African Diaspora Journal of Mathematics Special Issue in Memory of Prof. Ibni Oumar Mahamat Saleh Volume 8, Number 2, pp. 90–99 (2009)

ISSN 1539-854X www.african-j-math.org

P SEUDO A LMOST AUTOMORPHIC S OLUTIONS TO S OME N EUTRAL D ELAY I NTEGRAL E QUATIONS OF A DVANCED T YPE T OKA D IAGANA∗ Department of Mathematics, Howard University, Washington, DC 20059, USA

Abstract This paper is concerned with the existence of pseudo almost automorphic solutions to some neutral delay integral equations with delay. Some reasonable sufficient conditions are given, which do ensure the existence and uniqueness of pseudo almost automorphic solutions to those integral equations. Next we make extensive use of the previous result to characterize pseudo almost automorphic solutions to the logistic equation.

AMS Subject Classification: 43A60; 35B15; 47B55. Keywords: pseudo almost automorphic function; almost periodic function; neutral delay integral equation, logistic equation, integral equation of advanced type.

1

Introduction

Consider the integral equation u(t) = f (u(h1 (t))) +

Z ∞ t

Q(s, u(s), u(h2 (s)))C(t − s)ds + g(t),

t ∈R

(1.1)

where f , g, h1 , h2 ,C : R 7→ R are continuous functions with hi (R) = R for i = 1, 2, and Q : R × R × R 7→ R is jointly continuous. Letting, h1 (t) = h2 (t) = t − p (p > 0) in (1.1), one obtains the so-called neutral delay integral equation of advanced type u(t) = f (u(t − p)) +

Z ∞

Q(s, u(s), u(s − p)C(t − s)ds + g(t),

(1.2)

t

which was introduced in the literature by Burton [2] as an intermediate step while he was studying the existence of (periodic) bounded solutions to the logistic equation u0 (t) = au(t) + αu0 (t − p) − q(t, u(t), u(t − p)) where a, α (a > 0 and 0 ≤ |α| < 1) are constants. ∗ E-mail

address: [email protected]

(1.3)

Pseudo Almost Automorphic Solutions to Some Neutral Delay Integral Equations 91 In Diagana et al. [6], the existence of pseudo almost periodic solutions to (1.1) was established under some reasonable assumptions. The previous result was, subsequently utilized to obtain the existence of pseudo almost periodic solutions to the logistic equation. More recently, Diagana [4] has extended the previous results to the weighted pseudo almost periodic realm. The main motivation of this paper is to extend the above-mentioned (pseudo almost periodic) results to the pseudo almost automorphic setting. More precisely, under some suitable assumptions, we obtain the existence of pseudo almost automorphic solutions to (1.1) (Theorem 3.2). Next we make extensive use of the main result to prove the existence of pseudo almost automorphic solutions to the logistic equation (Theorem 3.4). The concept of pseudo almost automorphy, which is the central tool in this paper, was introduced in the literature a few year ago by Liang et al. [11, 12]. The pseudo almost automorphy is a generalization of both the classical almost automorphy due to Bochner [1] and that of pseudo almost periodicity due to Zhang [16, 17, 18]. It has recently generated several developments and extensions. For the most recent developments, we refer the reader to [3, 10, 11, 12, 13]. One should mention that more recently, in Diagana [3], the concept of S p -pseudo almost automorphy (or Stepanov-like pseudo almost automorphy) was also introduced, which in fact generalizes in a natural fashion the concept of pseudo almost automorphy. This paper is organized as follows. In Section 2, we introduce the background on pseudo almost automorphic functions needed in the sequel. Section 3 is devoted to the main results. In Section, an example is given to illustrate the main results.

2

Pseudo Almost Automorphic Functions

Let (X, k · k), (Y, k · kY ) be two Banach spaces. Let BC(R, X) (respectively, BC(R × Y, X)) denote the collection of all X-valued bounded continuous functions (respectively, the class of jointly bounded continuous functions F : R × Y 7→ X). The space BC(R, X) equipped with the sup norm k · k∞ is a Banach space. Furthermore, C(R, Y) (respectively, C(R × Y, X)) denotes the class of continuous functions from R into Y (respectively, the class of jointly continuous functions F : R × Y 7→ X). Definition 2.1. A function f ∈ C(R, X) is said to be almost automorphic if for every sequence of real numbers (s0n )n∈N , there exists a subsequence (sn )n∈N such that g(t) := lim f (t + sn ) n→∞

is well defined for each t ∈ R, and lim g(t − sn ) = f (t)

n→∞

for each t ∈ R. Note that the function g in Definition 2.1 is measurable, but not necessarily continuous. Moreover, if g is continuous, then f is uniformly continuous. If the convergence above is uniform in t ∈ R, then f is almost periodic. Denote by AA(X) the collection of all almost automorphic functions from R into X. Note that AA(X) equipped with the sup norm, k · k∞ , turns out to be a Banach space. Among other things, almost automorphic functions satisfy the following properties.

92

T. Diagana

Theorem 2.2. [14] If ϕ, ϕ1 , ϕ2 ∈ AA(X), then (i) ϕ1 + ϕ2 ∈ AA(X), (ii) λϕ ∈ AA(X) for any scalar λ, (iii) ϕα ∈ AA(X) where ϕα : R → X is defined by ϕα (·) = ϕ(· + α), © ª (iv) the range Rϕ := ϕ(t) : t ∈ R is relatively compact in X, thus ϕ is bounded in norm, (v) if ϕn → ϕ uniformly on R where each ϕn ∈ AA(X), then ϕ ∈ AA(X) too. Moreover, if ψ ∈ L1 (R), then ϕ ∗ ψ ∈ AA(R), where ϕ ∗ ψ is the convolution of ϕ with ψ on R. Example 2.3. A classical example of an almost automorphic function, which is not almost periodic is the function defined by µ ¶ 1 ϕα (t) = cos , t ∈ R, 2 + sin αt + sint where α is an irrational. It can be shown that ϕα is not uniformly continuous, and hence is not almost periodic. Definition 2.4. A continuous function Φ : R × X 7→ Y is said to be almost automorphic in t ∈ R for each z ∈ X if for every sequence of real numbers (σn )N there exists a subsequence (sn )N of (σn )N such that Ψ(t, z) := lim Φ(t + sn , z) in Y n7→∞

is well defined for each t ∈ R and each z ∈ X and Φ(t, u) = lim Ψ(t − sn , z) in Y n7→∞

for each t ∈ R and for every z ∈ X. The collection of such functions will be denoted by AA(X, Y). Theorem 2.5. [15] Let F : R × X 7→ Y be an almost automorphic function in t ∈ R for each z ∈ X and assume that F satisfies a Lipschitz condition in z uniformly in t ∈ R. Let φ : R 7→ X be almost automorphic. Then the function Φ : R 7→ Y defined by Φ(t) := F(t, φ(t)) is almost automorphic. For more on almost automorphic functions and related issues, we refer the reader to the book by N’Gu´er´ekata [14]. Define the classes of functions PAP0 (X) and AA0 (R × X) respectively as follows: ½ ¾ Z 1 T PAP0 (X) := u ∈ BC(R, X) : lim ku(s)kds = 0 , T →∞ 2T −T and AA0 (R × Y) is the collection of all functions F ∈ BC(R × Y, X) such that 1 T →∞ 2T lim

Z T −T

kF(t, u)kdt = 0

uniformly in u ∈ K, where K ⊂ Y is any bounded subset.

Pseudo Almost Automorphic Solutions to Some Neutral Delay Integral Equations 93 Definition 2.6. A function f ∈ C(R, X) is called pseudo almost automorphic if it can be expressed as f = h + ϕ, where h ∈ AA(X) and ϕ ∈ PAP0 (X). The collection of such functions will be denoted by PAA(X). Definition 2.7. A function F ∈ C(R × Y, X) is said to be pseudo almost automorphic if it can be expressed as F = G + Φ, where G ∈ AA(R × Y) and ϕ ∈ AA0 (R × Y). The collection of such functions will be denoted by PAA(R × Y). Theorem 2.8. [12] The space PAA(X) equipped with the sup norm k·k∞ is a Banach space. Theorem 2.9. [12] If f : R × Y 7→ X belongs to PAA(R × Y) and if x 7→ f (t, x) is uniformly continuous on any bounded subset K of Y for each t ∈ R, then the function defined by h(t) = f (t, ϕ(t)) belongs to PAA(X) provided ϕ ∈ PAA(Y).

3

Main Results

Throughout the rest of the paper, we suppose that X = Y = R equipped the classical absolute value. Though when we deal with the pseudo almost automorphy of the function Q it would be more convenient to choose X = R × R. (H.1) The function f , g : R 7→ R are pseudo almost automoprhic and f satisfies, | f (x) − f (y)| ≤ α . |x − y|,

0 ≤ α < 1,

for all x, y ∈ R; (H.2) The function hi : R 7→ R is continuous, hi (R) = R, and u(hi ) ∈ PAA(R) (i = 1, 2) whenever u ∈ PAA(R); (H.3) The function Q : R×(R×R) 7→ R, (t, x, y) 7→ Q(t, x, y) is pseudo almost automorphic. The function (x, y) 7→ Q(t, x, y) is uniformly continuous on any bounded subset K of R2 for each t ∈ R. Setting Q = Q1 + Q2 with Q1 ∈ AA(R2 , R) and Q2 ∈ AA0 (R × R2 ), we suppose that Q2 (·, v(·), v(h2 (·))) ∈ L1 (R) for each v ∈ PAA(R) where h2 is the function appearing in (H.2). Furthermore, there exists 0 ≤ k ≤ 1 such that |Q(t, x, y) − Q(t, w, z)| ≤ (k . |x − w| + (1 − k) . |y − z|) for all x, y, z, w ∈ R; (H.4) 0 <

Z ∞ 0

|C(−s)|ds = C0 < ∞.

Our main result requires the following technical lemma: Lemma 3.1. Under assumptions (H.2)-(H.3)-(H.4), the function defined by Γu(t) := maps PAA(R) into itself.

Z ∞ t

Q(s, u(s), u(h2 (s)))C(t − s)ds

94

T. Diagana

Proof. Let u ∈ PAA(R). First of all, note that t 7→ u(h2 (t)) is pseudo almost automorphic, by (H.2). Using (H.3) it follows that s 7→ Q(s, u(s), u(h2 (s))) is pseudo almost automorphic, by Theorem 2.9. Now write Q = Q1 + Q2 where Q1 ∈ AA(R2 , R) and Q2 ∈ AA0 (R × R2 ). Consequently, Γu = Γ1 (u) + Γ2 (u) where Γ1 u(t) := and Γ2 u(t) :=

Z ∞

Q1 (s, u(s), u(h2 (s)))C(t − s)ds

t

Z ∞ t

Q2 (s, u(s), u(h2 (s)))C(t − s)ds.

To complete the proof, it suffices to prove that Γ1 u ∈ AA(R) and Γ2 u ∈ PAP0 (R). Since Q1 (·, u(·), u(h2 (·))) ∈ AA(R), for every sequence of real numbers (s0n )n∈N , there exists a subsequence (sn )n∈N such that L(t) := lim Q1 (t + sn , u(t + sn ), u(h2 (t + sn ))) n→∞

is well defined for each t ∈ R, and lim L(t − sn ) = Q1 (t, u(t), u(h2 (t)))

n→∞

for each t ∈ R. Now Γ1 u(t + sn )) =

Z ∞ t

Q1 (r + sn , u(r + sn ), u(h2 (r + sn )))C(t − r)dr,

by setting r = s − sn . Using assumption (H.4) and the Lebesgue Dominated Convergence Theorem it follows Z that Γ1 u(t + sn ) converges to N(t) :=



L(r)C(t − r)dr.

t

Arguing as previously, one can easily see that N(t − sn ) converges to Γ1 u(t). The next step consists of showing that Γ2 u ∈ PAP0 (R). This was done in Diagana et al.[6]. Though for the sake of clarity, the proof will be reproduced here. First of all, s 7→ Γ2 (u)(s) is a bounded continuous function. Thus to complete the proof we need to show that Z r 1 lim |Γ2 u(t)| dt = 0. r→∞ 2r −r Clearly,

Z r

1 lim r→∞ 2r where

1 I := lim r→∞ 2r

µZ

Z r −r

1 J := lim r→∞ 2r

dt Z r −r

t

r

−r

|Γ2 u(t)| dt ≤ I + J, ¶

|Q2 (s, u(s), u(h2 (s)))| . |C(t − s)|ds , and

Z ∞

dt r

|Q2 (s, u(s), u(h2 (s)))| . |C(t − s)|ds.

To show that I = J = 0, we make use of the following arguments:

Pseudo Almost Automorphic Solutions to Some Neutral Delay Integral Equations 95 Z s

(A.0)

−r

|C(t − s)|dt =

Z r+s 0

|C(−v)|dv ≤ C0 for all r + s ≥ 0;

(A.1) Q2 (·, u(·), u(h2 (·))) ∈ AA0 (R); (A.2) Q2 (·, u(·), u(h2 (·))) ∈ L1 (R). Indeed, by changing the order of integration we obtain: Z

r 1 |Q2 (s, u(s), u(h2 (s)))| ds I = lim r→∞ 2r −r Z C0 r |Q2 (s, u(s), u(h2 (s)))| ds ≤ lim r→∞ 2r −r = 0,

µZ

s

−r

¶ |C(t − s)|dt

by (A.0)-(A.1). Similarly, Z

Z

∞ r 1 |Q2 (s, u(s), u(h2 (s)))| ds |C(t − s)|dt r→∞ 2r r −r Z ∞ Z s+r 1 = lim |Q2 (s, u(s), u(h2 (s)))|ds |C(−v)|dv r→∞ 2r r s−r

where φr (s) = Now

J =

lim

=

lim

1 2r

Z ∞

r→∞ r

Z s+r

|Q2 (s, u(s), u(h2 (s)))| . φr (s) ds,

|C(−v)|dv.

s−r

φr (s) ≤ Z s+r

by using the fact that s−r

C0 , 2r

|C(−v)|dv ≤ C0 for all s ≥ r. And hence φr (s) 7→ 0 as r 7→ ∞.

Since Q2 (·, u(·) u(h2 (·))) ∈ L1 (R) it follows that Z ∞

lim

r→∞ r

|Q2 (s, u(s), u(h2 (s)))| . φr (s) ds = 0,

by using (A.2) and the Lebesgue Dominated Convergence Theorem. Therefore, Γ2 u ∈ PAP0 (R). Theorem 3.2. Under assumptions (H.1)-(H.2)-(H.3)-(H.4), equation (1.1) has a unique pseudo almost automorphic solution whenever α +C0 < 1. Proof. Let u ∈ PAA(R). Define the nonlinear operator Λ(u)(t) := f (u(h1 (t))) +

Z ∞ t

Q(s, u(s)u(h2 (s)))C(t − s)ds + g(t),

t ∈ R.

First of all, let us mention that f (u(h1 (·))) ∈ PAA(R), which follows immediately from Theorem 2.9. Thus, in view of the previous facts and Lemma 3.1, it easily follows that Λ maps PAA(R) into itself and that Γ1 u and Γ2 u are respectively the almost automorphic

96

T. Diagana

and ergodic perturbation components of Λ(u). To complete the proof, we must show that Λ : PAA(R) 7→ PAA(R) has a unique fixed-point. For u, v ∈ PAP(R), |Λ(u)(t) − Λ(v)(t)| ≤ αku − vk∞ + ≤ αku − vk∞ +

Z ∞ t

Z ∞ t

|(Q(s, u(s), u(h2 (s))) − Q(s, v(s), v(h2 (s))))| . |C(t − s)|ds [k|u(s) − v(s)| + (1 − k)|u(h2 (s) − v(h2 (s))] . |C(t − s)|ds

≤ αku − vk∞ + ku − vk∞ . and hence

Z ∞

|C(t − s)|ds,

t

kΛ(u) − Λ(v)k∞ ≤ (α +C0 ) . ku − vk∞ . Therefore, by the Banach fixed-point principle, the operator Λ has a unique fixed point whenever α + C0 < 1, which obviously is the only pseudo almost automorphic solution to (1.1). Letting h1 (t) = h2 (t) = t − p, one can easily see that (H.2) holds, and hence the next corollary is a straightforward consequence of Theorem 3.2. (In assumption (H.3), we suppose that the ergodic component Q2 of Q is given such that Q2 (·, v(·), v(· − p)) ∈ L1 (R) for each v ∈ PAA(R).) Corollary 3.3. Under assumptions (H.1)-(H.3)-(H.4), equation (1.2) has a unique pseudo almost automorphic solution whenever α +C0 < 1. The rest of this paper is devoted to the existence and uniqueness of pseudo almost automorphic solutions to (1.3). In what follows we define the function q(t, ˜ x, y) := q(t, x, y) − aαy for all t ∈ R and x, y ∈ R, where a, α and q are respectively the constants and the function appearing in (1.3). We require the following assumption: (H.5) The function q˜ : R × (R × R) 7→ R, (t, x, y) 7→ q(t, ˜ x, y) is pseudo almost automorphic in t ∈ R uniformly if (x, y) ∈ R×R. Setting q˜ = q1 +q2 where q1 ∈ AP(R×R×R, R) and q2 ∈ AP0 (R × R × R, R), we suppose that q2 (·, v(·), v(· − p))) ∈ L1 (R) for each v ∈ PAA(R). Furthermore, suppose that α, a > 0, and |q(t, x, y) − q(t, w, z)| ≤ (1 − aα)|x − w| for all t, x, y, z, w ∈ R. Theorem 3.4. Under assumption (H.5), the logistic equation, (1.3), has a unique pseudo 1 almost automorphic solution whenever α + < 1. a

Pseudo Almost Automorphic Solutions to Some Neutral Delay Integral Equations 97 Proof. One follows along the same lines as in [2]. We are interested in bounded solutions only. Thus if u is a bounded solution to (1.3), then d [(u(t) − αu(t − p)) e−at ] = [aαu(t − p) − q(t, u(t), u(t − p))]e−at . dt Clearly, u(t) = αu(t − p) +

Z ∞

[q(s, u(s), u(s − p)) − aαu(s − p)]ea(t−s) ds,

(3.1)

t

for each t ∈ R, by lim [(u(t) − αu(t − p)) e−at ] = 0 (u is bounded). t→∞

To complete the proof, in (1.1), take f (t) = αt, h1 (t) = h2 (t) = t − p, C(t) = eat , g(t) = 0, and Q(t, u(t), u(t − p)) = q(t, ˜ u(t), u(t − p)), ∀t ∈ R, and follow along the same lines as in the proof of Theorem 3.2.

4

Example

Fix once and for all p > 0. Consider the integral equation K(t) + u(t) = 1 + |u(t − p)|

Z ∞ t

2

q(t)e−(t−s) ds + g(t), t ∈ R, (4.1) 1 + α1 |u(s)| + α2 |u(s − p)|

where K ∈ PAA(R), q = q1 + q2 with q1 ∈ AA(R) and q2 ∈ PAP0 (R) ∩ L1 (R), α1 ∈ (0, 1) with α1 + α2 = 1, and ¶ µ 1 1 + g(t) = cos . 2 + sin πt + sint 1 + t2 In addition to above we assume that sup |K(t)| = kKk∞ < 1. t∈R

Theorem 4.1. Under assumptions previous assumptions, equation (4.1) has a unique pseudo almost automorphic solution whenever √ π kKk∞ < 1 − . 2 2

It is not hard to check that assumptions (H.1)-(H.2)-(H.3) hold. Moreover C(s) = e−s √ π and hence assumption (H.4) hold with C0 = 2 . Thus it remains to check that the condition α + C0 < 1 of Theorem 3.2 √is satisfied. Indeed, here one can take α = kKk∞ ∈ (0, 1) and therefore α +C0 = kKk∞ + 2π < 1 whether kKk∞ is chosen small enough, that is, √ π kKk∞ < 1 − ≈ 0.11377. 2

98

T. Diagana

References [1] S. Bochner, Continuous mappings of almost automorphic and almost periodic functions, Proc. Nat. Acad. Sci. USA 52 (1964), pp. 907-910. [2] T. A. Burton, Basic Neutral Integral Equations of Advanced Type, Nonlinear Anal. 31 (1998), no. 3/4, pp. 295–310. [3] T. Diagana, Existence of Pseudo Almost Automorphic Solutions to Some Abstract Differential Equations with S p -Pseudo Almost Automorphic Coefficients. Nonlinear Anal. 70 (2009), no. 11, pp. 3781-3790. [4] T. Diagana, Weighted Pseudo Almost Periodic Solutions to Some Neutral Delay Integral Equation of Advanced Type. Nonlinear Anal. 70 (2009), no. 1, pp. 298-304. [5] T. Diagana, Pseudo almost periodic functions in Banach spaces. Nova Science Publishers, Inc., New York, 2007. [6] T. Diagana and C. M. Mahop, Pseudo almost periodic solutions to a neutral delay integral equation. Cubo 9 (2007), no. 1, pp. 47-55. [7] T. Diagana and E. Hern`andez M., Existence and Uniqueness of Pseudo Almost Periodic Solutions to Some Abstract Partial Neutral Functional-Differential Equations and Applications, J. Math. Anal. Appl. 327(2007), no. 2, pp. 776–791. [8] T. Diagana, N. Henr´ıquez, and E. Hern`andez, Almost automorphic mild solutions to some partial neutral functional-differential Equations and Applications. Nonlinear Anal. 69 (2008), no. 5, pp. 1485-1493. [9] K. Ezzinbi, S. Fatajou and G. M. NGu´er´ekata, Pseudo almost automorphic solutions for dissipative differential equations in Banach spaces. J. Math. Anal. Appl. 351 (2009), no. 2, 765-772. [10] K. Ezzinbi, S. Fatajou and G. M. NGu´er´ekata, Pseudo almost automorphic solutions to some neutral partial functional differential equations in Banach space. Nonlinear Anal. 70 (2009), no. 4, 2009. [11] J. Liang, J. Zhang, and T-J. Xiao, Composition of Pseudo Almost Automorphic and Asymptotically almost automorphic functions. J. Math. Anal. Appl. 340 (2008), no. 1493-1499. [12] T-J. Xiao, J. Liang, J. Zhang, Pseudo almost automorphic solutions to semilinear differential equations in Banach spaces. Semigroup Forum 76 (2008), no. 3, 518–524. [13] J. Liang, G. M. N’Gu´er´ekata, T-J. Xiao, and J. Zhang, Some properties of pseudo almost automorphic functions and applications to abstract differential equations. Nonlinear Anal. (n press). [14] G. M. N’Gu´er´ekata, Almost automorphic functions and almost periodic functions in abstract spaces, Kluwer Academic / Plenum Publishers, New York-London-Moscow, 2001.

Pseudo Almost Automorphic Solutions to Some Neutral Delay Integral Equations 99 [15] G. M. N’Gu´er´ekata, Existence and uniqueness of almost automorphic mild soilution to some semilinear abstract differential equations, Semigroup Forum 69 (2004), pp. 80-86. [16] C. Y. Zhang, Pseudo Almost Periodic Solutions of Some Differential Equations, J. Math. Anal. Appl. 151 (1994), pp. 62–76. [17] C. Y. Zhang, Pseudo Almost Periodic Solutions of Some Differential Equations II, J. Math. Anal. Appl. 192 (1995), pp. 543–561. [18] C. Y. Zhang, Integration of Vector-Valued Pseudo Almost Periodic Functions, Proc. Amer. Math. Soc. 121 (1994), pp. 167–174.

Pseudo almost automorphic solutions to some neutral ...

African Diaspora Journal of Mathematics. Special Issue in Memory of Prof. Ibni Oumar Mahamat Saleh. Volume 8, Number 2, pp. 90–99 (2009). ISSN 1539-854X.

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