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# Existence of nontrivial solutions for p-Kirchhoff type equations

Chunhan Liu*, Jianguo Wang and Qingling Gao

Author Affiliations

Department of Mathematics, Qilu Normal University, Jinan, 250013, P.R. China

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Boundary Value Problems 2013, 2013:279  doi:10.1186/1687-2770-2013-279

 Received: 13 July 2013 Accepted: 29 November 2013 Published: 30 December 2013

This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this paper, the linking theorem and the mountain pass theorem are used to show the existence of nontrivial solutions for the p-Kirchhoff equations without assuming Ambrosetti-Rabinowitz type growth conditions, nontrivial solutions are obtained.

MSC: 35J60, 35J25.

##### Keywords:
linking theorem; mountain pass theorem; nontrivial solutions

### 1 Introduction

In this paper, we consider the nonlocal elliptic problem of the p-Kirchhoff type given by

{ [ M ( Ω | u | p d x ) ] p 1 ( p u ) = f ( x , u ) , in  Ω , u = 0 , on  Ω , (1)

where Ω R N is a bounded domain, and p u = div ( | u | p 2 u ) is the p-Laplacian with 1 < p < N .

Recently, the equation

{ ( a + b Ω | u | 2 d x ) u = f ( x , u ) , in  Ω , u = 0 , on  Ω , (2)

began to attract the attention of several researchers only after Lion [1] had proposed an abstract framework for this problem. Perera and Zhang [2] obtained a nontrivial solution of (2) by using the Yang index and critical group. They revisited (2) via invariant sets of decent flow and obtained the existence of a positive solution, a negative, and a sign-changing solutions in [3].

The study of Kirchhoff-type equations has been extended to the following case involving the p-Laplacian:

{ M ( 1 p Ω | u | p d x ) div ( | u | p 2 u ) = f ( x , u ) , in  Ω , u = 0 , on  Ω ,

for details see [4-6]. One of the authors has done some related work on this field. Liu [7] gave infinite solutions to the following equation via the fountain theorem and the dual fountain theorem:

{ [ M ( Ω ( | u | p + λ ( x ) | u | p ) d x ) ] p 1 ( p u + λ ( x ) | u | p 2 u ) = f ( x , u ) , in  Ω , | u | p 2 u ν = η | u | p 2 , on  Ω .

However, to the best of our knowledge, there have been few papers dealing with equation (1) using the linking theorem and the mountain pass theorem. This paper will make some contribution to this research field.

It is well known (see [8]) that the eigenvalue problem

{ p u = λ | u | p 2 u in  Ω , u = 0 on  Ω ,

has the first eigenvalue λ 1 > 0 , which is simple, and has an associated eigenfunction ϕ 1 > 0 . It is also known that λ 1 is an isolated point of σ ( p ) , the spectrum of p , which contains at least an eigenvalue sequence { λ n } and 0 < λ 1 < λ 2 λ 3 λ n .

Let

W 0 1 , p ( Ω ) = { u L p ( Ω ) : Ω | u | p d x <  and  u | Ω = 0 }

be a Banach space with the norm u = u p = ( Ω | u | p d x ) 1 p for u W 0 1 , p ( Ω ) . W = span { ϕ 1 } be the one-dimensional eigenspace associated with λ 1 , where ϕ 1 = 1 . Let W 0 1 , p ( Ω ) = W V , where V = { u W 0 1 , p ( Ω ) : Ω u ϕ 1 p 1 d x = 0 } , there exists λ ¯ > λ 1 such that

Ω | u | p d x λ ¯ Ω | u | p d x for  u V .

When p = 2 , we can take λ ¯ = λ 2 , the second eigenvalue of −△ in H 0 1 ( Ω ) .

In this paper, the weak solutions of (1) are the critical points of the energy functional

Φ ( u ) = 1 p M ˆ ( Ω | u | p d x ) Ω F ( x , u ) d x ,

where M ˆ ( u ) = 0 u [ M ( s ) ] p 1 d s , F ( x , u ) = 0 u f ( x , s ) d s . Obviously, Φ ( u ) C 1 ( W 0 1 , p ( Ω ) , R ) and for all u , v W 0 1 , p ( Ω ) ,

Φ ( u ) , v = [ M ( Ω | u | p d x ) ] p 1 Ω ( | u | p 2 u v ) d x Ω f ( x , u ) v d x .

In this paper we use the following notation: L p ( Ω ) denotes the Lebesgue space with the norm | | p ; | Ω | denotes the Lebesgue measure of the set Ω R N ; , is the dual pairing of the space ( W 0 1 , p ( Ω ) ) and W 0 1 , p ( Ω ) ; → (resp. ⇀) denotes strong (resp. weak) convergence. C 0 , C 1 , C 2 , denote positive constants (possibly different).

Definition 1[9]

Let Φ C 1 ( X , R ) , we say that Φ satisfies the Cerami condition at the level c R if any sequence { u n } X , along with

Φ ( u n ) c and ( 1 + u n ) Φ ( u n ) 0 as  n ,

possesses a convergent subsequence; Φ satisfies the (C) condition if Φ satisfies ( C ) c for all c R .

Definition 2[9]

A subset A of E is link (with respect to Φ ) to B of E if A B = , for every Γ Φ , there is t [ 0 , 1 ] such that Γ ( t , A ) B .

Let X = Y Z be a Banach space with dim Y < . Let ρ > r > 0 , and let z Z be such that u = r . Define

M = { u = y + λ z : u ρ , λ 0 , y Y } , M 0 = { u = y + λ z : y Y , u = ρ  and  λ 0  or  u ρ  and  λ = 0 } , N = { u Z : u = r } .

Let Φ C 1 ( X , R ) be such that

b = inf N Φ > a = max M 0 Φ .

If Φ satisfies the (PS) condition with

c = inf γ Γ max u M Φ ( γ ( u ) ) , Γ = { γ C ( M , X ) : γ | M 0 = id } ,

thencis a critical value of Φ.

Remark 1 If Φ satisfies the (C) condition, then Theorem 1 still holds.

Theorem 2[10] (Mountain pass theorem)

LetXbe a real Banach space, and let Φ C 1 ( X , R ) satisfy the (C) condition. Suppose, for some α < β , ρ > 0 and u X , u > ρ ,

max { Φ ( 0 ) , Φ ( u ) } α < β inf u = ρ Φ ( u ) .

Then Φ has a critical value c β > 0 characterized by

c = inf γ Γ max τ [ 0 , 1 ] Φ ( γ ( τ ) ) ,

where

Γ = { γ C ( [ 0 , 1 ] , X ) : γ ( 0 ) = 0 , γ ( 1 ) = u } .

### 2 Main results

In this section, we give our main theorem. Near the origin, we make the following assumptions.

Suppose that M : R + R + is a continuous function satisfying the following conditions:

( m 0 ) there exists a constant m 0 > 0 such that M ( t ) m 0 for all t 0 ;

( m 1 ) there exists a constant m 1 > 0 such that M ( t ) m 1 for all t 0 and M ˆ ( t ) a [ M ( t ) ] p 1 t , a > 1 .

Caratheodory function f satisfies:

( F 0 ) For some p < q < p = N p N p , there exists a constant C > 0 such that

| f ( x , t ) | C ( 1 + | t | q 1 ) for all  x Ω , t R .

( F 1 ) There exist 0 < α < m 0 p 1 λ 1 ( a 1 ) and σ [ 0 , p ] such that

lim inf | t | f ( x , t ) t p F ( x , t ) | t | σ > α uniformly in  x Ω .

( F 2 ) lim | t | p F ( x , t ) | t | p = + uniformly in x Ω .

( F 2 ) lim inf | t | p F ( x , t ) | t | p > m 1 p 1 λ 1 uniformly in x Ω .

( F 3 ) lim sup | t | 0 p F ( x , t ) | t | p < m 0 p 1 λ ¯ uniformly in x Ω .

( F 3 ) lim sup | t | 0 p F ( x , t ) | t | p < m 0 p 1 λ 1 uniformly in x Ω .

( F 4 ) F ( x , t ) m 1 p 1 λ 1 p | t | p , ( x , t ) Ω × R 1 .

The main results of this paper are the following.

Theorem 3Assume that ( m 0 ), ( m 1 ) and ( F 0 )-( F 4 ) hold, then problem (1) has at least one nontrivial weak solution in W 0 1 , p ( Ω ) .

Theorem 4Assume that ( m 0 ), ( m 1 ) and ( F 0 ), ( F 1 ), ( F 2 ) , ( F 3 ) hold, then problem (1) has at least one nontrivial weak solution in W 0 1 , p ( Ω ) .

### 3 Proofs of theorems

First, we give several lemmas.

Lemma 1[7]

Under assumptions ( m 0 ) and ( F 0 ), any bounded sequence { u n } W 0 1 , p ( Ω ) such that Φ ( u ) 0 in ( W 0 1 , p ( Ω ) ) as n has a convergent subsequence.

Lemma 2Under assumptions ( m 0 ) and ( F 1 ), the functional Φ ( u ) satisfies the (C) condition.

Proof Let { u n } W 0 1 , p ( Ω ) , for every c > 0 ,

Φ ( u n ) c and ( 1 + u n ) Φ ( u n ) 0 as  n . (3)

We claim that { u n } is bounded in W 0 1 , p ( Ω ) . For this purpose, we can suppose that u n . By ( F 1 ), there exists r > 1 such that

f ( x , t ) t p F ( x , t ) α | t | σ , | t | > r . (4)

For large n, set Ω n = { x Ω : | u | > r } , (3) and (4) imply that there exists M 1 > 0 such that

p ( 1 + c ) p Φ ( u n ) Φ ( u n ) , u n = M ˆ ( Ω | u n | p d x ) Ω p F ( x , u n ) d x [ [ M ( Ω | u n | p d x ) ] p 1 Ω ( | u n | p ) d x Ω f ( x , u n ) u n d x ] ( a 1 ) m 0 p 1 u n p + Ω [ f ( x , u n ) u n p F ( x , u n ) ] d x = ( a 1 ) m 0 p 1 u n p + Ω n [ f ( x , u n ) u n p F ( x , u n ) ] d x + Ω Ω n [ f ( x , u n ) u n p F ( x , u n ) ] d x ( a 1 ) m 0 p 1 u n p Ω n α | u n | σ d x M 1 ( a 1 ) m 0 p 1 u n p Ω n α | u n | p d x M 1 [ ( a 1 ) m 0 p 1 α λ 1 ] u n p M 1 .

This is a contradiction. Then { u n } is bounded in W 0 1 , p ( Ω ) . By Lemma 1, we see that { u n } has a convergent subsequence in W 0 1 , p ( Ω ) . □

Proof of Theorem 3 We obtain from assumptions ( m 0 ), ( F 0 ) and ( F 3 ) that for some ε > 0 small, there exists C 1 > 0 such that

F ( x , u ) m 0 p 1 ( λ ¯ ε ) p | u | p + C 1 | u | q , x Ω .

Taking u V , using the inequality Ω | u | p d x λ ¯ Ω | u | p d x and the Sobolev inequality | u | q q K u q , we have

Φ ( u ) = 1 p M ˆ ( Ω | u | p d x ) Ω F ( x , u ) d x m 0 p 1 p u p m 0 p 1 ( λ ¯ ε ) p Ω | u | p d x C 1 Ω | u | q d x m 0 p 1 p u p m 0 p 1 ( λ ¯ ε ) p λ ¯ u p C 1 K u q = m 0 p 1 p ε λ ¯ u p C 1 K u q .

Then there exists r > 0 such that b = inf u V , u = r Φ ( u ) > 0 .

Let z V , z = r and M 0 = { u = v + λ z : v W : u = ρ  and  λ 0 , or  u ρ  and  λ = 0 } , N = { u V : u = r } .

For every u M 0 , if u = v + λ z , u ρ and λ = 0 , then u = v W . By ( F 4 ), we know that

Φ ( u ) = 1 p M ˆ ( Ω | u | p d x ) Ω F ( x , u ) d x m 1 p 1 p u p Ω F ( x , u ) d x m 1 p 1 p λ 1 Ω | u | p d x Ω F ( x , u ) d x Ω [ m 1 p 1 p λ 1 | u | p d x F ( x , u ) ] d x 0 .

Since dim W R z < , there exists C 0 > 1 such that

u p C 0 | u | p p , u W R z .

By ( F 0 ) and ( F 2 ), there exists ρ > 0 such that F ( x , u ) m 1 p 1 p C 0 2 | u | p , | u | > ρ . Let M = max { 0 , inf | u | ρ F ( x , u ) } , then we have

F ( x , u ) m 1 p 1 p C 0 2 | u | p M .

Therefore, u W R z , we have

Φ ( u ) = 1 p M ˆ ( Ω | u | p d x ) Ω F ( x , u ) d x m 1 p 1 p u p m 1 p 1 C 0 2 p Ω | u | p d x + M | Ω | m 1 p 1 p u p m 1 p 1 C 0 p u p + M | Ω | ( C 0 1 ) m 1 p 1 p u p + M | Ω | .

Hence, for ρ = u large enough, we have Φ ( u ) .

Then there exists ρ > r > 0 such that

inf N Φ ( u ) > 0 = max M 0 Φ ( u ) .

By Lemmas 1 and 2, Φ satisfies the (C) condition. Then the conclusion follows from Theorem 1 and Remark 1. □

Remark 2 (i) There exists R > 0 such that p F ( x , u ) u f ( x , u ) , | u | R , x Ω , which implies | u | R , x Ω ,

f ( x , u ) u p F ( x , u ) 0 > α | u | σ .

Then

lim inf | t | f ( x , t ) t p F ( x , t ) | t | σ > α uniformly in  x Ω .

Hence ( F 1 ) is much weaker than Ambrosetti-Rabinowitz type growth conditions.

(ii) If f ( x , u ) u p F ( x , u ) + , as | u | uniformly in x Ω , then

lim inf | t | f ( x , t ) t p F ( x , t ) | t | σ 0 > α uniformly in  x Ω .

Example 1 Set

f ( x , t ) = { 0 , t < 0 ; m 0 p 1 λ ˜ t p 1 , 0 t 1 ; t p 1 ln t + m 0 p 1 λ ˜ t p 1 , t > 1 ,

where λ 1 < λ ˜ < λ ¯ , m 1 p 1 λ 1 + 1 < m 0 p 1 λ ˜ . Then it is easy to verify that f ( x , t ) satisfies ( F 0 )-( F 4 ) with σ = p . When t < 0 , we can use odd expansion to f ( x , t ) .

Example 2 Set

f ( x , t ) = { 0 , t < 0 ; m 0 p 1 λ ˜ t p 1 , 0 t < 2 π ; m 1 p 1 λ 1 p [ q ( x , t ) t p + p q ( x , t ) t p 1 ] + m 0 p 1 λ ˜ t p 1 , t 2 π ,

where

q ( x , t ) = { t sin t , t [ 2 k π , ( 2 k + 1 ) π ) ; sin t t , t [ ( 2 k + 1 ) π , ( 2 k + 2 ) π ) ( k = 1 , 2 , ) ,

q ( x , t ) = 2 π t q ( x , s ) d s , λ 1 < λ ˜ < λ ¯ , m 1 p 1 λ 1 < m 0 p 1 λ ˜ . Then it is easy to verify that f ( x , t ) satisfies ( F 0 )-( F 4 ) with σ = p . When t < 0 , we can use odd expansion to f ( x , t ) .

Proof of Theorem 4 We obtain from assumptions ( m 0 ), ( F 0 ), ( F 3 ) that for some ε > 0 small, there exists C 2 > 0 such that

F ( x , u ) m 0 p 1 p ( λ 1 ε ) | u | p + C 2 | u | q for all  x Ω .

Taking u W 0 1 , p ( Ω ) , using the inequality Ω | u | p d x λ 1 Ω | u | p d x and the Sobolev inequality | u | q q K u q , we have

Φ ( u ) = 1 p M ˆ ( Ω | u | p d x ) Ω F ( x , u ) d x m 0 p 1 p u p m 0 p 1 ( λ 1 ε ) p Ω | u | p d x C 2 Ω | u | q d x m 0 p 1 p u p m 0 p 1 ( λ 1 ε ) p λ 1 u p C 2 K u q = m 0 p 1 p ε λ 1 u p C 2 K u q .

Then there exists ρ > 0 such that inf u = ρ Φ ( u ) > 0 .

By ( F 0 ) and ( F 2 ) , there exist β > λ 1 , ε > 0 , ρ > 0 , and β ε > λ 1 such that | u | > ρ ,

F ( x , u ) m 1 p 1 ( β ε ) p | u | p .

Let M 1 = max { 0 , inf u ρ F ( x , u ) } , then we have

F ( x , u ) m 1 p 1 ( β ε ) p | u | p M 1 .

For u W , we have

Φ ( u ) = 1 p M ˆ ( Ω | u | p d x ) Ω F ( x , u ) d x m 1 p 1 p u p m 1 p 1 ( β ε ) p Ω | u | p d x + M 1 | Ω | .

Therefore, for β ε > λ 1 , let u = t ϕ 1 , t R , we have

Φ ( t ϕ 1 ) m 1 p 1 p | t | p m 1 p 1 ( β ε ) p Ω | t ϕ 1 | p d x + M 1 | Ω | m 1 p 1 p | t | p m 1 p 1 ( β ε ) p λ 1 | t | p + M 1 | Ω | = m 1 p 1 p ( 1 β ε λ 1 ) | t | p + M 1 | Ω | as  t .

Hence there exists u 1 = t 1 ϕ 1 W 0 1 , p ( Ω ) , u 1 > ρ such that Φ ( u 1 ) 0 .

Then

inf u = ρ Φ ( u ) > 0 max { Φ ( 0 ) , Φ ( u 1 ) } .

Summing up Lemma 1 and Lemma 2, Φ ( u ) satisfies all the conditions of Theorem 2, then the conclusion follows from Theorem 2. □

Remark 3 The result of Theorem 1.1 in [11] corresponds to our results for the case m 0 = m 1 = 1 and ( F 2 + ) μ replaces ( F 1 ). It is easy to see that ( F 1 ) is much weaker than ( F 2 + ) μ , hence the results of Theorems 3 and 4 extend the results of [11].

Example 3 Set

f ( x , t ) = { 0 , t < 0 ; m 0 p 1 μ 2 t p 1 , 0 t 1 ; m 1 p 1 μ 1 t p 1 m 1 p 1 μ 1 + m 0 p 1 μ 2 t , t > 1 ,

where p is odd, μ 1 > λ 1 > μ 2 ,

f ( x , t ) = { m 1 p 1 μ 1 t p 1 + m 1 p 1 μ 1 + m 0 p 1 μ 2 t , t < 1 ; m 0 p 1 μ 2 t p 1 , | t | 1 ; m 1 p 1 μ 1 t p 1 m 1 p 1 μ 1 + m 0 p 1 μ 2 t , t > 1 ,

where p is even, μ 1 > λ 1 > μ 2 .

Then similar to [12], it is easy to verify that f ( x , t ) satisfies ( F 0 ), ( F 1 ), ( F 2 ) , ( F 3 ) with σ = p .

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

CL and JW obtained the results in a joint research. All the authors read and approved the final manuscript.

### Acknowledgements

This work was supported by the National Nature Science Foundation of China (10971179) and the Project of Shandong Province Higher Educational Science and Technology Program (J09LA55, J12LI53).

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