):509-522, 2014) and Gan and Chen (Acta Math. Sci. 28(2):269-281, 2008).

MSC: 60F15

Keywords: complete convergence; Rosenthal type inequality; weighted sums

1 Introduction

In this paper we are interested in the complete convergence of a sequence of random vari-ables which satisfies the Rosenthal type inequality. First let us recall some definitions and well-known results.

1.1 Complete convergence

The following concept of complete convergence of a sequence of random variables was introduced first by Hsu and Robbins [], which plays an important role in limit theory of probability. A random sequence{Xn,n≥}is said to converge completely to the constant C(writeXnCcompletely) if for anyε> ,

n=

P|XnC|>ε

<∞.

From the Borel-Cantelli lemma, this implies thatXnCalmost surely (a.s.). For the case

of i.i.d. random variables, Hsu and Robbins [] proved that the sequence of arithmetic means of the random variables converges completely to the expected value if the variance of the summands is finite. Somewhat later, Erdös [] proved the converse. These results are summarized as follows.

Hsu-Robbins-Erdös strong law. Let{Xn,n≥}be a sequence of i.i.d. random variables

with mean zero and setSn= n

i=Xi,n≥, thenEX<∞is equivalent to the condition

that

n=

P|Sn|>εn

<∞ for allε> .

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The result of Hsu-Robbins-Erdös’ strong law is a fundamental theorem in probability theory and has been intensively investigated in several directions by many authors in the past decades. One of the most important results is Baum and Katz’ [] strong law of large numbers.

Baum and Katz strong law. Letαp≥,p> , and let{Xn}be a sequence of i.i.d. random

variables andE|X|p<∞. If <α≤, assume thatEX= . Then

n=

nαp–P

max ≤in j

i=

Xi >εnα

<∞ for allε> .

The Baum and Katz strong law bridges the integrability of summands and the rate of con-vergence in the Marcinkiewicz-Zygmund strong law of large numbers.

It is well known that the analysis of weighted sums plays an important role in the statis-tics, such as jackknife estimate, nonparametric regression function estimate and so on. Many authors considered the complete convergence of the weight sums of random vari-ables. Thrum [] studied the almost sure convergence of weighted sums of i.i.d. random variables; Liet al.[] obtained complete convergence of weighted sums without the iden-tically distributed assumption. Liang and Su [] extended the results of Thrum [], and Liet al.[] showed the complete convergence of weighted sums of negatively associated sequence. The reader can refer to further literature on complete convergence of weighted sums, such as Xueet al.[] for the NSD sequence, Gan and Chen [] for the NOD se-quence and so on.

1.2 Rosenthal type inequality

The Rosenthal type inequality is expressed as follows: let {Zn,n≥} be a sequence of

random variables, for anyr≥ and everyn≥, there exists a positive constantCsuch that

E

max ≤jn j

i=

(ZiEZi)

rC

n

i=

E|ZiEZi|r+ n

i=

E(ZiEZi) r

. (.)

Many dependent sequences satisfy the Rosenthal type inequality. We refer to Shao [] for a negatively associated sequence; Utev and Peligrad [] for aρ˜-mixing sequence; Shen [] and Stout [] for an extended negatively dependent sequence (END); Hu [] and Wanget al.[] for a negatively superadditive dependent sequence (NSD); Asadianet al.

[] and Wu [] for a negatively orthant dependent sequence (NOD); Yuan and An [] for an asymptotically almost negatively associated sequence (AANA).

The concept of stochastic domination is presented as follows.

Definition . A sequence {Xn,n≥} of random variables is said to be stochastically

dominated by a random variableXif there exists a positive constantCsuch that

P|Xn|>x

CP|X|>x

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In the present paper, we shall study the complete convergence of weighted sums of ran-dom sequence under the assumption that the ranran-dom variables satisfy the Rosenthal type inequality. Our main results are stated in Section  and the proofs are given in Section . Throughout this paper, letCdenote a positive constant, which may take different values whenever it appears in different expressions.an=O(bn) means|an/bn| ≤CandI(·) stands

for the indicator function.

2 Main results

Theorem . Let{Xn,n≥}be a sequence of random variables with zero means,which

is stochastically dominated by a random variable X with E|X|p<∞for some p≥.Let {ani, ≤in,n≥} be an array of real numbers satisfying|ani| ≤C for≤in and n≥,where C is a positive constant.Let{bn,n≥}and{cn,n≥} be two sequences of positive constants such that,for some r≥max{,p},

n cpn

→ and k

n=

nbn=O

cpk,

n=

nrbn

cr n

<∞ and

n=k nbn

cr n

=Ocpkr.

(.)

Suppose that Rosenthal type inequality of Zni:=aniXiI(|Xi| ≤cn) (≤in)holds for the above r.Then we have

n=

bnP

max ≤jn j

i=

aniXi >εcn

<∞ for allε> . (.)

Remark . Under the conditions of Theorem ., if we takebn=npα–,cn=for / <

α≤ and> , and suppose that the Rosenthal type inequality ofZni:=aniXiI(|Xi| ≤)

(≤in) holds for

⎧ ⎨ ⎩

r>max{p,αp–α––}, ifp≥,

r= , if  <p< ;

then it is easy to see that the conditions in (.) hold. Hence we have

n=

npα–P

max ≤jn j

i=

aniXi >εnα

<∞ for allε> . (.)

Remark . Under the conditions of Theorem ., letbn=ns–,cn=ns/pfors>p,p> ,

and let the Rosenthal type inequality ofZni:=aniXiI(|Xi| ≤ns/p) (≤in) hold for ⎧

⎨ ⎩

r>max{p, –s –sp}

, ifp≥,

(4)

then it is clear that the conditions in (.) hold. Hence we have

n=

ns–P

max ≤jn j

i=

aniXi >εns/p

<∞ for allε> . (.)

Remark . Under the conditions of Theorem ., if we takebn=

logn

n ,cn= (nlogn)

/p

for some ≤p≤, and suppose that Rosenthal type inequality ofZni :=aniXiI(|Xi| ≤

(nlogn)/p) (in) holds for

⎨ ⎩

r= , if ≤p< ,

r> , ifp= .

It is easy to check that the conditions in (.) hold. Hence we have

n=

logn

n P

max ≤jn j

i=

aniXi

>ε(nlogn)/p

<∞ for allε> . (.)

Remark . Under the conditions of Theorem ., if we takebn=nlog n,cn= (nlog logn)/p

for some ≤p≤, and suppose that Rosenthal type inequality ofZni :=aniXiI(|Xi| ≤

(nlog logn)/p) (in) holds for

⎨ ⎩

r= , if ≤p< ,

r> , ifp= .

It is easy to check that the conditions in (.) hold. Hence we have

n=

nlognP

max ≤jn j

i=

aniXi

>ε(nlog logn)/p

<∞ for allε> . (.)

Theorem . Let {Xn,n≥} be a sequence of random variables with zero means and

{ani, ≤in,n≥}be an array of real numbers satisfying|ani| ≤C for≤in and n≥, where C is a positive constant.Let{cn,n≥}be sequences of positive constants with cn↑ ∞ and{n(t),n≥}be a sequence of nonnegative and even functions such that for each n≥,

n(t) > as t> .Suppose that the Rosenthal type inequality of Zni:=aniXiI(|Xi| ≤cn)

(≤in)holds for r= .In addition,assume that

n(|t|) |t| ↑,

n(|t|)

t ↓ as|t| ↑ (.)

and

n=

n

i=

Ei(Xi)

i(cn)

<∞. (.)

Then we have

n=

P

max ≤jn j

i=

aniXi >εcn

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Theorem . Let {Xn,n≥} be a sequence of random variables with zero means and {ani, ≤in,n≥}be an array of real numbers satisfying|ani| ≤C for≤in and n≥, where C is a positive constant.Let{cn,n≥}be sequences of positive constants with cn↑ ∞ and{n(t),n≥}be a sequence of nonnegative and even functions such that for each n≥,

n(t) > as t> .Suppose that the Rosenthal type inequality of Zni:=aniXiI(|Xi| ≤cn)

(≤in)holds for r= . In addition, assume that for some ≤p<q≤ and each n≥,

n(|t|)

|t|pand

n(|t|)

tqas|t| ↑ (.) and

n=

n

i=

Ei(Xi)

i(cn)

<∞. (.)

Then we have

n=

P

max ≤jn j

i=

aniXi >εcn

<∞ for allε> . (.)

Corollary . Letn(t)be a positive even function satisfying(.)for some≤p<q and

q> .Under the conditions in Theorem.,suppose that the Rosenthal type inequality of Zni:=aniXiI(|Xi| ≤cn) (≤in)holds for r=q and

n=

cnr n

i=

EXir/

<∞ for r=q, (.)

we can obtain(.).

3 Proofs of main results

In order to prove the main theorems, we need the following lemma which includes the basic properties for stochastic domination. One can refer to Shen [], Wanget al.[], Wu [], or Shen and Wu [] for the proof.

Lemma . Let{Xn,n≥}be a sequence of random variables which is stochastically

dom-inated by a random variable X.Then for anyα> and b> ,

E|Xn|αI

|Xn| ≤b

CE|X|αI|X| ≤b+bαP|X|>b (.)

and

E|Xn|αI

|Xn| ≥b

CE|X|αI|X| ≥b. (.)

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Proof of Theorem . For ≤inandn≥, denote Xni =XiI(|Xi| ≤cn). Noting that EXi=  and by the conditionsncnp→ and|ani| ≤C, we have

c–n max ≤jn j

i=

aniEXni

=c–n max ≤jn j

i=

aniEXiI

|Xi|>cn

c–n max ≤jn

j

i=

aniEXiI

|Xi|>cn

c–n n

i=

|ani|E|Xi|I

|Xi|>cn

Cc–n n

i=

|ani|E|X|I

|X|>cn

Cnc–nE|X|I|X|>cn

CncnpE|X|p→ asn→ ∞.

Hence for anyε> , it follows that for allnlarge enough

c–n max ≤jn j

i=

aniEXni <

ε

. (.)

From (.), it is easy to see that

n=

bnP

max ≤jn j

i=

aniXi >εcn

C

n=

bn n

i=

P|Xi|>cn

+C

n=

bnP

max ≤jn j

i=

aniXni >εcn

C

n=

nbnP

|X|>cn

+C

n=

bnP

max ≤jn j

i=

ani

XniEXni >

εcn

=:CI+CJ. (.)

In order to prove (.), it suffices to prove thatI<∞andJ<∞. First, forI, by the condition (.), it is easy to check that

I =

n=

nbnP

|X|>cn

C

n=

nbn

k=n

Pck<|X| ≤ck+

C

k=

Pck<|X| ≤ck+

k

n=

(7)

C

k=

cpkPck<|X| ≤ck+

CE|X|p<∞. (.)

Second, we will showJ<∞. It follows by the Markov inequality and the Rosenthal type inequality that, forr≥,

J

ε

r

n=

bncnrE

max ≤jn j i= ani

XniEXni rCn=

bncnr

n

i=

aniEXniEXnir  + n i=

arniEXniEXnir

C

n=

bncnr

n

i=

aniEXnir  + n i=

arniEXnir

C

n=

bncnr n

i=

E|Xi|I

|Xi| ≤cnr

+C

n=

bncnr n

i=

E|Xi|rI

|Xi| ≤cn

=:CJ+CJ. (.)

For the casep≥, from Lemma ., Markov’s inequality, and the condition (.), we have

J≤C

n=

bncnr n

i=

E|X|I|X| ≤cn

+cnP|X|>cn r

C

n=

bncnr n

i=

E|X|I|X| ≤cn

+E|X|I|X|>cn r

C

n=

bncnrn r

<. (.)

Sincerp, it follows by Lemma . again, (.), and the condition (.) that

J =

n=

bncnr n

i=

E|Xi|rI

|Xi| ≤cn

C

n=

bncnr n

i=

E|X|rI|X| ≤cn

+crnP|X|>cn

=C

n=

nbncnrE|X|rI

|X| ≤cn

+C

n=

nbnP

|X|>cn

C

n=

nbncnr n

k=

E|X|rIck–<|X| ≤ck

(8)

=C

k=

E|X|rIck–<|X| ≤ck

n=k

nbncnr+C

C

k=

E|X|pIck–<|X| ≤ck

+C

CE|X|p+C<∞. (.)

For the case ≤p< , we haver≥ and we can taker=  in the Rosenthal type inequality. From a proof similar to (.), we get

JC

n=

bnc–n n

i=

E|Xi|I

|Xi| ≤cn

CE|X|p+C<∞.

Hence from the above discussions the claim (.) holds.

Proof of Theorem. For ≤inandn≥, defineXni =XiI(|Xi| ≤cn). From the

condi-tions (.),EXi= , and|ani| ≤C, we have

c–n max ≤jn j

i=

aniEXni

=c–n max ≤jn j

i=

aniEXiI

|Xi|>cn

c–n max ≤jn

j

i=

aniEXiI

|Xi|>cn

n

i=

c–n|ani|E|Xi|I

|Xi|>cn

C n

i=

Ei(|Xi|)

i(cn)

I|Xi|>cn

C n

i=

Ei(Xi)

i(cn)→

 asn→ ∞.

Hence, for anyε> , we have for allnlarge enough,

c–n max ≤jn j

i=

aniEXni <

ε

. (.)

From (.), it follows that

n=

P

max ≤jn j

i=

aniXi >εcn

C

n=

n

i=

P|Xi|>cn

+C

n=

P

max ≤jn j

i=

aniXni >εcn

(9)

C

n=

n

i=

P|Xi|>cn

+C

n=

P

max ≤jn j

i=

ani

XniEXni >

εcn

=:CI+CJ. (.)

Now it suffices to control the termsIandJ. For the termI, by the condition (.), we can get

n=

n

i=

P|Xi|>cn

≤ ∞

n=

n

i=

Ei(|Xi|)

i(cn)

=

n=

n

i=

Ei(Xi)

i(cn)

<∞. (.)

For the termJ, using the Markov inequality and the Rosenthal type inequality forr=  and the condition (.), (.), we have

J

ε

∞

n=

c–n E

max ≤jn j

i=

ani

XniEXni

C

n=

c–n n

i=

aniEXniEXni

C

n=

c–n n

i=

|ani|EXni

C

n=

c–n n

i=

E|Xi|I

|Xi| ≤cn

C

n=

n

i=

Ei(|Xi|)

i(cn)

I|Xi| ≤cn

C

n=

n

i=

Ei(Xi)

i(cn)

<∞. (.)

Hence the proof of Theorem . is completed.

Proof of Theorem. For ≤inandn≥, defineXni =XiI(|Xi| ≤cn). Similar to the

proof of Theorem ., it suffices to show that, for anyε> ,

c–n max ≤jn j

i=

aniEXni

→ asn→ ∞, (.) ∞

n=

n

i=

P|Xi|>cn

(10)

and

n=

P

max ≤jn j

i=

ani

XniEXni >

εcn

<∞. (.)

First, it follows from the conditions (.), (.),EXi= , and|ani| ≤Cthat

c–n max ≤jn j

i=

aniEXni

=c–

n maxjn j

i=

aniEXiI

|Xi|>cn

c–n max ≤jn

j

i=

aniEXiI

|Xi|>cn

n

i=

c–n|ani|E|Xi|I

|Xi|>cn

C n

i=

E|Xi|pI(|Xi|>cn) cpn

C n

i=

Ei(|Xi|)

i(cn)

I|Xi|>cn

C n

i=

Ei(Xi)

i(cn)→

 asn→ ∞. (.)

Second, by the condition (.), we known(t) is an increasing function ast> .

There-fore, by the condition (.)

n=

n

i=

P|Xi|>cn

≤ ∞

n=

n

i=

Ei(|Xi|)

i(cn)

=

n=

n

i=

Ei(Xi)

i(cn)

<∞. (.)

Finally, for ≤p<q≤, by the Markov inequality and the Rosenthal type inequality for

r=  and the conditions (.), (.), we have

n=

P

max ≤jn j

i=

ani

XniEXni >

εcn

C

n=

c–n n

i=

E|Xi|I

|Xi| ≤cn

≤ ∞

n=

n

i=

(11)

≤ ∞

n=

n

i=

Ei(|Xi|)

i(cn)

I|Xi| ≤cn

≤ ∞

n=

n

i=

Ei(Xi)

i(cn)

<∞. (.)

The proof of this theorem is completed.

Proof of Corollary. From (.), (.) in the proof of Theorem ., and the condition (.) holding for some ≤p<qandq> , we only need to show (.) holds. By Markov’s inequality, the Rosenthal type inequality and the conditions (.), (.), (.), we have, forr=q> ,

n=

P

max ≤jn j

i=

ani

XniEXni >

εcn

C

n=

cnr n

i=

E|Xi|I

|Xi| ≤cnr

+C

n=

cnr n

i=

E|Xi|rI

|Xi| ≤cn

C+C

n=

cnq n

i=

E|Xi|qI

|Xi| ≤cn

C+C

n=

n

i=

Ei(|Xi|)

i(cn)

I|Xi| ≤cn

C+C

n=

n

i=

Ei(Xi)

i(cn)

<∞, (.)

which completes the proof.

4 Conclusions

The present work is meant to establish some new results as regards complete convergence for weighted sums of random variables which satisfy the Rosenthal type inequality. These results extend some known results.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

All authors contributed equally to the manuscript, and they read and approved the final manuscript.

Author details

1College of Mathematics and Information Science, Henan Normal University, Xinxiang, Henan 453007, China.2Science College, Beijing Information Science and Technology University, Beijing, 100192, China.

Acknowledgements

The work is supported by Science and Technology Project of Beijing Municipal Education Commission (KM201611232019), NSFC (71501016) and IRTSTHN (14IRTSTHN023), NSFC (11471104).

Received: 6 April 2016 Accepted: 7 September 2016 References

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