Electronic Journal of Qualitative Theory of Differential Equations 2010, No. 58, 1-17; http://www.math.u-szeged.hu/ejqtde/

ON THE EXISTENCE OF MILD SOLUTIONS TO SOME SEMILINEAR FRACTIONAL INTEGRO-DIFFERENTIAL EQUATIONS ´ EKATA ´ T. DIAGANA, G. M. MOPHOU, AND G. M. N’GUER Abstract. This paper deals with the existence of a mild solution for some fractional semilinear differential equations with non local conditions. Using a more appropriate definition of a mild solution than the one given in [12], we prove the existence and uniqueness of such solutions, assuming that the linear part is the infinitesimal generator of an analytic semigroup that is compact for t > 0 and the nonlinear part is a Lipschitz continuous function with respect to the norm of a certain interpolation space. An example is provided.

1. Introduction Let X be a Banach space and let T > 0. This paper is aimed at discussing about the existence and the uniqueness of a mild solution for the fractional semilinear integro-differential equation with nonlocal conditions in the form:  Z t  β   a(t − s)h(s, x(s)) ds, t ∈ [0, T ],  D x(t) = −Ax(t) + f (t, x(t)) + 0 (1)     x(0) + g(x) = x , 0

where the fractional derivative Dβ (0 < β < 1) is understood in the Caputo sense, the linear operator −A is the infinitesimal generator of an analytic semigroup (R(t))t≥0 that is uniformly bounded on X and compact for t > 0, the function a(·) is real-valued such that Z T a(s) ds < ∞, (2) aT = 0

the functions f, g and h are continuous, and the non local condition g(x) =

p X

ck x(tk ),

k=1

with ck , k = 1, 2, ...p, are given constants and 0 < t1 < t2 < ... < tp ≤ T . Let us recall that those nonlocal conditions were first utilized by K. Deng [4]. In his paper, K. Deng indicated that using the nonlocal condition x(0) + g(x) = x0 1991 Mathematics Subject Classification. 34K05; 34A12; 34A40. Key words and phrases. fractional abstract differential equation, sectorial operator.

EJQTDE, 2010 No. 58, p. 1

to describe for instance, the diffusion phenomenon of a small amount of gas in a transparent tube can give better result than using the usual local Cauchy Problem x(0) = x0 . Let us observe also that since Deng’s paper, such problem has attracted several authors including A. Aizicovici, L. Byszewski, K. Ezzinbi, Z. Fan, J. Liu, J. Liang, Y. Lin, T.-J. Xiao, H. Lee, etc. (see for instance [1, 2, 3, 4, 9, 8, 7, 14, 11, 13] and the references therein). This problem has been studied in Mophou and N’Gu´er´ekata [12]. In this paper, we revisit that work and use a more appropriate definition for mild solutions. Namely, we investigate the existence and the uniqueness of a mild solution for the fractional semilinear differential equation (1), assuming that f is defined on [0, T ] × Xα × Xα where Xα = D(Aα ) (0 < α < 1), the domain of the fractional powers of A. The rest of this paper is organized as follows. In Section 2 we give some known preliminary results on the fractional powers of the generator of an analytic compact semigroup. In Section 3, we study the existence and the uniqueness of a mild solution for the fractional semilinear differential equation (1). We give an example to illustrate our abstract results.

2. Preliminaries Let I = [0, T ] for T > 0 and let X be a Banach space with norm k · k. Let  B(X), k · kB(X) be the Banach space of all linear bounded operators on X and A : D(A) → X be a linear operator such that −A is the infinitesimal generator of an analytic semigroup of uniformly bounded linear operators (R(t))t≥0 , which is compact for t > 0. In particular, this means that there exists M > 1 such that (3)

supkR(t)kB(X) ≤ M. t≥0

Moreover, we assume without loss of generality that 0 ∈ ρ(A). This allows us to define the fractional power Aα for 0 < α < 1, as a closed linear operator on its domain D(Aα ) with inverse A−α (see [8]). We have the following basic properties for fractional powers Aα of A: Theorem 2.1. ([15], pp. 69 -75). Under previous assumptions, then: (i) Xα = D(Aα ) is a Banach space with the norm kxkα := kAα xk for x ∈ D(Aα ); (ii) R(t) : X → Xα for each t > 0; (iii) Aα R(t)x = R(t)Aα x for each x ∈ D(Aα ) and t ≥ 0; EJQTDE, 2010 No. 58, p. 2

(iv) For every t > 0, Aα R(t) is bounded on X and there exist Mα > 0 and δ > 0 such that kAα R(t)kB(X) ≤

(4)

Mα −δt e ; tα

(v) A−α is a bounded linear operator in X with D(Aα ) = Im(A−α ); and (vi) If 0 < α ≤ ν, then D(Aν ) ֒→ D(Aα ). Remark 2.2. Observe as in [9] that by Theorem 2.1 (ii) and (iii), the restriction Rα (t) of R(t) to Xα is exactly the part of R(t) in Xα . Let x ∈ Xα . Since kR(t)xkα = kAα R(t)xk = kR(t)Aα xk ≤ kR(t)kB(X) kAα xk = kR(t)kB(X) kxkα , and as t decreases to 0 kR(t)x − xkα = kAα R(t)x − Aα xk = kR(t)Aα x − Aα xk → 0, for all x ∈ Xα , it follows that (R(t))t≥0 is a family of strongly continuous semigroup on Xα and kRα (t)kB(X) ≤ kR(t)kB(X) for all t ≥ 0. Lemma 2.3. [9] The restriction Rα (t) of R(t) to Xα is an immediately compact semigroup in Xα , and hence it is immediately norm-continuous. Now, let Φβ be the Mainardi function: Φβ (z) =

+∞ X

(−z)n . n!Γ(−βn + 1 − β) n=0

Then (5a)

Φβ (t) ≥ 0 for all t > 0; Z ∞ Φβ (t)dt = 1;

(5b)

0

(5c)

Z

0



Γ(1 + η) , tη Φβ (t)dt = Γ(1 + βη)

∀η ∈ [0, 1].

For more details we refer to [10]. We set

(6)

Sβ (t)

=

Z



Φβ (θ)R(θtβ ) dθ,

0

(7)

Pβ (t)

=

Z



βθΦβ (θ)R(tβ θ)dθ

0

Then we have the following results EJQTDE, 2010 No. 58, p. 3

Lemma 2.4. [16] Let Sβ and Pβ be the operators defined respectively by (6) and (7). Then β kxk for all x ∈ X and t ≥ 0. (i) kSβ (t)xk ≤ M kxk; kPβ (t)xk ≤ M Γ(β + 1) (ii) The operators (Sβ (t))t≥0 and (Pβ (t))t≥0 are strongly continuous. (iii) The operators (Sβ (t))t>0 and (Pβ (t))t>0 are compact. Lemma 2.5. Let Sβ and Pβ be the operators defined respectively by (6) and (7). Then kSβ (t)xkα ≤ M kxkα , ∀x ∈ Xα , t ≥ 0,  −βα Γ(2 − α)   βMα t  kxk if x ∈ X, t > 0, Γ(1 + β(1 − α)) kPβ (t)xkα ≤ β   kxkα if x ∈ Xα , t > 0.  Γ(1 + β)

Proof. Using (3) and (5b) we have for any x ∈ Xα and t ≥ 0,

Z ∞

β

kSβ (t)xkα = Φβ (θ)R(θt )x dθx

0 α Z ∞

α

β

≤ Φβ (θ) A R(θt )x dθx 0Z ∞ ≤ M Φβ (θ) kAα xk dθ 0

= M kxkα , ∀x ∈ Xα .

In view of (4) and (5c), we can write for any t > 0,

Z ∞

β

kPβ (t)xkα = βθΦβ (θ)R(θt )x dθ

0 α Z ∞

α

β

≤ βθΦβ (θ) A R(θt )x dθ Z0 ∞ ≤ βθΦβ (θ)kAα R(θtβ )kB(X) kxk dθ 0 Z ∞ −αβ ≤ βMα t kxk θ1−α Φβ (θ) dθ 0

βMα t−βα Γ(2 − α) kxk , ∀x ∈ X Γ(1 + β(1 − α))

≤ and kPβ (t)xkα

= ≤ ≤ =

Z ∞

β

βθΦβ (θ)R(θt )x dθ

0 α Z ∞

α

β

βθΦβ (θ) A R(θt )x dθ 0 R∞ M kxkα 0 βθΦ(θ)dθ β , ∀x ∈ Xα . M kxkα Γ(1 + β)  EJQTDE, 2010 No. 58, p. 4

Definition 2.6. ([5, 6]) Let Sβ and Pβ be operators defined respectively by (6) and (7). Then a continuous function x : I → X satisfying for any t ∈ [0, T ] the equation x(t) (8)

= +

Sβ (t)(x0 − g(x))   Z t Z t (t − s)β−1 Pβ (t − s) (f (s, x(s)) − a(t − s)h(s, x(s)) ds, 0

0

is called a mild solution of the equation (1). In the sequel, we set (9)

Kx(t) :=

Z

t 0

a(t − s)h(s, x(s)) ds.

We set α ∈ (0, 1) and we will denote by Cα , the Banach space C([0, T ], Xα ) endowed with the supnorm given by kxk∞ := sup kxkα , t∈I

for x ∈ C.

3. Main Results In addition to the previous assumptions, we assume that the following hold. (H1 ) The function f : I × Xα → X is continuous and satisfies the following condition: there exists a function µ1 (t) ∈ L∞ (I, R+ ) such that kf (t, x, ) − f (t, y)k ≤ µ1 (t)kx − ykα for all t ∈ I, x, y ∈ Xα . (H2 ) The function h : I × Xα → X is continuous and satisfies the following condition: there exists a function µ2 (t) ∈ L∞ (I, R+ ) such that kh(t, x, ) − h(t, y)k ≤ µ2 (t)kx − ykα for all t ∈ I, x, y ∈ Xα . (H3 ) The function g : Cα → Xα is continuous and there exists a constant b such that kg(x) − g(y)kα ≤ bkx − yk∞ for all x, y ∈ Cα . Theorem 3.1. Suppose assumptions (H1 )-(H3 ) hold and that Ωα,β,T < 1 where # "  βMα Γ(2 − α)T β(1−α)  kµ1 kL∞ (I,R+ ) + aT kµ2 kL∞ (I,R+ ) . Ωα,β,T = M b + Γ(1 + β(1 − α))(β(1 − α)) If x0 ∈ Xα , then (1) has a unique mild solution x ∈ Cα . EJQTDE, 2010 No. 58, p. 5

Proof. Define the nonlinear integral operator F : Cα → Cα by (F x)(t)

= Sβ (t) (x0 − g(x)) , Z t (t − s)β−1 Pβ (t − s) [f (s, x(s)) + Kx(s)] ds. + 0

where K is given by (9). In view of Lemma 2.4- (ii), the integral operator F is well defined. Now take t ∈ I and x, y ∈ Cα . We have k(F x)(t) − (F y)(t)kα

≤ + +

kS (t) (g(x) − g(y)) kα Z tβ (t − s)β−1 kPβ (t − s) (f (s, x(s)) − f (s, y(s)))kα ds 0 Z t (t − s)β−1 kPβ (t − s) (K x(s) − K y(s))kα ds 0

which according to Lemma 2.5 and (H3 ) gives k(F x)(t) − (F y)(t)kα ≤ M bkx − yk∞ Z t βMα Γ(2 − α) (t − s)β(1−α)−1 k(f (s, x(s)) − f (s, y(s)))k ds + Γ(1 + β(1 − α)) Z 0 t βMα Γ(2 − α) + (t − s)β(1−α)−1 k(K x(s) − K y(s))k ds Γ(1 + β(1 − α)) 0 Since (H2 ) and (2) hold, we can write Z s kKx(s) − Ky(s)k = a(s − τ ) kh(τ, x(τ )) − h(τ, y(τ ))k dτ Z0 s a(s − τ )µ2 (τ ) kx(τ ) − y(τ )k dτ ≤ 0



aT kµ2 kL∞ (I,R+ ) kx − yk∞ .

Thus, using (H1 ) we obtain k(F x)(t) − (F y)(t)kα ≤M bkx − yk∞ Z βMα Γ(2 − α)kx − yk∞ t + (t − s)β(1−α)−1 µ1 (s) ds Γ(1 + β(1 − α)) 0 βMα Γ(2 − α)T β(1−α) aT kµ2 kL∞ (I,R+ ) kx − yk∞ + Γ(1 + β(1 − α))(β(1 − α) ≤M bkx − yk∞ βMα Γ(2 − α)T β(1−α) + kx − yk∞ kµ1 kL∞ (I,R+ ) Γ(1 + β(1 − α))(β(1 − α)) β(1−α) βMα Γ(2 − α)T aT + kx − yk∞ kµ2 kL∞ (I,R+ ) Γ(1 + β(1 − α))(β(1 − α)) ≤Ωα,β,T kx − yk∞ EJQTDE, 2010 No. 58, p. 6

So we get k(F x)(t) − (F y)(t)k∞



Ωα,β,T (t)kx − yk∞ .

Since Ωα,β,T < 1, the contraction mapping principle enables us to say that, F has a unique fixed point in Cα , Z t x(t) = Sβ (t) (x0 − g(x)) + (t − s)β−1 Pβ (t − s) [f (s, x(s)) + K x(s)] ds 0

which is the mild solution of (1).



Now we assume that (H4 ) The function f : I × Xα → X is continuous and satisfies the following condition: there exists a positive function µ1 ∈ L∞ (I, R+ ) such that kf (t, x)k ≤ µ1 (t), (H5 ) The function h : I × Xα → X is continuous and satisfies the following condition: there exists a positive function µ2 ∈ L∞ (I, R+ ) such that kh(t, x)k ≤ µ2 (t), (H6 ) The function g ∈ C(Cα , Xα ) is completely continuous and there exist λ, γ > 0 such that kg(x)kα ≤ λkxk∞ + γ. Theorem 3.2. Suppose that assumptions (H4 )-(H6 ) hold. If x0 ∈ Xα and (10)

Mλ <

1 2

then (1.1) has a mild solution on [0, T ]. Proof. Define the integral operator F : Cα → Cα by (F x)(t)

= +

Sβ (t) (x0 − g(x)) , Z t (t − s)β−1 Pβ (t − s) [f (s, x(s)) + Kx(s)] ds, 0

and choose r such that r

≥ +

 T β(1−α) βMα Γ(2 − α)  kµ1 kL∞ (I,R+ ) + aT kµ2 kL∞ (I,R+ ) Γ(1 + β(1 − α))(β(1 − α)) 2M (kx0 kα + γ).

2

Let Br = {x ∈ Cα : kxk∞ ≤ r}. We proceed in three main steps. EJQTDE, 2010 No. 58, p. 7

Step 1. have

We show that F (Br ) ⊂ Br . For that, let x ∈ Br . Then for t ∈ I, we k(F x)(t)kα

≤ + +

kS (t) (x0 − g(x))kα Z tα (t − s)β−1 kPα (t − s)f (s, x(s))kα ds Z0 t (t − s)β−1 kPα (t − s)K x(s)kα ds 0

which according to (H4 )-(H6 ) and Lemma 2.5 gives k(F x)(t)kα

≤ + + ≤ + +

M (kx0 kα + λkxk∞ + γ) Z t βMα Γ(2 − α) (t − s)β(1−α)−1 kf (s, x(s))k ds Γ(1 + β(1 − α)) Z0 t βMα Γ(2 − α) (t − s)β(1−α)−1 kKx(s)k ds Γ(1 + β(1 − α)) 0 M (kx0 kα + λkxk∞ + γ) Z t βMα Γ(2 − α) (t − s)β(1−α)−1 µ1 (s) ds Γ(1 + β(1 − α)) Z0 Z s t βMα Γ(2 − α) β(1−α)−1 (t − s) a(s − τ )µ2 (τ )dτ ds. Γ(1 + β(1 − α)) 0 0

Consequently, using the inequality M λ < choice of r above, we get k(F x)(t)kα

1 2,

which yields M λkxk∞ <

r 2

and the



M (kx0 kα + λkxk∞ + γ) kµ1 kL∞ (I,R+ ) T β(1−α) βMα Γ(2 − α) + (β(1 − α) Γ(1 + β(1 − α)) kµ2 kL∞ (I,R+ ) T β(1−α) βMα Γ(2 − α)aT . + (β(1 − α) Γ(1 + β(1 − α)) In view of (10) and the choice of r, we obtain k(F x)k∞

≤ r.

Step 2. We prove that F is continuous. For that, let (xn ) be a sequence of Br such that xn → x in Br . Then f (s, xn (s)) h(t, xn (s))

→ f (s, x(s)), → h(t, x(s)),

n → ∞, n→∞

as both f and h are jointly continuous on I × Xα . Now, for all t ∈ I, we have kF xn − F xkα

≤ + +

kS (t)(g(xn ) − g(x))kα

Zβ t



(t − s)β−1 Pβ (t − s) (Kxn (s) − Kx(s)) ds

α

Z 0t



(t − s)β−1 S(t − s) (f (s, xn (s)) − f (s, x(s))) ds ,

0

α

EJQTDE, 2010 No. 58, p. 8

which in view of Lemma 2.5 gives kF xn − F xkα ≤ M kg(xn ) − g(x)kα Z t βMα Γ(2 − α) + (t − s)β(1−α)−1 kf (s, xn (s)) − f (s, x(s))k ds Γ(1 + β(1 − α)) Z 0 t βMα Γ(2 − α) (t − s)β(1−α)−1 kKxn (s) − Kx(s)k ds + Γ(1 + β(1 − α)) 0 for all t ∈ I. Therefore, on the one hand using (2), (H4 ) and (H5 ), we get for each t∈I kf (s, xn (s)) − f (s, x(s))k



kKxn (s) − Kx(s)k

≤ ≤ ≤

2µ1 (s) for s ∈ I, Z s a(s − τ )kh(τ, xn (τ )) − h(τ, x(τ ))kdτ, 0 s Z 2 a(s − τ )µ2 (τ )dτ 0

2aT kµ2 kL∞ (I,R+ ) for s ∈ I;

and on the other hand using the fact that the functions s 7→ 2µ1 (s)(t − s)β(1−α)−1 and s 7→ (t − s)β(1−α)−1 are integrable on I, by means of the Lebesgue Dominated Convergence Theorem yields t

Z

Z0 0

t

(t − s)β(1−α)−1 kf (s, xn (s)) − f (s, x(s))k ds → 0, (t − s)β(1−α)−1 kKxn (s) − Kx(s)k ds → 0.

Hence, since g(xn ) → g(x) as n → ∞ because g is completely continuous on Cα , it can easily be shown that

lim k(F xn ) − (F x)k∞ = 0,

n→∞

as n → ∞. In other words, F is continuous. Step 3. We show that F is compact. To this end, we use the Ascoli-Arzela’s theorem. For that, we first prove that {(F x)(t) : x ∈ Br } is relatively compact in Xα , for all t ∈ I. Obviously,{(F x)(0) : x ∈ Br } is compact. EJQTDE, 2010 No. 58, p. 9

Let t ∈ (0, T ]. For each h ∈ (0, t), ǫ > 0 and x ∈ Br , we define the operator Fh,ǫ by (Fh,ǫ x)(t)

= Sβ (t) (x0 − g(x)) Z t−h Z ∞ + (t − s)β−1 βθΦβ (θ)R((t − s)β θ)f (s, x(s))dθ ds 0 ǫ Z t−h Z ∞ + (t − s)β−1 βθΦβ (θ)R((t − s)β θ)Kx(s)dθ ds 0

ǫ

= Sβ (t) (x0 − g(x)) Z t−h Z ∞ + R(hβ ǫ) (t − s)β−1 βθΦβ (θ)R((t − s)β θ − hβ ǫ)f (s, x(s))dθ ds Z 0t−h Zǫ ∞ β−1 β + R(h ǫ) (t − s) βθΦβ (θ)R((t − s)β θ − hβ ǫ)Kx(s)dθ ds. 0

ǫ

Then the sets {(Fh,ǫ x)(t) : x ∈ Br } are relatively compact in Xα since by Lemma 2.3, the operators Rα (t), t > 0 are compact on Xα . Moreover, using (H1 ) and (4) we have k(F x)(t) − (Fh,ǫ x)(t)kα ≤ Z t Z ǫ

(t − s)β−1 βθΦβ (θ) R((t − s)β θ)f (s, x(s)) α dθ ds+ 0Z Z 0t ∞

β−1 (t − s) βθΦβ (θ) R((t − s)β θ)f (s, x(s)) α dθ ds+ Zt−h Z ǫǫ t

β−1 (t − s) βθΦβ (θ) R((t − s)β θ)Kx(s) α dθ ds+ 0Z Z 0t ∞

β−1 (t − s) βθΦβ (θ) R((t − s)β θ)Kx(s) α dθ ds. t−h

ǫ

Then using (4) and (H4 ), we obtain k(F x)(t) − (Fh,ǫ x)(t)kα

t

Z ǫ (t − s)β(1−α)−1 µ1 (s) θ1−α Φβ (θ)dθ ds 0Z Z 0t ∞ β(1−α)−1 + βMα (t − s) µ1 (s) βθ1−α Φβ (θ)dθ ds ǫ Zt−h Z t ǫ + βMα (t − s)β(1−α)−1 βθ1−α Φβ (θ) kKx(s)k dθ ds 0Z Z 0t ∞ β(1−α)−1 + βMα (t − s) βθ1−α Φβ (θ) kKx(s)k dθ ds. ≤ βMα

Z

t−h

ǫ

Since by (H5 ) and (2), kKx(s)k ≤ ≤ ≤

Z

s

Z0 s 0

a(s − τ )kh(τ, x(τ ))kdτ a(s − τ )µ2 (τ )dτ

aT kµ2 kL∞ (I,R+ ) ,

EJQTDE, 2010 No. 58, p. 10

using (5c), we deduce for all ǫ > 0 that k(F x)(t) − (Fh,ǫ x)(t)kα

≤ + + +

Z tβ(1−α) βMα kµ1 kL∞ (I,R+ ) ǫ 1−α θ Φβ (θ)dθ β(1 − α) 0 β(1−α) h βMα Γ(2 − α)kµ1 kL∞ (I,R+ ) β(1 − α)Γ(1 + β(1 − α)) Z tβ(1−α) βMα kµ2 kL∞ (I,R+ ) aT ǫ 1−α θ Φβ (θ)dθ β(1 − α) 0 β(1−α) h βMα Γ(2 − α)aT kµ2 kL∞ (I,R+ ) . β(1 − α)Γ(1 + β(1 − α))

In other words k(F x)(t) − (Fh,ǫ x)(t)kα

≤ +

hβ(1−α) βMα Γ(2 − α)kµ1 kL∞ (I,R+ ) β(1 − α)Γ(1 + β(1 − α)) hβ(1−α) βMα Γ(2 − α)aT kµ2 kL∞ (I,R+ ) . β(1 − α)Γ(1 + β(1 − α))

Therefore, the set {(F x)(t) : x ∈ Br } is relatively compact in Xα for all t ∈ (0, T ] and since it is compact at t = 0 we have the relatively compactness in Xα for all t ∈ I. Now, let us prove that F (Br ) is equicontinuous. By the compactness of the set g(Br ), we can prove that the functions F x, x ∈ Br are equicontinuous a t = 0. For 0 < t2 < t1 ≤ T , we have k(F x)(t1 ) − (F x)(t2 )kα ≤ k(Sβ (t1 ) − Sβ (t2 )) (x0 − g(x))kα

Z t2

β−1

+ (t1 − s) (Pβ (t1 − s) − Pβ (t2 − s)) (f (s, x(s)) + Kx(s)) ds

α

Z 0t2

 β−1 β−1

P (t − s)(f (s, x(s)) + Kx(s)) ds (t − s) − (t − s) + β 2 1 2

α

Z 0t1

β−1

+ (t1 − s) Pβ (t1 − s)(f (s, x(s)) + Kx(s)) ds t2

α

≤ I1 + I2 + I3 + I4 ,

where

I1

=

I2

=

I3

=

I4

=

k(Sβ (t1 ) − Sβ (t2 )) (x0 − g(x))kα

Z t2

β−1

(t1 − s) (Pβ (t1 − s) − Pβ (t2 − s)) (f (s, x(s)) + Kx(s)) ds

α

Z 0t2

 β−1 β−1

Pβ (t2 − s)(f (s, x(s)) + Kx(s)) ds (t1 − s) − (t2 − s)

α

Z 0t1

β−1

(t1 − s) Pβ (t1 − s)(f (s, x(s)) + Kx(s)) ds

t2

α

EJQTDE, 2010 No. 58, p. 11

Actually, I1 , I2 , I3 and I4 tend to 0 independently of x ∈ Br when t2 → t1 . Indeed, let x ∈ Br and G = sup kg(x)kα . In view of Lemma 2.5, we have x∈Cα

I1

= Z k(Sβ (t1 ) − Sβ (t2 )) (x0 − g(x))kα



≤ Φβ (θ) R(θtβ1 ) − R(θtβ2 ) kx0 − g(x)kα dθ Z0 ∞

B(X)

Φβ (θ) R(θtβ1 ) − R(θtβ2 ) ≤ (kx0 kα + G)dθ B(X)

0

from which we deduce that lim I1 = 0 since by Lemma 2.3 the function t 7→ t2 →t1

kRα (t)kα is continuous for t ≥ 0

I2

Z



t2 0



(t1 − s)β−1 (Pβ (t1 − s) − Pβ (t2 − s)) (f (s, x(s)) + Kx(s)) ds. α

Therefore using the continuity of Pβ (t) (Lemma 2.4) and the fact that both f and K are bounded we conclude that lim I2 = 0 t2 →t1

I3

≤ ≤ +

Z

t2

 (t2 − s)β−1 − (t1 − s)β−1 kPβ (t2 − s)(f (s, x(s)) + Kx(s))kα ds 0 Z t2  βMα Γ(2 − α) (t2 − s)β−1 − (t1 − s)β−1 (t2 − s)−αβ kf (s, x(s))k ds Γ(1 + β(1 − α)) Z 0 t2  βMα Γ(2 − α) (t2 − s)β−1 − (t1 − s)β−1 (t2 − s)−αβ kKx(s)k ds. Γ(1 + β(1 − α)) 0

Since −(t2 −s)−αβ (t1 −s)β−1 ≤ −(t1 −s)β(1−α)−1 because (t1 −s)−αβ ≤ (t2 −s)−αβ , we deduce that I3

≤ + ≤ +

Z  βMα Γ(2 − α)kµ1 kL∞ (I,R+ ) t2  (t2 − s)β(1−α)−1 − (t1 − s)β(1−α)−1 ds Γ(1 + β(1 − α)) 0Z  aT βMα Γ(2 − α)kµ1 kL∞ (I,R+ ) t2  (t2 − s)β(1−α)−1 − (t1 − s)β(1−α)−1 ds Γ(1 + β(1 − α)) 0 βMα Γ(2 − α)kµ1 kL∞ (I,R+ ) (t1 − t2 )β(1−α) β(1 − α)Γ(1 + β(1 − α)) aT βMα Γ(2 − α)kµ1 kL∞ (I,R+ ) (t1 − t2 )β(1−α) . β(1 − α)Γ(1 + β(1 − α))

Hence lim I3 = 0 since β(1 − α) > 0. t2 →t1

EJQTDE, 2010 No. 58, p. 12

I4

≤ ≤ ≤ ≤ ≤

Z

t1

(t1 − s)β−1 kPβ (t1 − s)(f (s, x(s) + Kx(s))kα ds Z t1 βMα Γ(2 − α) (t1 − s)β(1−α)−1 kf (s, x(s)) + Bx(s))k ds Γ(1 + β(1 − α)) t2 Z s Z t1 βMα Γ(2 − α) (t1 − s)β(1−α)−1 (µ1 (s) + a(s − τ )kh(τ, x(τ ))kdτ )ds Γ(1 + β(1 − α)) t2 0 Z t1 βMα Γ(2 − α) (t1 − s)β(1−α)−1 ds (kµ1 kL∞ (I,R+ ) + aT kµ2 kL∞ (I,R+ ) ) Γ(1 + β(1 − α)) t2 (t1 − t2 )β(1−α) βMα Γ(2 − α) (kµ1 kL∞ (I,R+ ) + aT kµ2 kL∞ (I,R+ ) ) β(1 − α)Γ(1 + β(1 − α)) t2

Since β(1 − α) > 0, we deduce that lim I4 = 0. t2 →t1

In short, we have shown that F (Br ) is relatively compact, for t ∈ I, {F x : x ∈ Br } is a family of equicontinuous functions. Hence by the Arzela-Ascoli Theorem, F is compact. By Schauder fixed point theorem F has a fixed point x ∈ Br , which obviously is a mild solution to (1).  4. Example

Let X = L2 [0, π] equipped with its natural norm and inner product defined respectively for all u, v ∈ L2 [0, π] by Z π Z π 1/2 kukL2 [0,π] = |u(x)|2 dx and hu, vi = u(x)v(x)dx. 0

0

Consider the following integro-partial differential equation

 β Z t ∂2u cos(tx) ∂ u   (t, x) = (t, x) + + e−|t−s| cos(u(s, x)) ds,  β 2 2 (t, x)  ∂t ∂x 1 + u  0       u(t, 0) = u(t, π) = 0, t ∈ [0, 1] (E)       N Z π  X    u(0, x) + δ0 cos(x − y)u(tk , y)dy = u0 (x), x ∈ [0, π]  k=0

0

where t ∈ [0, 1], x ∈ [0, π], 0 < t1 < t2 < ... < tN ≤ 1, and δ0 > 0. First of all, note that f, h, a are given by

cos(tx) , a(t) = e−|t| , and h(t, u(t, x)) = cos(u(s, x)), 1 + u2 (t, x) Z 1 e−|t| dt = and hence in (H4 ) and (H5 ) we take µ1 (t) = µ2 (t) = π. Moreover, a1 = f (t, u(t, x)) =

1 − e−1 .

0

EJQTDE, 2010 No. 58, p. 13

Let A be the operator given by Au = −u′′ with domain D(A) := {u ∈ L2 ([0, π]) : u′′ ∈ L2 ([0, π]), u(0) = u(π) = 0}. It is well known that A has a discrete spectrum with eigenvalues of the form n2 , n ∈ N, and corresponding normalized eigenfunctions given by zn (ξ) :=

r

2 sin(nξ). π

In addition to the above, the following properties hold: (a) {zn : n ∈ N} is an orthonormal basis for L2 [0, π]; (b) The operator −A is the infinitesimal generator of an analytic semigroup R(t) which is compact for t > 0. The semigroup R(t) is defined for u ∈ L2 [0, π] by R(t)u =

∞ X

n=1

2

e−n t hu, zn izn .

(c) The operator A can be rewritten as

Au =

∞ X

n=1

n2 hu, zn izn

for every u ∈ D(A). Moreover, it is possible to define fractional powers of A. In particular, (d) For u ∈ L2 [0, π] and α ∈ (0, 1), −α

A

∞ X 1 hu, zn izn ; u= 2α n n=1

(e) The operator Aα : D(Aα ) ⊆ L2 [0, π] 7→ L2 [0, π] given by α

A u=

∞ X

n=1

n2α hu, zn izn , ∀u ∈ D(Aα ),

∞ n o X where D(Aα ) = u ∈ L2 [0, π] : n2α hu, zn izn ∈ L2 [0, π] . n=1

EJQTDE, 2010 No. 58, p. 14

Clearly for all t ≥ 0 and 0 6= u ∈ L2 [0, π], |R(t)u| =

∞ X

|

n=1

∞ X



n=1

e−t

=

2

e−n t hu, zn izn |

e−t |hu, zn izn | ∞ X

n=1

|hu, zn izn |

e−t |u|



and hence kR(t)kB(L2 [0,π]) ≤ 1 for all t ≥ 0. Here we take M = 1. Set

g(u)(ξ) := δ0

N Z X k=0

Suppose α ∈ (0, 21 ) and (11)

π

0

cos(ξ − y)u(tk , y)dy.

√ 6 δ0 < 2 . 2π N

Now

kAα g(u)(ξ)k2L2 [0,π]

=

X

n≥1

≤ = =

X

n≥1

n4α kzn k2L2 [0,π] |hg(u)(ξ), zn i|2 n2 |hg(u)(ξ), zn i|2

Z π 2X | g(u)(ξ) n sin(nξ)dξ|2 π 0 n≥1 X 1 Z π ∂2 | g(u)(ξ) zn (ξ)dξ|2 n2 0 ∂ξ 2

n≥1

≤ ≤ ≤

π2 ∂ 2 g(u)(ξ)k2L2 [0,π] k 6 ∂ξ 2 π2 kg(u)(ξ)k2L2 [0,π] 6 π2 δ02 N 2 π 2 kuk2∞ 6

EJQTDE, 2010 No. 58, p. 15

δ0 π 2 N √ and µ = 0. Therefore, the 6 condition M λ < 21 holds under assumption (11). Using Theorem 3.2 and inequality Eq. (11) it follows that the system (E) at least one mild solution. and hence kg(u)kα ≤ λkuk∞ + µ where λ =

Acknowledgements: This work was completed when the second author was visiting Morgan State University in Baltimore, MD, USA in May 2010. She likes to thank Prof. N’Gu´er´ekata for the invitation.

References 1. S. Aizicovici and M. McKibben, Existence results for a class of abstract nonlocal Cauchy problems, Nonlinear Analysis, TMA 39 (2000), 649-668. 2. A. Anguraj, P. Karthikeyan and G. M. N’Gu´ er´ ekata, Nonlocal Cauchy problem for some fractional abstract differential equations in Banach spaces, Comm. Math. Analysis, 6,1(2009), 31-35. 3. L. Byszewski, Theorems about the existence and uniqueness of solutions of a semilinear evolution nonloncal Cauchy problem, J. Math. Anal. Appl., 162, (1991),494-505. 4. K. Deng, Exponential decay of solutions of semilinear parabolic equations with nonlocal initial conditions, J. Math. Analysis Appl., 179 (1993), 630-637. 5. M. EL-Borai, Some probability densities and fundamental solutions of fractional evolution equations. Chaos, Solitons and Fractals 14 (2002) 433-440. 6. A. Debbouche and M. M. El-Borai, Weak almost periodic and optimal mild solutions of fractional evolution equations, J. Diff. Eqns., Vol. 2009(2009), No. 46, pp. 1-8. 7. K. Ezzinbi and J. Liu, Nondensely defined evolution equations with nonlocal conditions, Math. Computer Modelling, 36 (2002), 1027-1038. 8. Z. Fan, Existence of nondensely defined evolution equations with nonlocal conditions, Nonlinear Analysis, (in press). 9. Hsiang Liu, Jung-Chan Chang Existence for a class of partial differential equations with nonlocal conditions, Nonlinear Analysis, TMA, (in press). 10. F. Mainardi, P. Paradis and R. Gorenflo, Probability distributions generated by fractional diffusion equations, FRACALMO PRE-PRINT www.fracalmo.org. 11. G. M. Mophou, O. Nakoulima and G. M. N’Gu´ er´ ekata, Existence results for some fractional differential equations with nonlocal conditions, Nonlinear Studies, Vol.17, n0.1, pp.15-22 (2010). 12. G. M. Mophou and G. M. N’Gu´ er´ ekata, Mild solutions for semilinear fractional differential equations, Electronic J. Diff. Equ., Vol.2009, No.21, pp.1-9 (2009). 13. G. M. N’Gu´ er´ ekata, Existence and uniqueness of an integral solution to some Cauchy problem with nonlocal conditions, Differential and Difference Equations and Applications, 843-849, Hindawi Publ. Corp., New York, 2006. 14. G. M. N’Gu´ er´ ekata, A Cauchy Problem for some fractional abstract differential equation with nonlocal conditions, Nonlinear Analysis, T.M.A., 70 Issue 5, (2009), 1873-1876. 15. A. Pazy, Semigroups of Linear Operators and Applications to Partial Differential Equations, Springer-Verlag, New York, 1983.

EJQTDE, 2010 No. 58, p. 16

16. Y. Zhou and F. Jiao, Existence of mild solutions for fractional neutral evolution equations, Computer and Mathematics with Applications, 59(2010), 1063-1077.

(Received May 19, 2010)

Toka Diagana, Department of Mathematics, Howard University, 2441 6th Street NW, Washington, DC, 20009, USA E-mail address: [email protected] Gis` ele M. Mophou,Universit´ e des Antilles et de la Guadeloupe, D´ epartement de Math´ ematiques et Informatique, Universit´ e des Antilles et de La Guyane, Campus Fouil` -Pitre Guadeloupe (FWI) lole 97159 Pointe-a E-mail address: [email protected] Gaston M. N’Gu´ er´ ekata, Department of Mathematics, Morgan State University, 1700 E. Cold Spring Lane, Baltimore, M.D. 21251, USA E-mail address: Gaston.N’[email protected]

EJQTDE, 2010 No. 58, p. 17

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