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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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EXPLICIT BOUNDS ON SOME NONLINEAR RETARDED INTEGRAL INEQUALITIES

MAN-CHUN TAN AND ZHI-HONG LI

Department of Mathematics Jinan University, Guangzhou 510632 People’s Republic of China

EMail:tanmc@jnu.edu.cn

Received: 21 November, 2007

Accepted: 10 July, 2008

Communicated by: S.S. Dragomir 2000 AMS Sub. Class.: 26D10, 26D15.

Key words: Integral inequality; retarded; nonlinear; explicit bound.

Abstract: In this paper some new retarded integral inequalities are established and explicit bounds on the unknown functions are derived. The present results extend some existing ones proved by Lipovan in [A retarded integral inequality and its appli- cations, J. Math. Anal. Appl. 285 (2003) 436-443].

Acknowledgement: The research was jointly supported by grants from the National Natural Science Foundation of China (No. 50578064) and the Natural Science Foundation of Guangdong Province, China (No.06025219).

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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Contents

1 Introduction 3

2 Main Results 4

3 Application 16

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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1. Introduction

During the past decades, studies on integral inequalities have been greatly enriched by the recognition of their potential applications in various applied sciences [1] – [6]. Recently, integral inequalities with delays have received much attention from researchers [7] – [12]. In this paper, we establish some new retarded integral in- equalities and derive explicit bounds on unknown functions, the results of which improve some known ones in [9].

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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2. Main Results

Throughout the paper, R denotes the set of real numbers and R+ = [0,+∞).

C(M, S) denotes the class of all continuous functions from M to S. C1(M, S) denotes the class of functions with continuous first derivative.

Theorem 2.1. Suppose that p > q ≥ 0 andc ≥ 0 are constants, andu, f, g, h ∈ C(R+,R+). Let w ∈ (R+,R+) be nondecreasing with w(u) > 0on (0,∞), and α ∈ C1(R+,R+) be nondecreasing with α(t) ≤ t on R+. Then the following integral inequality

(2.1) up(t)≤c2+ 2 Z α(t)

0

f(s)uq(s) Z s

0

g(τ)w(u(τ))dτ

+h(s)uq(s)

ds, t∈R+

implies for0≤t ≤T, (2.2) u(t)≤

( G−1

"

G(ξ(t)) + 2(p−q) p

Z α(t)

0

f(s) Z s

0

g(τ)dτ ds

#)p−q1

holds, where

(2.3) ξ(t) =c

2(p−q)

p +2(p−q) p

Z α(t)

0

h(s)ds,

(2.4) G(r) =

Z r

r0

1 w

sp−q1 ds, r≥r0 >0,

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vol. 9, iss. 3, art. 82, 2008

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G−1denotes the inverse function ofG, andT ∈R+is chosen so that G(ξ(t)) + 2(p−q)

p

Z α(t)

0

f(s) Z s

0

g(τ)dτ ds∈Dom G−1

, for all0≤t ≤T.

Proof. The conditionsα ∈ C1(R+,R+)andα(t) ≤t imply thatα(0) = 0. Firstly we assume thatc >0. Define the nondeceasing positive functionz(t)by

z(t) :=c2+ 2 Z α(t)

0

f(s)uq(s) Z s

0

g(τ)w(u(τ))dτ

+h(s)uq(s)

ds.

Thenz(0) =c2and by (2.1) we have

(2.5) u(t)≤[z(t)]p1 ,

and consequentlyu(α(t))≤[z(α(t))]1p ≤[z(t)]1p. By differentiation we get z0(t) = 2uq(α(t))

"

f(α(t))

Z α(t)

0

g(τ)w(u(τ))dτ

!

+h(α(t))

# α0(t)

≤2 [z(t)]qp

"

f(α(t))

Z α(t)

0

g(τ)w(u(τ))dτ

!

+h(α(t))

# α0(t).

Hence

z0(t)

[z(t)]pq ≤2f(α(t))α0(t) Z α(t)

0

g(τ)w(u(τ))dτ + 2h(α(t))α0(t).

Integrating both sides of last relation on[0, t]yields p

p−q[z(t)]p−qp ≤ p

p−q[z(0)]p−qp +2 Z α(t)

0

h(s)ds+2 Z α(t)

0

f(s) Z s

0

g(τ)w(u(τ))dτ ds,

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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which can be rewritten as (2.6) [z(t)]

p−q

p ≤c

2(p−q)

p +2(p−q)

p

Z α(t)

0

h(s)ds + 2(p−q)

p

Z α(t)

0

f(s) Z s

0

g(τ)w(u(τ))dτ ds.

LetT1(≤T)be an arbitrary number. For0≤t≤T1, from (2.3) and (2.6) we have (2.7) [z(t)]

p−q

p ≤ξ(T1) + 2(p−q) p

Z α(t)

0

f(s) Z s

0

g(τ)w(u(τ))dτ ds.

Denoting the right-hand side of (2.7) bym(t), we knowu(t)≤[z(t)]1p ≤[m(t)]p−q1 . Sincewis nondecreasing, we obtain

w[u(τ)]≤wh

(z(τ))1pi

≤wh

(z(α(t)))1pi

≤wh

(z(t))1pi

, for τ ∈[0, α(t)].

Hence

m0(t) = 2(p−q)

p f(α(t))α0(t) Z α(t)

0

g(τ)w(u(τ))dτ

≤ 2(p−q) p wh

(z(t))1pi

f(α(t))α0(t) Z α(t)

0

g(τ)dτ

≤ 2(p−q) p wh

(m(t))p−q1 i

f(α(t))α0(t) Z α(t)

0

g(τ)dτ . That is

(2.8) m0(t)

w[(m(t))p−q1 ]

≤ 2(p−q)

p f(α(t))α0(t) Z α(t)

0

g(τ)dτ .

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Integrating both sides of the last inequality on[0, t]and using the definition (2.4), we get

(2.9) G(m(t))−G(m(0))≤ 2(p−q) p

Z α(t)

0

f(s) Z s

0

g(τ)dτ ds.

Takingt=T1in inequality (2.9) and usingu(t)≤[m(t)]p−q1 , we have

u(T1)≤ (

G−1

"

G[ξ(T1)] + 2(p−q) p

Z α(T1)

0

f(s) Z s

0

g(τ)dτ ds

#)p−q1 . SinceT1(≤T)is arbitrary, we have proved the desired inequality (2.2).

The case c = 0 can be handled by repeating the above procedure with ε > 0 instead ofcand subsequently lettingε →0. This completes the proof.

Remark 1. If c = 0 and h(t) ≡ 0 hold, G(ξ(t)) = G(0) in (2.4) is not defined.

In such a case, the upper bound on solutions of the integral inequality (2.1) can be calculated as

u(t)≤ lim

ε→0+

( G−1

"

G(ε) + 2(p−q) p

Z α(t)

0

f(s) Z s

0

g(τ)dτ ds

#)p−q1 .

From Theorem2.1, we can easily derive the following corollaries.

Corollary 2.2. Suppose thatu, h ∈ C(R+,R+) andc ≥ 0is a constant. Letα ∈ C1(R+,R+)be nondecreasing withα(t)≤tonR+. Then the following inequality

u2(t)≤c2+ 2 Z α(t)

0

h(s)u(s)ds,

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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implies

u(t)≤c+ Z α(t)

0

h(s)ds.

Remark 2. Ifα(t)≡t, from Corollary2.2we get the Ou-Iang inequality.

Corollary 2.3. Suppose thatu, f, g, h ∈ C(R+,R+), andc ≥ 0is a constant. Let w ∈ (R+,R+)be nondecreasing with w(u) > 0on(0,∞), andα ∈ C1(R+,R+) be nondecreasing withα(t)≤tonR+. Then the following inequality

u2(t)≤c2+ 2 Z α(t)

0

f(s)u(s) Z s

0

g(τ)u(τ)dτ

+h(s)u(s)

ds

implies

u(t)≤ξ(t) exp

Z α(t)

0

f(s) Z s

0

g(τ)dτ

ds

!

whereξ(t) = c+Rα(t)

0 h(s)ds.

Theorem 2.4. Suppose that p > q ≥ 0 andc ≥ 0 are constants, andu, f, g, h ∈ C(R+,R+). Let w ∈ (R+,R+) be nondecreasing with w(u) > 0on (0,∞), and α ∈ C1(R+,R+) be nondecreasing with α(t) ≤ t on R+. Then the following integral inequality

(2.10) up(t)≤c2+ 2 Z α(t)

0

f(s)uq(s)

w(u(s)) +

Z s

0

g(τ)w(u(τ))dτ

+h(s)uq(s)

ds, t∈R+

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vol. 9, iss. 3, art. 82, 2008

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implies for0≤t ≤T (2.11) u(t)≤

( G−1

"

G(ξ(t))+2(p−q) p

Z α(t)

0

f(s)

1+

Z s

0

g(τ)dτ

ds

#)p−q1 ,

where ξ(t) and G(r) are defined by (2.3) and (2.4), respectively, and T ∈ R+ is chosen so that

G(ξ(t))+2(p−q) p

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds∈Dom G−1

, for all0≤t≤T.

Proof. Firstly we assume thatc >0. Define the nondeceasing positive function by z(t) :=c2+ 2

Z α(t)

0

f(s)uq(s)

w(u(s))+

Z s

0

g(τ)w(u(τ))dτ

+h(s)uq(s)

ds,

thenz(0) =c2and by (2.10) we have

(2.12) u(t)≤[z(t)]p1 ,

and

z0(t) = 2uq(α(t))

"

f(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

!

+h(α(t))

# α0(t)

≤2 [z(t)]qp

"

f(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

!

+h(α(t))

# α0(t).

Hence z0(t)

[z(t)]qp ≤2h(α(t))α0(t) + 2f(α(t))α0(t) w(u(α(t)) + Z α(t)

0

g(τ)w(u(τ))dτ

! .

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vol. 9, iss. 3, art. 82, 2008

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Integrating both sides of the last inequality on[0, t], we get p

p−q[z(t)]

p−q

p ≤ p

p−q[z(0)]

p−q p + 2

Z α(t)

0

h(s)ds + 2

Z α(t)

0

f(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds.

Using (2.3), we get [z(t)]

p−q

p ≤ξ(t) + 2(p−q) p

Z α(t)

0

f(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds.

LetT1(≤ T) be an arbitrary number. From last inequality we know the following relation holds fort∈[0, T1],

[z(t)]

p−q

p ≤ξ(T1) + 2(p−q) p

Z α(t)

0

f(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds.

Letting

(2.13) m(t) = ξ(T1) + 2(p−q) p

Z α(t)

0

f(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds, we get[z(t)]

p−q

p ≤m(t). Sincewis nondecreasing, we have w[u(α(t))]≤wh

(z(α(t)))1pi

≤wh

(z(t))1pi

≤wh

(m(t))p−q1 i and

w[u(τ)]≤wh

(z(τ))1pi

≤wh

(z(α(t)))1pi

≤wh

(z(t))1pi

, for τ ∈[0, α(t)].

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From (2.13), by differentiation we obtain m0(t) = 2(p−q)

p f(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

! α0(t)

≤ 2(p−q)

p f(α(t)) (

w

[m(t)]p−q1 +

Z α(t)

0

g(τ)w

[m(t)]p−q1

) α0(t)

=w

[m(t)]p−q1

2(p−q)

p f(α(t)) 1 + Z α(t)

0

g(τ)dτ

! α0(t).

Hence

m0(t) w

[m(t)]p−q1 ≤ 2(p−q)

p f(α(t)) 1 + Z α(t)

0

g(τ)dτ

! α0(t).

Integrating both sides of the last inequality on[0, t], from (2.4) we get G(m(t))≤G(m(0)) + 2(p−q)

p

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds.

Hence

(2.14) m(t)≤G−1

"

G(ξ(T1)) + 2(p−q) p

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds

# .

Takingt=T1in inequality (2.14) and usingu(t)≤[m(t)]p−q1 , we have u(T1)≤

( G−1

"

G(ξ(T1)) + 2(p−q) p

Z α(T1)

0

f(s)

1 + Z s

0

g(τ)dτ

ds

#)p−q1 .

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SinceT1(≤T)is arbitrary we have proved the desired inequality (2.11).

If c = 0, the result can be proved by repeating the above procedure withε > 0 instead ofcand subsequently lettingε →0. This completes the proof.

Remark 3. Theorem 2.1 of Lipovan in [9] is special case of above Theorem 2.4, under the assumptions thatp= 2,q = 1andg(t)≡0.

Theorem 2.5. Suppose that p > q ≥ 0 andc ≥ 0 are constants, andu, f, g, h ∈ C(R+,R+). Let w ∈ (R+,R+) be nondecreasing with w(u) > 0on (0,∞), and α, β ∈ C1(R+,R+)be nondecreasing with α(t) ≤ t, β(t) ≤ t on R+. Then the following integral inequality

(2.15) up(t)≤c2+ 2 Z α(t)

0

f(s)uq(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds + 2

Z β(t)

0

h(s)uq(s)w(u(s))ds, t∈R+ implies for0≤t ≤T

(2.16) u(t)≤ (

G−1

"

G(c2(p−q)p ) + 2(p−q) p

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds

+2(p−q) p

Z β(t)

0

h(s)ds

#)p−q1 ,

whereG(r)is defined by (2.4) andT ∈R+is chosen so that

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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G c

2(p−q) p

+2(p−q) p

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds +2(p−q)

p

Z β(t)

0

h(s)ds∈Dom G−1

, for all 0≤t≤T.

Proof. The conditions that α, β ∈ C1(R+,R+)are nondecreasing with α(t) ≤ t, β(t)≤timply thatα(0) = 0andβ(0) = 0.

Let us first assume thatc >0. Denoting the right-hand side of (2.15) byz(t), we knowz(t)is nondecreasing,z(0) =c2andu(t)≤[z(t)]1p. Consequently we have

u(α(t))≤[z(α(t))]1p ≤[z(t)]1p and u(β(t))≤[z(β(t))]1p ≤[z(t)]1p . Sincewis nondecreasing, we obtain

z0(t) = 2f(α(t))uq(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

! α0(t) + 2h(β(t))uq(β(t))w(u(β(t)))β0(t)

≤2 [z(t)]pq [f(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

! α0(t) +h(β(t))w(u(β(t)))β0(t)].

Hence z0(t)

[z(t)]qp ≤2f(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

! α0(t)

+ 2h(β(t))w(u(β(t)))β0(t).

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Integrating both sides on[0, t], we get p

p−q[z(t)]p−qp ≤ p

p−q[z(0)]p−qp + 2

Z α(t)

0

f(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds + 2

Z β(t)

0

h(s)w(u(s))ds, which can be rewritten as

(2.17) [z(t)]

p−q

p ≤c

2(p−q) p

+2(p−q) p

Z α(t)

0

f(s)

w(u(s)) + Z s

0

g(τ)w(u(τ))dτ

ds + 2(p−q)

p

Z β(t)

0

h(s)w(u(s))ds.

Denoting the right-hand side of (2.17) bym(t), we know[z(t)]

p−q

p ≤m(t)and m0(t) = 2(p−q)

p f(α(t)) w(u(α(t))) + Z α(t)

0

g(τ)w(u(τ))dτ

! α0(t) + 2 (p−q)

p h(β(t))w(u(β(t)))β0(t)

≤ 2(p−q)

p f(α(t)) w

z1p(α(t)) +

Z α(t)

0

g(τ)w

z1p (τ) dτ

! α0(t) + 2 (p−q)

p h(β(t))w

z1p(β(t)) β0(t)

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≤w

z1p(t)2(p−q) p

"

f(α(t)) 1 + Z α(t)

0

g(τ)dτ

!

α0(t) +h(β(t))β0(t)

#

≤w

mp−q1 (t)

2(p−q) p

"

f(α(t)) 1 + Z α(t)

0

g(τ)dτ

!

α0(t) +h(β(t))β0(t)

# .

The above relation gives m0(t)

w

mp−q1 (t) ≤ 2(p−q) p

"

f(α(t)) 1 + Z α(t)

0

g(τ)dτ

!

α0(t) +h(β(t))β0(t)

# .

Integrating both sides on[0, t]and using definition (2.4) we get G(m(t))

≤G(m(0)) +2 (p−q) p

"

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds+ Z β(t)

0

h(s)ds

#

≤G c

2(p−q) p

+2 (p−q) p

"

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds+ Z β(t)

0

h(s)ds

# .

Using the relationu(t)≤[z(t)]1p ≤[m(t)]p−q1 , we get the desired inequality (2.16).

If c = 0, the result can be proved by repeating the above procedure withε > 0 instead ofcand subsequently lettingε →0. This completes the proof.

Remark 4. Theorem 2 of Lipovan in [9] is a special case of Theorem 2.5 above, under the assumptions thatp= 2,q = 1,g(t)≡0andβ(t)≡t.

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3. Application

Example 3.1. Consider the delay integral equation (3.1) x5(t) =x20+2

Z α(t)

0

x3(s)M

s, x(s), Z s

0

N(s, τ, w(|x(τ)|))dτ

+h(s)x3(s)

ds.

Assume that

(3.2) |M(s, t, v)| ≤f(s)|v|, |N(s, t, v)| ≤g(t)|v|,

wheref, g, h, αandware as defined in Theorem2.1. From (3.1) and (3.2) we obtain

|x(t)|5 ≤x20+ 2 Z α(t)

0

|x(s)|3f(s) Z s

0

g(τ)w(|x(τ)|)dτ +h(s)|x(s)|3

ds.

Applying Theorem2.1to the last relation, we get an explicit bound on an unknown function

(3.3) |x(t)| ≤ (

G−1

"

G(ξ(t)) + 4 5

Z α(t)

0

f(s) Z s

0

g(τ)dτ ds

#)12 ,

where

ξ(t) =

5

q x40

+ 4 5

Z α(t)

0

h(s)ds.

In particular, ifω(t)≡tholds in (3.1), from (2.4) we derive

(3.4) G(t) =

Z t

0

1 ω

sp−q1 ds = Z t

0

1 sp−q1

ds= Z t

0

s12ds= 2√ t

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and

(3.5) G−1(t) = 1

4t2. Substituting (3.4) and (3.5) into inequality (3.3), we get

|x(t)| ≤p

ξ(t) + 2 5

Z α(t)

0

f(s) Z s

0

g(τ)dτ.

Example 3.2. Consider the following equation (3.6) x8(t) = x20+ 2

Z α(t)

0

x4(s)

M(s, x(s), w(|x(s)|)) +

Z s

0

N(s, τ, w(|x(τ)|))dτ

ds+ 2 Z α(t)

0

h(s)x4(s) ds.

Assume that

(3.7) |M(s, t, v)| ≤f(s)|v|, |N(s, t, v)| ≤f(s)g(t)|v|,

wheref, g, h, αandware as defined in Theorem2.4. From (3.6) and (3.7) we obtain

|x(t)|8 ≤x20+ 2 Z α(t)

0

|x(s)|4f(s)

w(|x(s)|) +

Z s

0

g(τ)w(|x(τ)|)dτ

+h(s)|x(s)|4

ds.

By Theorem2.4we get an explicit bound on an unknown function (3.8) |x(t)| ≤

( G−1

"

G(ξ(t)) + Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds

#)14 ,

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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where

ξ(t) = |x0|+ Z α(t)

0

h(s)ds.

In particular, ifω(t)≡t3 holds in (3.6), from (2.4) we obtain

(3.9) G(t) =

Z t

0

1 ω

sp−q1 ds= Z t

0

1 sp−q3

ds = Z t

0

s34ds = 4t14

and

(3.10) G−1(t) = 1

256t4. Substituting (3.9) and (3.10) into (3.8) we get

|x(t)| ≤[ξ(t)]14 +1 4

Z α(t)

0

f(s)

1 + Z s

0

g(τ)dτ

ds.

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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References

[1] L. OU-IANG, The boundness of solutions of linear differential equationay00+ A(t)y= 0, Shuxue Jinzhan, 3 (1997), 409–415.

[2] Y.H. KIM, On some new integral inequalities for functions in one and two variables, Acta Math. Sinica, 21(2) (2005), 423–434.

[3] B.G. PACHPATTE, Inequalities for Differential and Integral Equations, Aca- demic Press, New York, 1998.

[4] B.G. PACHPATTE, Explicit bounds on certain integral inequalities, J. Math.

Anal. Appl., 267 (2002) 48–61.

[5] M.C. TAN AND E.H. YANG, Estimation of bounded solutions of integral inequalities involving infinite integration limits, J. Inequal. Pure and Appl.

Math., 7(5) (2006), Art. 189. [ONLINE: http://jipam.vu.edu.au/

article.php?sid=806].

[6] E.H. YANGANDM.C. TAN, A generalization of Constantin’s integral inequal- ity and its discrete analogue, J. Inequal. Pure and Appl. Math., 8(2) (2007), Art. 57. [ONLINE:http://jipam.vu.edu.au/article.php?sid=

870].

[7] W.N. LI, M.A. HANAND F.W. MENG, Some new delay integral inequalities and their applications, J. Comput. Appl. Math., 180 (2005), 191–200.

[8] O. LIPOVAN, A retarded Gronwall-like inequality and its applications, J . Math. Anal . Appl., 252 (2000), 389–401.

[9] O. LIPOVAN, A retarded integral inequality and its applications, J. Math. Anal.

Appl., 285 (2003) 436–443.

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Retarded Inequality Man-Chun Tan and Zhi-Hong Li

vol. 9, iss. 3, art. 82, 2008

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[10] Q.H. MAANDE.H. YANG, On some new nonlinear delay integral inequalities, J. Math. Anal. Appl., 252 (2000), 864–878.

[11] Q.H. MAANDJ. PE ˇCARI ´C, Some new nonlinear retarded integral inequalities and their applications, Math. Inequal. and Applics., 9(4) (2006), 617–632.

[12] Y.G. SUN, On retarded integral inequalities and their applications, J. Math.

Anal. Appl., 301 (2005) 265–275.

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