IMPROVEMENT OF THE NON-UNIFORM VERSION OF BERRY-ESSEEN INEQUALITY VIA PADITZ-SIGANOV THEOREMS

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Berry-Esseen Inequality Via Paditz-Siganov Theorems K. Neammanee and P. Thongtha

vol. 8, iss. 4, art. 92, 2007

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IMPROVEMENT OF THE NON-UNIFORM VERSION OF BERRY-ESSEEN INEQUALITY VIA

PADITZ-SIGANOV THEOREMS

K. NEAMMANEE AND P. THONGTHA

Department of Mathematics, Faculty of Science, Chulalongkorn University

Bangkok 10330, Thailand EMail:Kritsana.n@chula.ac.th

Received: 28 June, 2007

Accepted: 05 October, 2007 Communicated by: T.M. Mills 2000 AMS Sub. Class.: 60F05, 60G50.

Key words: Berry-Esseen inequality, Paditz-Siganov theorems, central limit theorem, uniform and non-uniform bounds.

Abstract: We improve the constant in a non-uniform bound of the Berry-Esseen inequality without assuming the existence of the absolute third moment by using the method obtained from the Paditz-Siganov theorems. Our bound is better than the results of Thongtha and Neammanee in 2007 ([14]).

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1. Introduction and Main Results

The Berry-Esseen inequality is one of the most important inequalities in the theory of probability. This inequality was independently discovered by two mathemati- cians, Andrew C. Berry ([2]) and Carl-Gustav Esseen ([5]) in 1941 and 1945 respectively. LetX1, X2, . . . , Xnbe independent random varibles with zero mean and Pn

i=1EX_i² = 1. DefineWn =X1+X2+· · ·+Xn.ThenVarWn = 1. LetFnbe the distribution function ofW_nandΦthe standard normal distribution function, i.e.,

F_n(x) = P(W_n≤x) and Φ(x) = 1

√2π Z x

−∞

e⁻^t

2 2 dt.

The central limit theorem shows thatF_nconverges pointwise toΦasn → ∞and the bounds of this convergence are,

(1.1) sup

x∈R

|P(Wn≤x)−Φ(x)| ≤C0 n

X

i=1

E|X_i|³ and

(1.2) |P(W_n ≤x)−Φ(x)| ≤ C₁ 1 +|x|³

n

X

i=1

E|X_i|³

for uniform and non-uniform versions respectively, where bothC₀ ansC₁ are positive constants and stated under the assumption thatE|X_i|³ <∞fori= 1,2, . . . , n.

In the case of identicalX_i’s, Siganov ([11]) and Chen ([5]) improved the constant down to 0.7655 and 0.7164, respectively. For non-uniform bounds, Nageav ([7]) was the first to obtain (1.2) and Michel ([6]) calculated the constant to be 30.84.

Without assuming identically distributedX_i⁰s, Beek ([15]) sharpened the constant down to 0.7975 in 1972 for the uniform version. The best bound was found by Siganov ([11]) in 1986.

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Theorem 1.1 (Siganov,1986). Let X₁, X₂, . . . , X_n be independent random vari- ables such that EX_i = 0 and E|X_i|³ < ∞ for i = 1,2, . . . , n. Assume that Pn

i=1EX_i² = 1.Then sup

x∈R

|P(W_n≤x)−Φ(x)| ≤0.7915

n

X

i=1

E|X_i|³, whereW_n=X₁+X₂+· · ·+X_n.

For the non-uniform version, Bikelis ([1]) generalized (1.2) to this case and Paditz ([9]) calculatedC₁ to be 114.7 in 1977. He also improved his result down to 31.935 in 1989.

Theorem 1.2 (Paditz ([10]),1989). Under the assumptions of Theorem1.1, we have

|P(W_n≤x)−Φ(x)| ≤ 31.935 1 +|x|³

n

X

i=1

E|X_i|³.

In 2001, Chen and Shao ([3]) gave new versions of (1.1) and (1.2) without assuming the existence of third moments. Their results are

(1.3) sup

x∈R

|P(W_n≤x)−Φ(x)|

≤4.1

n

X

i=1

{E|X_i|²I(|X_i| ≥1|) +E|X_i|³I(|X_i|<1)}

and

(1.4) |P(W_n ≤x)−Φ(x)|

≤C₂

n

X

i=1

EX_i²I(|X_i| ≥1 +|x|)

(1 +|x|)² +E|X_i|³I(|X_i|<1 +|x|) (1 +|x|)³

,

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whereC₂is a positive constant andI(A)is an indicator random variable such that I(A) =

( 1 if A is true, 0 otherwise.

In 2005, Neammanee ([8]) combined the concentration inequality in ([3]) with a coupling approach to calculate the constant in (1.4), giving,

(1.5) |P(W_n ≤x)−Φ(x)|

≤C3 n

X

i=1

EX_i²I(|X_i| ≥1 +|^x₄|)

(1 +|^x₄|)² + E|X_i|³I(|X_i|<1 +|^x₄|) (1 +|^x₄|)³

,

whereC₃is 21.44 for large values ofxsuch that|x| ≥14.

Thongtha and Neammanee ([14]) improved the concentration inequality used in ([8]) and gave a better constant, i.e., 9.7 for|x| ≥14. The method which was used in ([8]) is Stein’s method which was first introduced by Stein ([12]) in 1972. In this work, we provide a better constant by using Paditz-Siganov theorems. The results are as follows.

Theorem 1.3. We have

≤C

n

X

i=1

EX_i²I(|X_i| ≥1 +|x|)

(1 +|x|)² + E|X_i|³I(|X_i|<1 +|x|) (1 +|x|)³

,

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where

C =











49.89 if 0≤ |x|<1.3, 59.45 if 1.3≤ |x|<2, 73.52 if 2≤ |x|<3, 76.17 if 3≤ |x|<7.98, 45.80 if 7.98≤ |x|<14, 39.39 if |x| ≥14.

To compare Theorem 1.3with the result of Thongtha and Neammanee ([14]) in (1.5), we give Corollary1.4.

Corollary 1.4. We have

|P(W_n≤x)−Φ(x)| ≤C

n

X

i=1

EX_i²I(|X_i| ≥1 +|^x₄|)

(1 +|^x₄|)² +E|X_i|³I(|X_i|<1 +|^x₄|) (1 +|^x₄|)³

,

where

C =











9.54 if 0≤ |x|<1.3, 19.74 if 1.3≤ |x|<2, 18.38 if 2≤ |x|<3, 14.63 if 3≤ |x|<7.98, 5.13 if 7.98≤ |x|<14, 3.55 if |x| ≥14.

We note from Corollary1.4 that our result is better than a bound from Thongtha and Neammanee in ([14]).

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2. Proof of the Main Results

In this section, we will prove Theorem 1.3 by using the Paditz-Siganov theorems.

Corollary1.4can be obtained easily from Theorem1.3. To prove these results, let Y_i,x =X_iI(|X_i|<1 +x), S_x =

n

X

i=1

Y_i,x,

α_x =

n

X

i=1

EX_j²I(|X_j| ≥1 +x), β_x =

n

X

i=1

E|X_j|³I(|X_j|<1 +x),

γ_x = β_x

2 and δ_x = α_x

(1 +x)² + β_x

(1 +x)³ forx >0.

Proposition 2.1. For eachn∈N, we have 1. Pn

i=1E|Y_i,x−EY_i,x|³ ≤β_x+_1+x^7α^x, 2. 1−2α_x ≤VarS_x ≤1,and

3. If α_x ≤0.11, then0< ^√ ¹

VarSx ≤1 + 1.452α_x. Proof. 1. By the fact that

|EX_iI(|X_i|<1 +x)|=|EX_iI(|X_i| ≥1 +x)|, (2.1)

E|X_i|² ≤

n

X

i=1

EX_i² = 1 and E²X_i ≤EX_i², we have

n

X

i=1

E|Yi,x−EYi,x|³ =

n

X

i=1

E|XiI(|Xi|<1 +x)−EXiI(|Xi|<1 +x)|³

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≤

n

X

i=1

[E|X_i|³I(|X_i|<1 +x) + 3EX_i²I(|X_i|<1 +x)|EX_iI(|X_i|<1 +x)|

+ 3E|X_iI(|X_i|<1 +x)|E²X_iI(|X_i|<1 +x)|+|EX_iI(|X_i|<1 +x)|³]

≤

n

X

i=1

E|X_i|³I(|X_i|<1 +x) + 3

n

X

i=1

|EX_iI(|X_i|<1 +x)|

+ 3

n

X

i=1

E|X_i||EX_iI(|X_i|<1 +x)||EX_iI(|X_i|<1 +x)|

+

n

X

i=1

E|X_i|²I(|X_i|<1 +x)|EX_iI(|X_i|<1 +x)|

≤β_x+ 3

n

X

i=1

|EX_iI(|X_i| ≥1 +x)|+ 3

n

X

i=1

E|X_i|²|EX_iI(|X_i| ≥1 +x)|

+

n

X

i=1

|EXiI(|Xi| ≥1 +x)|

≤β_x+ 3

n

X

i=1

E|X_i|I(|X_i| ≥1 +x) + 3

n

X

i=1

E|X_i|I(|X_i| ≥1 +x)

+

n

X

i=1

E|X_i|I(|X_i| ≥1 +x)

=β_x+ 7

n

X

i=1

E|X_i|I(|X_i| ≥1 +x)

≤β_x+ 7

n

X

i=1

E|X_i|²I(|X_i| ≥1 +x)

(1 +x) =β_x+ 7α_x (1 +x).

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2. By (2.1), we note that VarS_x =

n

X

i=1

VarY_i,x =

n

X

i=1

(EY_i,x² −E²Y_i,x)

=

n

X

i=1

EX_i²I(|X_i|<1 +x)−

n

X

i=1

E²X_iI(|X_i|<1 +x)

= 1−

n

X

i=1

EX_i²I(|X_i| ≥1 +x)−

n

X

i=1

E²X_iI(|X_i| ≥1 +x)

= 1−α_x−

n

X

i=1

E²X_iI(|X_i| ≥1 +x).

(2.2)

From this and the fact thatα_x ≥0, we haveVarS_x ≤1.

By (2.2), we have

VarS_x = 1−α_x−

n

X

i=1

E²X_iI(|X_i| ≥1 +x)

≥1−αx−

n

X

i=1

EX_i²I(|Xi| ≥1 +x) = 1−2αx. Hence,1−2α_x ≤VarS_x ≤1.

3. For0< t≤0.11, by using Taylor’s formula, we have

√ 1

1−2t = 1 + t

(1−2c)³² for some c∈(0,0.11]

≤1 + t

(1−2(0.11))³² ≤1 + 1.452t.

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From this fact and 2., we have 0< 1

√VarS_x ≤ 1

√1−2α_x ≤1 + 1.452α_x forα_x ≤0.11.

Proposition 2.2. For eachx >0, letY¯_i,x= ^Y^i,x^√_Var^−EY_S^i,x

x andS¯_x=Pn i=1Y¯_i,x. 1. If α_x ≤0.099and1.3≤x≤2,then

P

S¯x≤ x−ESx

√VarS_x

−Φ

x−ESx

√VarS_x

≤ 54.513αx

(1 +x)² + 41.195βx

(1 +x)³. 2. If (1 +x)²α_x < ¹₅, then

P

S¯_x ≤ x−ES_x

√VarS_x

−Φ(

x−ES_x

√VarS_x

≤ C₁α_x

(1 +x)² + C₂β_x (1 +x)³ where C1 = 57.186 C2 = 73.515 for 2≤x <3,

C₁ = 33.318 C₂ = 76.17 for 3≤x <7.98, C₁ = 3.976 C₂ = 45.8 for 7.98≤x <14, and

C₁ = 1.226 C₂ = 39.382 for x≥14.

Proof. 1. By Proposition 2.1(1) of ([14]) and Proposition2.1(2), we have (2.3) |ES_x| ≤ α_x

1 +x ≤0.043 and 1≥VarS_x ≥0.802 which imply

(2.4) 0≤ x−ES_x

√VarS_x ≤ 2 + 0.043

√0.802 = 2.2813.

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By Proposition2.1(1) and (2.3),

n

X

i=1

E|Y¯_i,x|³ =

n

X

i=1

E

Y_i,x−EY_i,x

√VarS_x

3

= 1

(VarS_x)³²

n

X

i=1

E|Y_i,x−EY_i,x|³

≤ 1

(VarS_x)³²

βx+ 7α_x 1 +x

= 1.3923β_x+ 4.2375α_x. (2.5)

Note thatS¯x =Pn

i=1Y¯i,xis the sum of independent random variables whose EY¯_i,x= 0 and Var ¯S_x = 1.

By (2.5) and Theorem1.1, P S¯_x ≤z

−Φ(z)

≤0.7915

n

X

i=1

E|Y¯_i,x|³

≤0.7915(1.3923β_x+ 4.2375α_x)

≤1.102β_x+ 3.354α_x for allz ∈R. From this fact, (2.3) and (2.4), we have

P

S¯x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

≤

1 +

x−ESx

√VarSx

3

(1.102β_x+ 3.354α_x)

1 +

x−ESx

√VarSx

3

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≤ (3.2813)³(1.102β_x+ 3.354α_x)

1 +

x−ESx

√VarSx

3

≤ 38.933βx+ 118.495αx

(0.957 +x)³

≤ 41.195β_x

(1 +x)³ + 125.379α_x (1 +x)³

≤ 41.195β_x

(1 +x)³ + 54.513α_x (1 +x)² where we use the fact that

1 +x

0.957 +x ≤1.019 for all 1.3< x <2 in the fourth inequality.

2. Case2≤x <3.

We can prove the result of this case by using the same argument as 1.

Case3≤x <7.98.

To bound P

S¯x ≤ ^√^x−ES_VarS^x

x

−Φ

√x−ESx

VarSx

in 1., we used Theorem1.1.

But in this case, we will use Theorem1.2.

We note that

(2.6) 0≤α_x ≤0.0125, 1≥VarS_x ≥0.975, and, by Proposition 2.1(1) of ([14]),|ESx| ≤0.00313.

Then, for3≤x≤7.98, 1

1 +

x−ESx

√VarSx

3 ≤ 2.29

1 + ^√^x−ES^x

VarSx

3 and

n

X

i=1

E|Y¯_i,x|³ ≤ 1.039β_x+1.819α_x.

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From these facts, (2.6) and Theorem1.2, we have

P

S¯_x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

≤ (31.935)Pn

i=1E|Y¯_i,x|³ 1 +

√x−ESx

VarSx

3 ≤ (31.935)(2.29)Pn

i=1E|Y¯_i,x|³

1 + ^√^x−ES_Var_S^x

x

3

≤ 73.131(1.039β_x+ 1.818α_x)

(0.99687 +x)³ ≤ (1.0008)³(75.983β_x+ 132.952α_x) (1 +x)³

≤ 76.17β_x

(1 +x)³ +133.27α_x

(1 +x)³ ≤ 76.17β_x

(1 +x)³ +33.318α_x (1 +x)² , where we use the fact that

1 +x

0.99687 +x ≤1.0008 for all 3≤x <7.98 in the fourth inequality.

Casex≥7.98.

We can prove the result of this case by using the same argument as the case3≤x <

7.98.

We are now ready to prove Theorem1.3.

Proof of Theorem1.3. It suffices to consider onlyx≥0as we can simply apply the results to−W_nwhenx <0.

Case 1. 0≤x <1.3.

Note that forx≥0,

EX_i²I(|X_i| ≥1)+E|X_i|³I(|X_i|<1)≤EX_i²I(|X_i| ≥1+x)+E|X_i|³I(|X_i|<1+x)

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and for0≤x≤1.3,(1 +x)³ ≤12.167.

From these facts and (1.3), we have

≤4.1

n

X

i=1

n

EX_i²I(|X_i| ≥1) +E|X_i|³I(|X_i|<1)o

≤4.1

n

X

i=1

n

EX_i²I(|X_i| ≥1 +x) +E|X_i|³I(|X_i|<1 +x)o

≤ 4.1(12.167) (1 +x)³

n

X

i=1

n

EX_i²I(|Xi| ≥1 +x) +E|Xi|³I(|Xi|<1 +x) o

≤49.89

n

X

i=1

EX_i²I(|X_i| ≥1 +x)

(1 +x)² +E|X_i|³I(|X_i|<1 +x) (1 +x)³

.

Before proving another case, we need the equation (2.7) |P(W_n ≤x)−Φ(x)| ≤ 4.931α_x

(1 +x)²+ P

S¯_x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

forα_x ≤0.11andx≥1.3.

By (2.9) of ([14]), it suffices to show that forαx ≤0.11andx≥1.3, (2.8) |P(S_x ≤x)−Φ(x)| ≤ 3.319α_x

(1 +x)²+ P

S¯_x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

.

By Proposition2.1(1) and Proposition2.1(2), we have x−ES_x

√VarSx

≥x−ES_x≥x− α_x (1 +x),

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which implies

min

x,x−ESx

√VarS_x

≥x− αx

1 +x. From this and the fact that

Φ(b)−Φ(a) = 1

√2π Z b

a

e^−t

2

2 dt ≤ 1

√2πe^a²² Z b

a

1dt = (b−a)

√2πe^a²² for0< a < b, we have

|P(S_x ≤x)−Φ(x)|

≤ P

S¯x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

+

Φ

x−ES_x

√VarS_x

−Φ(x)

≤ P

S¯x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

+ 1

√2πe¹²

h min

x,^√^x−ESx_Var_Sx i2

√ x

VarS_x −x− ES_x

√VarS_x

≤ P

S¯_x ≤ x−ES_x

√VarS_x

−Φ

x−ES_x

√VarS_x

+ 1

√2πe¹²(^x−1+x^αx )²

√ x

VarS_x −x− ES_x

√VarS_x (2.9) .

Note that forx≥1.3 e^x

2

2 ≥0.933(1 +x), e^x

2

2 ≥0.193(1 +x)³ and

e¹²^(x−^1+x^αx⁾² ≥e^x

2 2 −( ^x

1+x)αx ≥0.89e^x

2 2 .

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From these facts, Proposition2.1(1) , Proposition2.1(3) andα_x ≤0.11, we have 1

√2πe¹²(^x−_1+x^αx )²

√ x

VarS_x −x− ESx

√VarS_x

≤ 1

√2π

0.89e^x²²

√ x

VarS_x −x

+ 1

√2π

0.89e^x²²

ES_x

√VarS_x

≤ 1.452α_xx

√2π(0.89)(0.193)(1 +x)³ + α_x (1 +x)

(1 + 1.452α_x)

√2π(0.89)(0.933)(1 +x)

≤ 3.373α_xx

(1 +x)³ + 0.558α_x

(1 +x)² ≤ 3.931α_x (1 +x)². From this fact, (2.8) and (2.9), we have (2.7)

Case 2.1.3≤x <2.

By the fact that|P(Wn≤x)−Φ(x)| ≤0.55([3, pp. 246]), we can assume_(1+x)^α^x 2 ≤ 0.011, i.e.α_x ≤0.099.

From this fact, (2.7) and Proposition2.2(1), we have

|P (Wn ≤x)−Φ(x)| ≤ 4.931αx

(1 +x)² + P

S¯x ≤ x−ESx

√VarS_x

−Φ

x−ESx

√VarS_x

≤ 4.931α_x

(1 +x)² + 41.195β_x

(1 +x)³ + 54.513α_x (1 +x)²

= 59.444α_x

(1 +x)² +41.195β_x

(1 +x)³ ≤59.444δx. Case 3. 2≤x≤14.

Subcase 3.1. (1 +x)²α_x ≥ ¹₅.

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Using the same argument of subcase 1.1 in Theorem 1.2 of ([14]) and the facts that (2.10) 1 +x

x = 1 + 1

x ≤1.5 and e^x

2

2 ≥0.92x³ for 2≤x≤14, we can show that

|P(W_n≤x)−Φ(x)| ≤37.408δ_x. Subcase 3.2. (1 +x)²α_x < ¹₅.

Note that forx≥2,we have

0≤αx≤ 1

5(1 +x)² ≤0.023 ≤0.11.

By (2.7) and Proposition2.2(2), we obtain the required bounds.

Case 4. x >14.

Follows the argument of case 3 on replacing the inequalities e^x

2

2 ≥60x³ and 1 +x

x = 1 + 1

x ≤1.071 in (2.10).

Proof of Corollary1.4. If0≤x <1.3.

We used the same argument as case 1 of Theorem1.3 and the fact that(1 + ^x₄)³ ≤ 2.327to getC = 9.54.

Suppose thatx≥1.3.By the fact that δ_x ≤

1 + ^x₄ 1 +x

2

δ^x

4,

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we have

δ_x ≤











0.332δ^x

4 if 1.3≤x <2, 0.250δ^x

4 if 2≤x <3, 0.192δ^x

4 if 3≤x <7.98, 0.112δ^x

4 if 7.98≤x <14, 0.090δ^x

4 if x≥14.

Then Corollary1.4follows from this fact and Theorem1.3.

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References

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