Research Article | Open Access

Lu-Chuan Ceng, Ngai-Ching Wong, Jen-Chih Yao, "Well-Posedness for a Class of Strongly Mixed Variational-Hemivariational Inequalities with Perturbations", *Journal of Applied Mathematics*, vol. 2012, Article ID 712306, 21 pages, 2012. https://doi.org/10.1155/2012/712306

# Well-Posedness for a Class of Strongly Mixed Variational-Hemivariational Inequalities with Perturbations

**Academic Editor:**Ya Ping Fang

#### Abstract

The concept of well-posedness for a minimization problem is extended to develop the concept of well-posedness for a class of strongly mixed variational-hemivariational inequalities with perturbations which includes as a special case the class of variational-hemivariational inequalities with perturbations. We establish some metric characterizations for the well-posed strongly mixed variational-hemivariational inequality and give some conditions under which the strongly mixed variational-hemivariational inequality is strongly well-posed in the generalized sense. On the other hand, it is also proven that under some mild conditions there holds the equivalence between the well posedness for a strongly mixed variational-hemivariational inequality and the well-posedness for the corresponding inclusion problem.

#### 1. Introduction

It is well known that the classical notion of well-posedness for the minimization problem (MP) is due to Tykhonov [1], which has been known as the Tykhonov well-posedness. Let be a Banach space and be a real-valued functional on . The problem (MP), that is, , is said to be well posed if there exists a unique minimizer and every minimizing sequence converges to the unique minimizer. Furthermore, the notion of generalized Tykhonov well-posedness is also introduced for the problem (MP), which means the existence of minimizers and the convergence of some subsequence of every minimizing sequence toward a minimizer. Clearly, the concept of well-posedness is inspired by numerical methods producing optimizing sequences for optimization problems and plays a crucial role in the optimization theory. Therefore, various concepts of well-posedness have been introduced and studied for optimization problems. For more details, we refer to [2–8] and the references therein.

On the other hand, the concept of well-posedness has been extended to other related problems, such as variational inequalities [5, 9–14], saddle-point problem [15], inclusion problems [10, 11], and fixed-point problems [10, 11]. An initial notion of well-posedness for variational inequalities is due to Lucchetti and Patrone [5]. They introduced the notion of well-posedness for variational inequalities and proved some related results by means of Ekeland’s variational principle. Since then, many authors have been devoted to generating the concept of well-posedness from the minimization problem to various variational inequalities. In [2], Crespi et al. gave the notions of well-posedness for a vector optimization problem and a vector variational inequality of the differential type, explored their basic properties, and investigated their links. Lignola [13] introduced two concepts of well-posedness and -well-posedness for quasivariational inequalities and investigated some equivalent characterizations of these two concepts. Recently, Fang et al. [11] generalized the concepts of well-posedness and -well-posedness to a generalized mixed variational inequality which includes as a special case the classical variational inequality and discussed its links with the well-posedness of corresponding inclusion problem and the well-posedness of corresponding fixed-point problem. They also derived some conditions under which the mixed variational inequality is well posed. For further results on the well-posedness for variational inequalities and equilibrium problems, we refer to [5, 8, 11, 13, 16–18] and the references therein.

In 1983, in order to formulate variational principles involving energy functions with no convexity and no smoothness, Panagiotopoulos [19] first introduced the hemivariational inequality which is an important and useful generalization of variational inequality and investigated it by using the mathematical notion of the generalized gradient of Clarke for nonconvex and nondifferentiable functions [20]. The hemivariational inequalities have been proved very efficient to describe a variety of mechanical problems, for instance, unilateral contact problems in nonlinear elasticity, problems describing the adhesive and frictional effects, and nonconvex semipermeability problems (see, for instance, [19, 21, 22]). Therefore, in recent years all kinds of hemivariational inequalities have been studied by many authors [14, 21, 23–29], and the study of hemivariational inequalities has emerged as a new and interesting branch of applied mathematics. However, there are very few researchers extending the well-posedness to hemivariational inequalities. In 1995, Goeleven and Mentagui [14] first introduced the notion of well-posedness for hemivariational inequalities and established some basic results concerning the well-posed hemivariational inequality.

Very recently, Xiao and Huang [30] generalized the well-posedness of minimization problems to a class of variational-hemivariational inequalities with perturbations, which includes as special cases the classical hemivariational inequalities and variational inequalities. Under appropriate conditions, they derived some metric characterizations for the well-posed variational-hemivariational inequality and presented some conditions under which the variational-hemivariational inequality is strongly well posed in the generalized sense. Meantime, they also proved that the well-posedness for a variational-hemivariational inequality is equivalent to the well-posedness for the corresponding inclusion problem.

In this paper, we extend the notion of well-posedness for minimization problems to a class of strongly mixed variational-hemivariational inequalities with perturbations, which includes as a special case the class of variational-hemivariational inequalities with perturbations in [30]. Under very mild conditions, we establish some metric characterizations for the well-posed strongly mixed variational-hemivariational inequality and give some conditions under which the strongly mixed variational-hemivariational inequality is strongly well-posed in the generalized sense. On the other hand, it is also proven that the well-posedness for a strongly mixed variational-hemivariational inequality is equivalent to the well-posedness for the corresponding inclusion problem.

#### 2. Preliminaries

Throughout this paper, unless stated otherwise, we always suppose that is a real reflexive Banach space, where its dual space is denoted by and the generalized duality pairing between and is denoted by . We denote the norms of Banach spaces and by and , respectively. In what follows, let , and be four mappings, be a proper, convex, and lower semicontinuous functional, and be some given element. Denote by the efficient domain of functional, that is,

Consider the following strongly mixed variational-hemivariational inequality: find such that where denotes the generalized directional derivative in the sense of Clarke of a locally Lipschitz functional at in the direction (see [20]) given by

In particular, if , for all and the identity mapping of , then the problem (2.2) reduces to the following variational-hemivariational inequality of finding such that where is perturbation, which was first introduced and studied by Xiao and Huang [30].

Let be an open bounded subset of which is occupied by a linear elastic body and the boundary of the which is assumed to be appropriately regular (, i.e., a Lipschitzian boundary, is sufficient). We denote by the stress vector on , which can be decomposed into a normal component and a tangential component with respect to , that is, where is an appropriately defined stress tensor and is the outward unit normal vector on . Analogously, and denote the normal and the tangential components of the displacement vector with respect to . As pointed out in [30], the reaction-displacement law presents in compression ideal locking effect (the infinite branch ), that is, always , whereas is impossible. Specifically, where is a multivalued function defined as follows. Suppose that is a function such that , that is, a function essentially bounded on any bounded interval of . For any and , we define and . By the monotonicity of the functions and with respect to , we infer that the limits as exist, that is, Then, Furthermore, a locally Lipschitz function can be determined up to an additive constant by such that for each when the limits exist, where is the Clarke’s generalized gradient of locally Lipschitz function which will be specified in what follows.

Now, let , the normal cone to at , and the indicator of the set . Then (2.6) can be written as where is the subgradient of the convex functional in the sense of convex analysis, which will also be specified in what follows. By the definitions of the Clarke’s generalized gradient of locally Lipschitz function and the subgradient of the convex functional, (2.10) gives rise to the following variational-hemivariational inequality which is a special case of the variational-hemivariational inequality VHVI. Beyond question, the problem (2.11) is a special case of the strongly mixed variational-hemivariational inequality SMVHVI as well. More special cases of the SMVHVI are stated as follows.(i)If and , where denotes the indicator functional of a nonempty, convex subset of a function space defined on and is a locally Lipschitz continuous function, then the SMVHVI reduces to the following strongly mixed variational-hemivariational inequality: Remark that the SMVHVI (2.12) with and is equivalent to the VHVI which was considered by Goeleven and Mentagui in [14].(ii)If , then the SMVHVI (2.2) with reduces to the strongly mixed hemivariational inequality of finding such that Remark that the (2.13) with and is equivalent to the hemivariational inequality (HVI) studied intensively by many authors (see, e.g., [21, 22]).(iii)If , then the SMVHVI (2.2) with reduces to the strongly mixed variational inequality of finding such that Remark that the SMVI (2.14) with and is equivalent to the mixed variational inequality (see, e.g., [11, 31]) and the references therein).(iv)If , , and , then the SMVHVI (2.2) with reduces to the classical variational inequality: (v)If , , , and , then the SMVHVI (2.2) reduces to the global minimization problem: Let and denote the subgradient of convex functional in the sense of convex analysis (see [32]) and the Clarke’s generalized gradient of locally Lipschitz functional (see [20]), respectively. That is,

*Remark 2.1 (see [33]). *The Clarke’s generalized gradient of a locally Lipschitz functional at a point is given by

About the subgradient in the sense of convex analysis, the Clarke’s generalized directional derivative, and the Clarke’s generalized gradient, we have the following basic properties (see, e.g., [20, 30, 32, 33]).

Proposition 2.2. *Let be a Banach space and be a convex and proper functional. Then we have the following properties of :*(i)* is convex and weak*-closed;*(ii)*if is continuous at , then is nonempty, convex, bounded, and weak*-compact;*(iii)*if is Gateaux differentiable at , then , where is the Gateaux derivative of at .*

Proposition 2.3. *Let be a Banach space and be two convex functionals. If there is a point at which is continuous, then the following equation holds:
*

Proposition 2.4. *Let be a Banach space, , and a locally Lipschitz functional defined on . Then*(i)*the function is finite, positively homogeneous, subadditive, and then convex on ;*(ii)* is upper semicontinuous as a function of , as a function of alone, is Lipschitz continuous on ;*(iii)*;*(iv)* is a nonempty, convex, bounded, weak*-compact subset of ;*(v)*for every , one has*

Now we recall some important definitions and useful results.

*Definition 2.5 (see [34]). *Let be a real Banach space with its dual and be an operator from to its dual space . is said to be monotone if

*Definition 2.6 (see [34]). *A mapping is said to be hemicontinuous if for any , the function from into is continuous at .

It is clear that the continuity implies the hemicontinuity, but the converse is not true in general.

Theorem 2.7 (see [35]). *Let be nonempty, closed, and convex, nonempty, closed, convex, and bounded, proper, convex, and lower semicontinuous, and be arbitrary. Assume that, for each , there exists such that
**
Then, there exists such that
*

*Definition 2.8 (see [36]). *Let be a nonempty subset of . The measure, say , of noncompactness for the set is defined by
where means the diameter of set .

*Definition 2.9 (see [36]). *Let be nonempty subsets of . The Hausdorff metric between and is defined by
where with .

Let be a sequence of nonempty subsets of . We say that converges to in the sense of Hausdorff metric if . It is easy to see that if and only if for all section . For more details on this topic, we refer the reader to [36].

#### 3. Well-Posedness of the SMVHVI with Metric Characterizations

In this section, we generalize the concept of well-posedness to the strongly mixed variational-hemivariational inequality SMVHVI with perturbations, establish its metric characterizations, and derive some conditions under which the strongly mixed variational-hemivariational inequality is strongly well-posed in the generalized sense in Euclidean space .

*Definition 3.1. *A sequence is said to be an approximating sequence for the SMVHVI if there exists a nonnegative sequence with as such that

*Definition 3.2. *The SMVHVI is said to be strongly (resp., weakly) well posed if the SMVHVI has a unique solution in and every approximating sequence converges strongly (resp., weakly) to the unique solution.

*Remark 3.3. *Strong well-posedness implies weak well-posedness, but the converse is not true in general.

*Definition 3.4. *The SMVHVI is said to be strongly (resp., weakly) well posed in the generalized sense if the SMVHVI has a nonempty solution set in and every approximating sequence has a subsequence which converges strongly (resp., weakly) to some point of the solution set .

*Remark 3.5. *Strong well-posedness in the generalized sense implies weak well-posedness in the generalized sense, but the converse is not true in general.

*Definition 3.6. *Let and be two mappings. Then(i) is said to be monotone with respect to the first argument of if there holds (ii) is said to be continuous with respect to the first argument of if for each the mapping from into is continuous;(iii) is said to be hemicontinuous with respect to the first argument of if for all and , the function from into is continuous at .

For any , we define the following two sets:

Lemma 3.7. *Suppose that is both monotone and hemicontinuous with respect to the first argument of is a proper, convex, and lower semicontinuous functional. Then for all .*

* Proof. * Let . Then, by the monotonicity of the mapping with respect to the first argument of , we have for all
This implies that . Thus, we get the inclusion .

Next let us show that . Indeed, for any , we have
For any and , putting in (3.5), we obtain
Since the Clarke's generalized directional derivative is positively homogeneous with respect to and is convex, it follows that
Taking the limit for (3.7) as , we obtain from the hemicontinuity of the mapping with respect to the first argument of that
By the arbitrariness of , we conclude that , which implies that . This completes the proof.

Lemma 3.8. *Suppose that is continuous with respect to the second argument of , is continuous, and is a proper, convex, and lower semicontinuous functional. Then is closed in for all .*

*Proof. * Let be a sequence such that in . Then
Since is continuous with respect to the second argument of , is continuous, is lower semicontinuous, and the Clarke's generalized directional derivative is upper semicontinuous with respect to , we deduce that , , and
Taking the for (3.9) as , we obtain from (3.10) that
which implies that . Therefore, is closed in . This completes the proof.

Corollary 3.9. *Suppose that is both monotone and hemicontinuous with respect to the first argument of and is continuous with respect to the second argument of . Let be continuous and be a proper, convex, and lower semicontinuous functional. Then, for all , is closed in .*

Theorem 3.10. *Suppose that is both monotone and hemicontinuous with respect to the first argument of and is continuous with respect to the second argument of . Let be continuous and a proper, convex, and lower semicontinuous functional. Then, the SMVHVI is strongly well posed if and only if
*

*Proof. *“Necessity”. Suppose that the SMVHVI is strongly well posed. Then the SMVHVI has a unique solution which lies in and so for all . If as , then there exist a constant , a nonnegative sequence with and such that
Since , it is known that and are both approximating sequences for the SMVHVI. From the strong well-posedness of the SMVHVI, it follows that both and converge strongly to the unique solution of the SMVHVI, which is a contradiction to (3.13).

“Sufficiency”. Let be an approximating sequence for the SMVHVI. Then there exists a nonnegative sequence with such that
which implies that . By condition (3.12), is a Cauchy sequence and so converges strongly to some point . Since the mapping is monotone with respect to the first argument of , the mapping is continuous with respect to the second argument of , is continuous, the Clarke's generalized directional derivative is upper semicontinuous with respect to , and is lower semicontinuous, it follows from (3.14) that
Furthermore, since is also hemicontinuous with respect to the first argument of and is convex, by the argument similar to that in Lemma 3.7 we can readily prove that
which implies that solves the SMVHVI.

To complete the proof of Theorem 3.10, we need only to prove that the SMVHVI has a unique solution. Assume by contradiction that the SMVHVI has two distinct solutions and . Then it is easy to see that for all and
which is a contradiction. Therefore, the SMVHVI has a unique solution. This completes the proof.

Corollary 3.11 (see [30, Theorem 3.1]). *Suppose that is a monotone and hemicontinuous mapping, is a continuous mapping, and is a proper, convex, and lower semicontinuous functional. Then, the VHVI is strongly well posed if and only if
*

*Proof. *In Theorem 3.10, put , for all and the identity mapping of . Then from the monotonicity and hemicontinuity of it follows that is both monotone and hemicontinuous with respect to the first argument of . Moreover, from the continuity of it follows that is continuous with respect to the second argument of . Thus, utilizing Theorem 3.10, we obtain the desired result.

Theorem 3.12. *Suppose that is both monotone and hemicontinuous with respect to the first argument of and is continuous with respect to the second argument of . Let be continuous and be a proper, convex, and lower semicontinuous functional. Then, the SMVHVI is strongly well posed in the generalized sense if and only if
*

*Proof. *“Necessity”. Suppose that the SMVHVI is strongly well posed in the generalized sense. Then the solution set of the SMVHVI is nonempty and for any . Furthermore, the solution set of the SMVHVI also is compact. In fact, for any sequence , it follows from for any that is an approximating sequence for the SMVHVI. Since the SMVHVI is strongly well posed in the generalized sense, has a subsequence which converges strongly to some point of the solution set . Thus, the solution set of the SMVHVI is compact. Now let us show that as . From for any , we get
Taking into account the compactness of the solution set , we obtain from (3.20) that
In order to prove that as , it is sufficient to show that as . Assume by contradiction that as . Then there exist a constant , a sequence with and such that
where is the closed ball centered at 0 with radius . Since is an approximating sequence for the SMVHVI and the SMVHVI is strongly well posed in the generalized sense, there exists a subsequence which converges strongly to some point which is a contradiction to (3.22). Then as .

“Sufficiency”. Assume that condition (3.19) holds. By Corollary 3.9, we conclude that is nonempty and closed for all . Observe that
Since as , by applying the theorem [36, page 412], it can be easily found that is nonempty and compact with
Let be an approximating sequence for the SMVHVI. Then there exists a nonnegative sequence with such that
and so by the definition of . It follows from (3.24) that
Since the solution set is compact, there exists such that
Again from the compactness of the solution set , has a subsequence converging strongly to some . It follows from (3.27) that
which implies that converges strongly to . Therefore, the SMVHVI is strongly well-posed in the generalized sense. This completes the proof.

Corollary 3.13 (see [30, Theorem 3.2]). *Suppose that is a monotone and hemicontinuous mapping, is a continuous mapping, and is a proper, convex, and lower semicontinuous functional. Then, the VHVI is strongly well posed in the generalized sense if and only if
**The following theorem gives some conditions under which the strongly mixed variational-hemivariational inequality is strongly well posed in the generalized sense in Euclidean space .*

Theorem 3.14. *Suppose that is both monotone and hemicontinuous with respect to the first argument of and is continuous with respect to the second argument of . Let be continuous and be a proper, convex, and lower semicontinuous functional. If there exists some such that is nonempty and bounded. Then the strongly mixed variational-hemivariational inequality SMVHVI is strongly well posed in the generalized sense.*

*Proof. *Suppose that is an approximating sequence for the SMVHVI. Then there exists a nonnegative sequence with as such that
Let be such that is nonempty and bounded. Then there exists such that for all . This implies that is bounded by the boundedness of . Thus, there exists a subsequence such that as . Since the mapping is monotone with respect to the first argument of , the mapping is continuous with respect to the second argument of , is continuous, the Clarke’s generalized directional derivative is upper semicontinuous with respect to , and is lower semicontinuous, it follows from (3.30) that
Meantime, since is also hemicontinuous with respect to the first argument of and is convex, by the argument similar to that in Lemma 3.7 we can readily prove that
which implies that solves the SMVHVI. Therefore, the SMVHVI is strongly well-posed in the generalized sense. This completes the proof.

Corollary 3.15 (see [30, Theorem 3.3]). *Suppose that is a monotone and hemicontinuous mapping, is a continuous mapping, and is a proper, convex, and lower semicontinuous functional. If there exists some such that is nonempty and bounded. Then the variational-hemivariational inequality VHVI is strongly well posed in the generalized sense.*

#### 4. Well-Posedness of Inclusion Problem

In this section, we first recall the concept of well-posedness for inclusion problems and then investigate the relations between the well-posedness for the strongly mixed variational-hemivariational inequality and the well-posedness for the corresponding inclusion problem. In what follows we always assume that is a set-valued mapping from real reflexive Banach space to its dual space . The inclusion problem associated with mapping is defined by

*Definition 4.1 (see [18, 37]). *A sequence is called an approximating sequence for the inclusion problem if or, equivalently, there exists a sequence such that as .

*Definition 4.2 (see [18, 37]). *We say that the inclusion problem is strongly (resp., weakly) well posed if it has a unique solution and every approximating sequence converges strongly (resp., weakly) to the unique solution of .

*Definition 4.3 (see [18, 37]). *We say that the inclusion problem is strongly (resp., weakly) well posed in the generalized sense if the solution set of the is nonempty and every approximating sequence has a subsequence which converges strongly (resp., weakly) to some point of the solution set for the .

The following two theorems establish the relations between the strong (resp., weak) well-posedness for the strongly mixed variational-hemivariational inequality and the strong (resp., weak) well-posedness for the corresponding inclusion problem.

Theorem 4.4. *Let , , and be four mappings, a locally Lipschitz functional, and a proper, convex, and lower semicontinuous functional. Then the strongly mixed variational-hemivariational inequality SMVHVI is strongly (resp., weakly) well posed if and only if the corresponding inclusion problem is strongly (resp., weakly) well posed.*

Theorem 4.5. *Let , , and be four mappings, a locally Lipschitz functional, and a proper, convex, and lower semicontinuous functional. Then the strongly mixed variational-hemivariational inequality SMVHVI is strongly (resp., weakly) well posed in the generalized sense if and only if the corresponding inclusion problem is strongly (resp., weakly) well posed in the generalized sense.*

Lemma 4.6. *Let , , and be four mappings, a locally Lipschitz functional, and a proper, convex, and lower semicontinuous functional. Then is a solution of the SMVHVI if and only if is a solution of the corresponding inclusion problem of finding such that
*

* Proof. * “Sufficiency”. Assume that is a solution of the inclusion problem . Then there exist and such that
By multiplying at both sides of the above equation (4.3), we obtain from the definitions of the Clarke’s generalized gradient for locally Lipschitz functional and the subgradient for convex functional that
which implies that is a solution of the SMVHVI.

“Necessity”. Suppose that is a solution of the SMVHVI. Then,
From the fact that
we deduce that there exists a such that
In terms of Proposition 2.4 (iv), is a nonempty, convex, bounded, weak*-compact subset of . Note that is a real reflexive Banach space. Hence, is a nonempty, convex, bounded, weak-compact subset in . Thus is a nonempty, closed, convex, and bounded subset in which implies that is nonempty, closed, convex, and bounded in . Since is a proper, convex, and lower semicontinuous functional, it follows from Theorem 2.7 with and (4.7) that there exists such that
For the sake of simplicity we write , and hence from (4.8) we have
which implies that . Consequently, it follows from that
which implies that is a solution of the inclusion problem . This completes the proof.

*Proof of Theorem 4.4. *“Necessity”. Assume that the SMVHVI is strongly (resp., weakly) well posed. Then there is a unique solution for the SMVHVI. By Lemma 4.6, also is the unique solution for the inclusion problem . Let be an approximating sequence for the . Then there exists a sequence such that as . And so there exist and such that
From the definitions of the Clarke’s generalized gradient for locally Lipschitz functional and the subgradient for convex functional, we obtain by multiplying at both sides of the above equation (4.11) that
Letting , we obtain that is an approximating sequence for the SMVHVI from (4.12) with as . Therefore, it follows from the strong (resp., weak) well-posedness of the SMVHVI that converges strongly (resp., weakly) to the unique solution . Thus, the inclusion problem is strongly (resp., weakly) well posed.

“Sufficiency”. Suppose that the inclusion problem is strongly (resp., weakly) well posed. Then the has a unique solution , which implies that is the unique solution for the SMVHVI by Lemma 4.6. Let be an approximating sequence for the SMVHVI. Then there exists a sequence with as such that
By the same argument as in the proof of Lemma 4.6, there exists a such that
and is nonempty, closed, convex, and bounded in . Then, it follows from (4.14) and Theorem 2.7 with , which is proper, convex, and lower semicontinuous, that there exists such that
For the sake of simplicity we write , and hence from (4.15) we have
Define functional as follows:
where and are two functionals on defined by
Clearly, is proper, convex, and lower semicontinuous and is a global minimizer of on . Thus, . Since the functionals and are continuous on and is proper, convex, and lower semicontinuous, it follows from Proposition 2.3 that
It is easy to calculate that
and so there exists a with such that
Letting , we have as . Moreover, since , it follows from (4.21) that
which implies that is an approximating sequence for the . Since the inclusion problem is strongly (resp., weakly) well posed, converges strongly (resp., weakly) to the unique solution . Therefore, the strongly mixed variational-hemivariational inequality SMVHVI is strongly (resp., weakly) well posed. This completes the proof.

*Proof of Theorem 4.5. *The proof is similar to that in Theorem 4.4 and so we omit it here.

Corollary 4.7 (see [30, Theorem 4.1]). *Let and be two mappings from Banach space to its dual , be a locally Lipschitz functional, and be a proper, convex, and lower semicontinuous functional. Then the variational-hemivariational inequality VHVI is strongly (resp., weakly) well posed if and only if the corresponding inclusion problem is strongly (resp., weakly) well posed.*

* Proof. * In Theorem 4.4, put the identity mapping of and , for all . Then, in terms of Theorem 4.4 we derive the desired result.

Corollary 4.8 ([30, Theorem 4.2]). *Let and be two mappings from Banach space to its dual , be a locally Lipschitz functional, and be a proper, convex, and lower semicontinuous functional. Then the variational-hemivariational inequality VHVI is strongly (resp., weakly) well posed in the generalized sense if and only if the corresponding inclusion problem is strongly (resp., weakly) well posed in the generalized sense.*

#### 5. Concluding Remarks

In this paper, we introduce some concepts of well-posedness for a class of strongly mixed variational-hemivariational inequalities with perturbations, which includes as a special case the class of variational-hemivariational inequalities in [30]. We establish some metric characterizations for the well-posed strongly mixed variational-hemivariational inequality and give some conditions under which the strongly mixed variational-hemivariational inequality is strongly well posed in the generalized sense in . On the other hand, we first recall the concept of well-posedness for inclusion problems and then investigate the relations between the strong (resp., weak) well-posedness for a strongly mixed variational-hemivariational inequality and the strong (resp., weak) well-posedness for the corresponding inclusion problem.

It is well known that there are many other concepts of well-posedness for optimization problems, variational inequalities, and Nash equilibrium problems, such as -well-posedness [17], well-posedness by perturbations [12], and Levitin-Polyak well-posedness [38]. However, we wonder whether the concepts mentioned as above can be extended to the strongly mixed variational-hemivariational inequality. Beyond question, this is an interesting problem.

#### Acknowledgments

This paper was partially supported by the National Science Foundation of China (11071169), Innovation Program of Shanghai Municipal Education Commission (09ZZ133), and Leading Academic Discipline Project of Shanghai Normal University (DZL707) (to L.-C. Ceng). This paper was partially supported by the Taiwan NSC Grant 99-2115-M-110-007-MY3 (to N.-C. Wang). This paper was partially supported by the Taiwan NSC Grant 99-2221-E-037-007-MY3 (to J.-C. Yao).

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