Shen et al. Advances in Difference Equations (2016) 2016:118 DOI 10.1186/s13662-016-0797-3

RESEARCH

Open Access

Fractional boundary value problems with p(t)-Laplacian operator Tengfei Shen, Wenbin Liu* and Ren Zhao *

Correspondence: [email protected] Department of Mathematics, China University of Mining and Technology, Xuzhou, 221116, P.R. China

Abstract The purpose of this paper is to discuss boundary value problems of fractional differential equations with p(t)-Laplacian operator. Some new existence, uniqueness, and multiplicity results were acquired by employing some fixed point theorems. Moreover, some examples are supplied to verify our main results. MSC: 26A33; 34A08; 34B15 Keywords: fractional differential equation; boundary value problem; p(t)-Laplacian operator; cone

1 Introduction Fractional differential equations have been applied in many research fields in recent years (see [–]). Leszczynski and Blaszczyk [] studied the following fractional mathematical model, which can be used to describe the height of granular material decreasing over time in a silo: C

DαT – Dαa+ h∗ (t) + βh∗ (t) = ,

t ∈ [, T],

where C DαT – and Dαa+ are respectively the right Caputo and left Riemann-Liouville fractional derivatives of order α ∈ (, ). Moreover, many valuable results related to boundary value problems (BVPs) or initial problems for fractional differential equations have been achieved by some scholars (see [–]). Bai and Lü [] considered the fractional BVP 

Dα+ x(t) = f (t, x(t)), x() = x() = ,

t ∈ (, ),

(.)

and obtained the existence and multiplicity of positive solutions by taking advantage of methods in cone. Here, Dα+ is the left Riemann-Liouville fractional derivative of order α ∈ (, ], and f : [, ] × R → R is a continuous function. Recently, many scholars have focused on fractional BVPs with p-Laplacian operator (see [–]). Chen and Liu [] dealt with the following BVP involving p-Laplacian operator: 

β

D+ ϕp (Dα+ x(t)) = f (t, x(t)), t ∈ [, ], x() = –x(), Dα+ x() = –Dα+ x(),

(.)

© 2016 Shen et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 2 of 10

where  < β, α ≤ ,  < α + β ≤ , Dα+ is the Caputo fractional derivative, ϕp (·) is the pLaplacian operator, and f : [, ] × R → R is a continuous function. However, many papers focused on the existence and multiplicity of solutions for fractional BVPs. The uniqueness of solution for fractional BVPs with p(t)-Laplacian operator has not been yet investigated. Thus, we deal with the following fractional BVP with p(t)Laplacian operator and obtain the uniqueness of its solution by the method in cone (see Theorem .):  β D+ ϕp(t) (Dα+ x(t)) + f (t, x(t)) = , t ∈ [, ], Dα+ x() = , x () = x() = x () = ,

(.)

where Dα+ is the Caputo fractional derivative,  < α < ,  < β < , ϕp(t) (·) is the p(t)Laplacian operator with p(t) ∈ C  [, ] such that p(t) > . Moreover, f does not need to satisfy the Lipschitz condition, so the problem becomes more complicated. An iterative scheme is shown to approximate it. Furthermore, we also discuss the existence and multiplicity of solutions for the BVP (.) (see Theorem . and Theorem .). Noting that when p(t) = p, it becomes the wellknown p-Laplacian operator, our results extend and enrich some existing papers. For the problems of integer differential equations with p(t)-Laplacian or p-Laplacian operator, we refer the readers to [–]).

2 Preliminaries Let E = C[, ] with norm x∞ = maxt∈[,] |x(t)|, and P be a cone of E, where P = {x ∈ E|x(t) ≥ }. Moreover, we define the partial ordering ≤ with respect to P by x ≤ y if only if y – x ∈ P. For u ∈ P such that u > θ (i.e., u (t) is unequal to zero identically), we denote   Pu = x|x ∈ E, ∃λ(x) > , μ(x) >  s.t. λ(x)u ≤ x ≤ μ(x)u . Lemma . ([]) For any (t, x) ∈ [, ] × R, ϕp(t) (x) = |x|p(t)– x is a homeomorphism from – R to R and strictly monotone increasing for any fixed t. Moreover, ϕp(t) (·) is continuous, sends bounded sets to bounded sets, and is defined by –p(t)

– (x) = |x| p(t)– x ϕp(t)

for x ∈ R \ {},

– ϕp(t) () = 

for x = .

Definition . ([]) Let E be a Banach space. A cone P ⊂ E is called normal if there is a constant N >  such that θ ≤ x ≤ y and x ≤ Ny for all x, y ∈ E. Lemma . ([, ]) Let P ⊂ E be a normal cone. Suppose that A : Pu → Pu is increasing and for any t ∈ (, ), there exists η(t) >  such that   A(tx) ≥ t  + η(t) Ax,

x ∈ Pu .

Then, A has a unique fixed point x∗ if and only if there exist w , v ∈ Pu such that w ≤ Aw ≤ Av ≤ v . Moreover, for any x ∈ [w , v ], letting xn = Axn– (n = , , . . .), we have xn → x∗ .

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 3 of 10

Define P(θ , b, d) = {x ∈ P|b ≤ θ (x) and x ≤ d} and Pc = {x ∈ P|x ≤ c}, where b, c, d > . Lemma . ([]) Let P be a cone of Banach space E, and T : Pc → Pc be a completely continuous map. Suppose that there exists a nonnegative continuous concave functional θ such that θ (x) ≤ x for x ∈ P and numbers  < a < b < d ≤ c satisfying the following conditions: (i) {x ∈ P(θ , b, d)|θ (x) > b} = ∅ and θ (Tx) > b for x ∈ P(θ , b, d). (ii) Tx < a for x ∈ Pa . (iii) θ (Tx) > b for x ∈ P(θ , b, c) with Tx > d. Then T has at least three fixed points x , x , x in Pc . Definition . ([]) The Riemann-Liouville fractional integral operator of order α of a function x is given by Iα+ x(t) =

 (α)



t

(t – s)α– x(s) ds, 

provided that the right-hand side are pointwise defined on [, ]. Definition . ([]) The Caputo fractional derivative of order α of a function x is given by d Dα+ x(t) = In–α +

n

 x(t) = dt n (n – α)



t

(t – s)n–α– x(n) (s) ds, 

where n = [α] + , provided that the right-hand side is pointwise defined on [, ]. Lemma . ([]) The general solution of the Caputo fractional differential equation Dα+ x(t) =  is given by x(t) = c + c t + c t  + · · · + cn– t n– , where ci ∈ R, i = , , , . . . , n – , n = [α] + . Lemma . ([]) Assume that Dα+ x(t) ∈ C[, ]. Then Iα+ Dα+ x(t) = x(t) + c + c t + c t  + · · · + cn– t n– , where ci = – x

(i) ()

i!

, i = , , , . . . , n – , n = [α] + .

Lemma . If y(t) ∈ C[, ], then the unique solution of  β D+ ϕp(t) (Dα+ x(t)) + y(t) = , t ∈ [, ], Dα+ x() = , x () = x() = x () = ,

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 4 of 10

can be expressed as the integral 



x(t) = 

β  – I+ y(s) ds, G(t, s)ϕp(s)

(.)

where  G(t, s) =

(–s)α– –(t–s)α– , (α) (–s)α– , (α)

 ≤ s ≤ t ≤ ,  ≤ t ≤ s ≤ .

Proof Based on Lemma ., we have   β ϕp(t) Dα+ x(t) = –I+ y(t) + c,

c ∈ R.

– Applying the operator ϕp(t) to both sides of this equality, we have

  β – Dα+ x(t) = ϕp(t) –I+ y(t) + c . – Combining this with Dα+ x() = , for fixed t = , we have ϕp() (c) = . By Lemma . we have that c =  and

 x(t) = – (α)



t 

β  – I+ y(s) ds + c + c t + c t  , (t – s)α– ϕp(s)

where ci ∈ R, i = , , . Since x () = x() = x () = , we get that  c = (α)



 

β  – I+ y(s) ds ( – s)α– ϕp(s)

and c = c = . Thus, we have x(t) = –

 (α)



t 

β  – I+ y(s) ds + (t – s)α– ϕp(s)

 (α)

 



β  – I+ y(s) ds. ( – s)α– ϕp(s)

Therefore, (.) holds. The proof is complete. Lemma . G(t, s) satisfies the following conditions: (D ) G(t, s) ∈ C([, ] × [, ]), G(t, s) ≥  for t, s ∈ [, ].   ( – t α– )( – s)α– ≤ G(t, s) ≤ (α) ( – t α– ) for t, s ∈ [, ]. (D ) (α) Proof Clearly, (D ) is satisfied. For s ≤ t and  < α < , we have ( – s)α– – (t – s)α– ≥ ( – s)α– – (t – ts)α–   =  – t α– ( – s)α– . For given t, we have   ∂G(t, s)  = (t – s)α– – ( – s)α– ≤ . ∂s (α – ) Thus, G(t, s) ≤

 ( – t α– ). (α)



Shen et al. Advances in Difference Equations (2016) 2016:118

Page 5 of 10

For t ≤ s, it is easy to get that   ( – s)α– ≥  – t α– ( – s)α– , ( – s)α– ≤ ( – t)α– ≤  – t ≤  – t α– . 

Thus, (D ) is satisfied.

3 Main result Define the nonnegative continuous concave functional θ by θ (Ax) = min Ax(t), t∈[τ ,–τ ]

  , x ∈ P, τ ∈ , 

where  Ax(t) = 



β   – I+ f s, x(s) ds. G(t, s)ϕp(s)

Let   :=   :=

 (α)

 





–  β s ds , (β + )  –    α– – β s ds . ( – s) ϕp(s) (β + ) 

– ( – s)α– ϕp(s)

 – ( – τ )α– (α)

Theorem . Assume that f : [, ] × [, ∞) → [, ∞) is continuous and there exist numbers  < a < b < d = c satisfying the following conditions: (I ) f (t, x) < ϕp(t) ( a) for [, ] × [, a]. (I ) f (t, x) > ϕp(t) ( b) for [τ ,  – τ ] × [b, c]. (I ) f (t, x) ≤ ϕp(t) ( c) for [, ] × [, c]. Then BVP (.) has three positive solutions. Proof To begin with, for u ∈ Pc , we will prove that A : Pc → Pc . By I we have

 



β  

– Ax = max

G(t, s)ϕp(s) I+ f s, x(s) ds

t∈[,]    β    – I+ f s, x(s) ds ( – s)α– ϕp(s) ≤ (α)       α– – β s ds = c. ( – s) ϕp(s) ≤  c (α)  (β + ) Thus, APc ⊂ Pc . It is easy to get that A is continuous by the continuity of f . Let be any – bounded open subset of Pc . Since ϕp(t) (·) and f are continuous, there exists a constant B >  β – such that |ϕp(t) (I+ f (t, x(t)))| ≤ B on [, ] × . Thus, we have Ax = max |Ax| ≤ t∈[,]

 (α)





( – s)α– B ds = 

B . (α + )

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 6 of 10

Thus, A is uniformly bounded. On the other hand, for all t , t ∈ [, ] such that t ≤ t and for all x ∈ , we have



 



 β    

–

Ax(t ) – Ax(t ) = G(t , s)ϕ – I β+ f s, x(s) ds – G(t , s)ϕp(s) I+ f s, x(s) ds

p(s) 

   



G(t , s) – G(t , s) ds ≤B 

≤

B  α α t –t . (α + )  

Thus, we have



Ax(t ) – Ax(t ) →  uniformly as t → t . Therefore, A is equicontinuous on , so that A : Pc → Pc is completely continuous by the Arzelà-Ascoli theorem. Similarly, by I we obtain that Ax < a for x ∈ Pa . So, condition (ii) of Lemma . is satisfied. Let x (t) = b+c . Clearly, x  ≤ c and θ (x ) > b. Thus,    x ∈ P(θ , b, d)|θ (x) > b = ∅. For x ∈ P(θ , b, d), by (I ) we have b ≤ x(t) ≤ c, t ∈ [τ ,  – τ ], and θ (Ax) = min Ax(t) t∈[τ ,–τ ]

≥

 min (α) t∈[τ ,–τ ]



  β  –  – t α– ( – s)α– ϕp(s) I+ f s, x(s) ds

 

 – ( – τ )α– > b (α)





 

– ( – s)α– ϕp(s)

 β s ds = b. (β + )

Thus, condition (i) of Lemma . holds. When d = c, condition (i) implies (iii) in Lemma .. Then BVP (.) has three positive solutions.  Example . Consider the following BVP: ⎧  ⎨D  ϕ (D  x(t)) + f (t, x(t)) = , t ∈ [, ], +  +  ⎩x () = x() = x () = , D + x() = .

(.)



Choose α =  , β =  , p(t) = , τ =  ,    x ≤ , t + x , f t, x(t) =   t +  x + ,, x > , a =  , b = , c = . By simple calculation we have 

 =

((  ))  (  ) 

So, we have

(  )



≈ .,

 =

((  ))  (  )  (  )

·

(  ) 

( – (  )) 

≈ ..

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 7 of 10

(I ) f (t, x(t)) ≤ . < . ≈ ϕ ( a) for [, ] × [,  ]; (I ) f (t, x(t)) ≥ ,. > ,. = ϕ ( b) for [  ,  ] × [, ]; (I ) f (t, x(t)) ≤ , < ,. = ϕ ( c) for [, ] × [, ]. Thus, Example . has three positive solutions. Theorem . Assume that f : [, ] × [, ∞) → (, ∞) is continuous and the following conditions hold: (I ) f (t, x) is increasing in x. (I ) There exists r ∈ (, ) such that f (t, rx) ≥ r

p(t)– 

 p(t)–  which implies f t, x ≤ r–  f (t, x). r

f (t, x),

Then BVP (.) has a unique positive solution. Proof Let (t) =  – t α– , λ(x) = β (I+ f (s, x(s))) ds.

 (α)



 (

β

– – s)α– ϕp(s) (I+ f (s, x(s))) ds, μ(x) =

 (α)

 

– ϕp(s) ×

For u ∈ P , we will prove that A : P → P . Indeed, by Lemma ., we

have Ax(t) ≥ (t) Ax(t) ≤ (t)

 (α)  (α)

 



β   – I+ f s, x(s) ds, ( – s)α– ϕp(s)



β   – I+ f s, x(s) ds. ϕp(s)





Clearly, we have λ(x) (t) ≤ Ax ≤ μ(x) (t). Thus, we that A : P → P , together with (I ) – and the monotone increasing of ϕp(t) (·), yields that A is an increasing operator. In view of (I ), we have A(rx) ≥ r

 



 

β   – I+ f s, x(s) ds G(t, s)ϕp(s)

   = r  + r–  – 



 

 β  – G(t, s)ϕp(s) I+ f s, x(s) ds.



We have η(r) := r–  –  > . Let 



b(t) = 

β   – I+ f s, (s) ds, G(t, s)ϕp(s)

 b = min ,  b = max ,

 (α)  (α)









  β  – ( – s)α– ϕp(s) I+ f s, (s) ds ,

 

– ϕp(s)



 β  I+ f s, (s) ds

 .

Thus, we have b (t) ≤ b(t) ≤ b (t). Let a = min{  , b }, a > , w (t) = a b(t), v (t) = a b(t). Since A is an increasing operator, we have Aw ≤ Av . On one hand,      p(t)–  f (t, w ) = f t, a b(t) ≥ f t, a b (t) ≥ (a b )  f t, (t) .

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 8 of 10



For a ≤ b , we have a ≤ (a b )  . Therefore, 





Aw (t) =

G(t, s)  







≥ (a b )   ≥ a

 





β   – I+ f s, w (s) ds ϕp(s)

β   – I+ f s, (s) ds G(t, s)ϕp(s)

β   – I+ f s, (s) ds G(t, s)ϕp(s)

= w (t). On the other hand,     f (t, v ) = f t, a b(t) ≤ f t, a b (t) ≤



 a  b

p(t)– 

  f t, (t) .

Hence,  Av (t) = 



β   – I+ f s, v (s) ds G(t, s)ϕp(s)



   β    – I+ f s, (s) ds ≤ G(t, s)ϕp(s) a  b    β   – I+ f s, (s) ds ≤ a G(t, s)ϕp(s) 

= v (t). Thus, we have w ≤ Aw ≤ Av ≤ v . In view of Lemma ., BVP (.) has a unique positive solution. Moreover, for any u ∈ [w , v ], letting un = Aun– (n = , , . . .), we have un → u∗ .  Example . Consider the following BVP: ⎧  ⎨D  ϕ  (D  x(t)) + x (t) sin t = , t ∈ [, ], + t + +  ⎩x () = x() = x () = , D  x() = ,

(.)

+

, β =  , p(t) = t  + , r =  , f (t, x(t)) = x (t) sin t. It is easy to verify that (I ) is where α =   satisfied. Moreover, 

 x 

 sin t =

  x sin t ≥ 

 t +  x sin t, 

(x) sin t = x sin t ≤ t

 + 

x sin t,

which yields that (I ) is satisfied. Thus, Example . has a unique positive solution. In order to state Theorem ., let PL := mint∈[,] p(t), PM := maxt∈[,] p(t). Theorem . Assume that f : [, ] × R → R is continuous and the following conditions hold.

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 9 of 10

(I ) There exist constants l , l >  such that



f (t, x) ≤ l + l |x|r– , 

(I )

 < r ≤ PL .  P –

K PL –  (α)((β+)) PM –

P



–

< , K = max{l L , l M }.

Then BVP (.) has a solution. Proof In the same way as in the proof of Theorem ., it is easy to prove that A : C[, ] → C[, ] is completely continuous. Denote   V = x ∈ X|x = λAx, λ ∈ (, ) . According to Schaefer’s fixed point theorem, we just need to prove that V is bounded. For x ∈ V , we have

β  

I + f t, x(t) ≤ 

 (β)  (β)

≤

 

t

 

(t – s)β– f s, x(s) ds



r–   (t – s)β– l + l x(s)





   l + l xr– ≤ ∞ . (β + ) By the inequality (x + y)p ≤ p (xp + yp ) for x, y, p >  we have





x(t) = λ Ax(t)

  β   – I+ f s, x(s) ds G(t, s)ϕp(s) ≤ 

 ≤ (α) Since

r– p(t)–







( – s)





 p(s)–

α–





((β + )) p(s)–





r–

p(s)– lp(s)– + lp(s)– x∞



ds.

∈ (, ], by the inequality xα ≤ x +  for x >  and α ∈ (, ], we have 



x∞ ≤

 PL – 

(α)((β + )) PM –

 

     ( – s)α– lp(s)– + lp(s)– x∞ +  ds.

By (I ) there exists a constant μ >  such that x∞ ≤ μ. Thus, the operator has a fixed point, which implies that BVP (.) has a solution.  Example . Consider the following BVP: ⎧  ⎨D  ϕ  (D  x(t)) +  +  x (t) = , t ∈ [, ], + t + +   ⎩x () = x() = x () = , D+ x() = ,

(.)

Shen et al. Advances in Difference Equations (2016) 2016:118

Page 10 of 10

where α =  , β =  , p(t) = t  + , r = , f (t, x(t)) =  +  x (t), l = , l =  . Clearly, (I ) holds. Moreover,   (  )((  )) 

< ,

which implies that (I ) is satisfied. Thus, BVP (.) has a solution. Competing interests The authors declare that they have no competing interests. Authors’ contributions The authors contributed equally in this article and approved the final manuscript. Acknowledgements The authors really appreciate the referee’s valuable suggestions and comments, which improved the former version of this paper. This research is supported by the National Natural Science Foundation of China (No. 11271364). Received: 29 November 2015 Accepted: 1 March 2016 References 1. Kilbas, AA, Srivastava, HM, Trujillo, JJ: Theory and Applications of Fractional Differential Equations. North-Holland Mathematics Studies, vol. 204. Elsevier, Amsterdam (2006) 2. Leszczynski, J, Blaszczyk, T: Modeling the transition between stable and unstable operation while emptying a silo. Granul. Matter 13, 429-438 (2011) 3. Bai, J, Feng, X: Fractional-order anisotropic diffusion for image denoising. IEEE Trans. Image Process. 16, 2492-2502 (2007) 4. Magin, R: Fractional calculus models of complex dynamics in biological tissues. Comput. Math. Appl. 59, 1586-1593 (2010) 5. Bai, Z, Lü, H: Positive solutions for boundary value problem of nonlinear fractional differential equation. J. Math. Anal. Appl. 311, 495-505 (2005) 6. Zhao, X, Chai, C, Ge, W: Positive solutions for fractional four-point boundary value problems. Commun. Nonlinear Sci. Numer. Simul. 16, 3665-3672 (2011) 7. Zhou, H, Kou, C, Xie, F: Existence of solutions for fractional differential equations with multi-point boundary conditions at resonance on a half-line. Electron. J. Qual. Theory Differ. Equ. 2011, 27 (2011) 8. Sun, J, Liu, Y, Liu, G: Existence of solutions for fractional differential systems with antiperiodic boundary conditions. Comput. Math. Appl. 64, 1557-1566 (2012) 9. Jiang, W: The existence of solutions to boundary value problems of fractional differential equations at resonance. Nonlinear Anal. 74, 1987-1994 (2011) 10. Hu, Z, Liu, W: Solvability for fractional order boundary value problem at resonance. Bound. Value Probl. 2011, 20 (2011) 11. Ford, N, Xiao, J, Yan, Y: A finite element method for time fractional partial differential equations. Fract. Calc. Appl. Anal. 14, 454-474 (2011) 12. Hu, Z, Liu, W, Liu, J: Existence of solutions of fractional differential equation with p-Laplacian operator at resonance. Abstr. Appl. Anal. 2014, 809637 (2014) 13. Chen, T, Liu, W: An anti-periodic boundary value problem for the fractional differential equation with a p-Laplacian operator. Appl. Math. Lett. 25, 1671-1675 (2012) 14. Chai, G: Positive solutions for boundary value problem of fractional differential equation with p-Laplacian operator. Bound. Value Probl. 2012 18 (2012) 15. Leibenson, L: General problem of the movement of a compressible fluid in a porous medium. Izv. Akad. Nauk Kirg. SSR, Geography and Geophysics 9, 7-10 (1983) (in Russian) 16. Zhang, Q, Wang, Y, Qiu, Z: Existence of solutions and boundary asymptotic behavior of p(r)-Laplacian equation multi-point boundary value problems. Nonlinear Anal. 72, 2950-2973 (2010) 17. Zhang, Q: Existence of solutions for weighted p(r)-Laplacian system boundary value problems. J. Math. Anal. Appl. 327, 127-141 (2007) 18. Fan, X, Zhang, Q, Zhao, D: Eigenvalues of p(x)-Laplacian Dirichlet problem. J. Math. Anal. Appl. 302, 306-317 (2005) 19. Ge, W: Boundary Value Problems for Ordinary Nonlinear Differential Equations. Science Press, Beijing (2007) 20. Zeidler, E: Nonlinear Functional Analysis and Its Applications I: Fixed-Point Theorems. Springer, Berlin (1985) 21. Wang, W, Liang, Z: Fixed point theorem of a class of nonlinear operators and applications. Acta Math. Sin. 48, 789-800 (2005) 22. Yang, L, Chen, H: Unique positive solutions for fractional differential equation boundary value problems. Appl. Math. Lett. 23, 1095-1098 (2010) 23. Leggett, R, Williams, L: Multiple positive fixed points of nonlinear operators on ordered Banach spaces. Indiana Univ. Math. J. 28, 673-688 (1979) 24. Mawhin, J: Topological degree and boundary value problems for nonlinear differential equations in topological methods for ordinary differential equations. Lect. Notes Math. 1537, 74-142 (1993) 25. Kuang, J: Applied Inequalities. Shandong Science and Technology Press, Shandong (2004)