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# Some eigenvalue results for perturbations of maximal monotone operators

- In-Sook Kim
^{1}Email author and - Jung-Hyun Bae
^{1}

**2013**:72

https://doi.org/10.1186/1029-242X-2013-72

© Kim and Bae; licensee Springer 2013

**Received:**19 September 2012**Accepted:**9 February 2013**Published:**28 February 2013

## Abstract

We study the solvability of a nonlinear eigenvalue problem for maximal monotone operators under a normalization observation. The investigation is based on degree theories for appropriate classes of operators, and a regularization method by the duality operator is used. Let *X* be a real reflexive Banach space with its dual ${X}^{\ast}$ and Ω be a bounded open set in *X*. Suppose that $T:D(T)\subset X\to {X}^{\ast}$ is a maximal monotone operator and $C:(0,\mathrm{\infty})\times \overline{\mathrm{\Omega}}\to {X}^{\ast}$ is a bounded demicontinuous operator satisfying condition (${S}_{+}$). Applying the Browder degree theory, we solve a nonlinear eigenvalue problem of the form $Tx+C(\lambda ,x)=0$. In the case where $Tx+\lambda Cx=0$, an eigenvalue result for generalized pseudomonotone densely defined perturbations is obtained by the Kartsatos-Skrypnik degree theory.

**MSC:**47J10, 47H05, 47H14, 47H11.

## Keywords

- eigenvalue
- maximal monotone operator
- perturbation
- degree theory

## 1 Introduction and preliminaries

Eigenvalue theory is closely related to the problem of solving nonlinear equations which was initiated by Krasnosel’skii [1] for compact operators. Regarding maximal monotone operators, it has been extensively investigated by many researchers in various aspects, with applications to evolution equations and differential equations; see, *e.g.*, [2–5]. The study was mostly based on degree theories for appropriate classes of operators and the usual method of regularization by means of the duality operator; see [6–10].

*X*be a real reflexive Banach space with its dual ${X}^{\ast}$ and Ω be a bounded open set in

*X*. Suppose that $T:D(T)\subset X\to {X}^{\ast}$ is a maximal monotone operator. We consider the nonlinear eigenvalue problem

*C*or the resolvents of the operator

*T*are compact, this problem was studied, for instance, by Guan-Kartsatos [11], Kartsatos [12], and Li-Huang [13], where the method is to use the Leray-Schauder degree theory. More generally, an implicit eigenvalue problem of the form

has no solution in $D(T)\cap \mathrm{\Omega}$, where *J* denotes the duality operator. This property has a close relation to the use of topological degree in eigenvalue theory by the regularization method. It is shown in [3] that two conditions about the weak closure of a certain set consisting of normalized vectors and the asymptotic behavior of the operator *C* at infinity of *λ*, called *normalized conditions*, are main ingredients in solving an eigenvalue problem. In this connection, we are now interested in finding eigenvectors under normalized conditions more concrete than property ($\mathcal{P}$).

The purpose of this paper is to establish the existence of solutions for the above eigenvalue problems under normalized conditions, motivated by the works of Kartsatos and Skrypnik [3, 4]. We first study implicit eigenvalue problem (IE), where *C* is assumed to be a bounded demicontinuous operator satisfying condition (${S}_{+}$). For this, a key tool is the Browder degree given in [6]. Next, we consider two types of the operator *C* for eigenvalue problem (E). For the one, we apply the Browder degree theory for nonlinear operators of the form $T+f$ with *T* maximal monotone and *f* bounded with condition (${S}_{+}$) introduced in [7]. In the other case, where *C* is a generalized pseudomonotone, quasibounded, and densely defined operator, we solve this problem by using the Kartsatos-Skrypnik degree theory for densely defined (${\tilde{S}}_{+}$)-perturbations of maximal monotone operators developed in [8].

This paper is organized as follows. In Section 2, we study the solvability of implicit eigenvalue problem (IE) with normalized conditions based on the Browder degree theory. Section 3 contains an eigenvalue result for problem (E) as a special case of (IE). In Section 4, we deal with eigenvalue problem (E) for densely defined perturbations of maximal monotone operators under normalized conditions.

Let *X* be a real Banach space with dual space ${X}^{\ast}$, Ω be a nonempty subset of *X*, and *Y* be another real Banach space. Let $\overline{\mathrm{\Omega}}$, intΩ, and *∂* Ω denote the closure, the interior, and the boundary of Ω in *X*, respectively. The symbol → (⇀) stands for strong (weak) convergence. An operator $F:\mathrm{\Omega}\to Y$ is said to be *bounded* if *F* maps bounded subsets of Ω into bounded subsets of *Y*. *F* is said to be *demicontinuous* if for every ${x}_{0}\in \mathrm{\Omega}$ and for every sequence $\{{x}_{n}\}$ in Ω with ${x}_{n}\to {x}_{0}$, we have $F{x}_{n}\rightharpoonup F{x}_{0}$.

*T*is said to be

*monotone*if

*T*is said to be

*maximal monotone*if it is monotone and it follows from $(x,{x}^{\ast})\in X\times {X}^{\ast}$ and

that $x\in D(T)$ and $Tx={x}^{\ast}$.

*T*is said to be

*generalized pseudomonotone*if for every sequence $\{{x}_{n}\}$ in $D(T)$ with ${x}_{n}\rightharpoonup {x}_{0}$, $T{x}_{n}\rightharpoonup {h}^{\ast}$ and

we have ${x}_{0}\in D(T)$, $T{x}_{0}={h}^{\ast}$, and ${lim}_{n\to \mathrm{\infty}}\u3008T{x}_{n},{x}_{n}\u3009=\u3008T{x}_{0},{x}_{0}\u3009$.

*T*satisfies condition (${\tilde{S}}_{+}$) if for every sequence $\{{x}_{n}\}$ in $D(T)$ with ${x}_{n}\rightharpoonup {x}_{0}$, $T{x}_{n}\rightharpoonup {h}^{\ast}$ and

we have ${x}_{n}\to {x}_{0}$, ${x}_{0}\in D(T)$, and $T{x}_{0}={h}^{\ast}$.

*T*satisfies condition (${S}_{+}$) if for every sequence $\{{x}_{n}\}$ in $D(T)$ with ${x}_{n}\rightharpoonup {x}_{0}$ and

we have ${x}_{n}\to {x}_{0}$.

*T*satisfies condition (${S}_{0}$) on a set $M\subset D(T)$ if for every sequence $\{{x}_{n}\}$ in

*M*with ${x}_{n}\rightharpoonup {x}_{0}$ and $T{x}_{n}\rightharpoonup {h}^{\ast}$ and

we have ${x}_{n}\to {x}_{0}$.

We say that *T* satisfies condition (${S}_{q}$) on a set $M\subset D(T)$ if for every sequence $\{{x}_{n}\}$ in *M* with ${x}_{n}\rightharpoonup {x}_{0}$ and $T{x}_{n}\to {h}^{\ast}$, we have ${x}_{n}\to {x}_{0}$.

We say that *T* satisfies condition (${T}_{\mathrm{\infty}}^{(0)}$) on a set $M\subset D(T)$ if for every sequence $\{{x}_{n}\}$ in *M* with $\parallel T{x}_{n}\parallel \to \mathrm{\infty}$, we have ${\parallel T{x}_{n}\parallel}^{-1}T{x}_{n}\rightharpoonup 0$.

It is obvious from the definitions that (${S}_{+}$) implies (${S}_{0}$) and (${S}_{0}$) implies (${S}_{q}$). Note that if *T* satisfies condition (${\tilde{S}}_{+}$) and *X* is reflexive, then *T* is generalized pseudomonotone. For the above definitions, we refer to, *e.g.*, [3, 4, 8], and [5].

Let $C:[0,\mathrm{\infty})\times M\to {X}^{\ast}$ be an operator, where *M* is a subset of *X*. Then $C(t,x)$ is said to be *continuous in* *t* *uniformly* with respect to $x\in M$ if for every ${t}_{0}\in [0,\mathrm{\infty})$ and for every sequence $\{{t}_{n}\}$ in $[0,\mathrm{\infty})$ with ${t}_{n}\to {t}_{0}$, we have $C({t}_{n},x)\to C({t}_{0},x)$ uniformly with respect to $x\in M$.

*C*satisfies condition (${S}_{+}$) if for every $\lambda \in (0,\mathrm{\infty})$ and for every sequence $\{{x}_{n}\}$ in

*M*with ${x}_{n}\rightharpoonup {x}_{0}$ and

we have ${x}_{n}\to {x}_{0}$.

Throughout this paper, *X* will always be an infinite dimensional real reflexive Banach space which has been renormed so that *X* and ${X}^{\ast}$ are locally uniformly convex.

*duality operator*if

where $\psi :[0,\mathrm{\infty})\to [0,\mathrm{\infty})$ is continuous, strictly increasing, $\psi (0)=0$, and $\psi (t)\to \mathrm{\infty}$ as $t\to \mathrm{\infty}$ called a *gauge function*. If *ψ* is the identity map *I*, then $J:={J}_{I}$ is called a *normalized duality operator*. It is known in [14] that ${J}_{\psi}$ is continuous, bounded, surjective, strictly monotone, maximal monotone and satisfies condition (${S}_{+}$).

The following demiclosedness property of maximal monotone operators will be frequently used; see [5].

**Lemma 1.1** *Let* $T:D(T)\subset X\to {X}^{\ast}$ *be a maximal monotone operator*. *Then for every sequence* $\{{x}_{n}\}$ *in* $D(T)$, ${x}_{n}\to x$ *in* *X* *and* $T{x}_{n}\rightharpoonup {x}^{\ast}$ *in* ${X}^{\ast}$ *imply that* $x\in D(T)$ *and* $Tx={x}^{\ast}$.

## 2 Implicit eigenvalue problem

In this section, we establish the existence of a solution for a nonlinear implicit eigenvalue problem under normalized conditions by using the Browder degree theory for class (${S}_{+}$).

Recall that a mapping $H:[0,1]\times \overline{\mathrm{\Omega}}\to {X}^{\ast}$ is of class (${S}_{+}$) if the following condition holds:

we have ${u}_{j}\to {u}_{0}$ and $H({t}_{j},{u}_{j})\rightharpoonup H({t}_{0},{u}_{0})$; see [2, 6].

As mentioned in the introduction, Kartsatos and Skrypnik [4] gave the following result provided that property ($\mathcal{P}$) is fulfilled. In a more concrete situation, we adopt the normalization method considered in [3].

**Theorem 2.1** *Let* Ω *be a bounded open set in* *X* *with* $0\in \mathrm{\Omega}$. *Let* $T:D(T)\subset X\to {X}^{\ast}$ *be a maximal monotone operator such that the closure of* Ω *is included in the interior of* $D(T)$ *with* $T(0)=0$ *and* *T* *satisfies condition* (${T}_{\mathrm{\infty}}^{(0)}$) *on* $\overline{\mathrm{\Omega}}$. *Assume that* $C:[0,\mathrm{\infty})\times \overline{\mathrm{\Omega}}\to {X}^{\ast}$ *is demicontinuous*, *bounded and satisfies condition* (${S}_{+}$) *such that* $C(0,x)=0$ *for all* $x\in \overline{\mathrm{\Omega}}$ *and* $C(t,x)$ *is continuous in* *t* *uniformly with respect to* $x\in \overline{\mathrm{\Omega}}$. *Further assume that*

*There exists a positive number*$\mathcal{N}$

*such that the weak sequential closure of the set*

*does not contain the origin*0,

*where*

(c2) ${lim}_{\lambda \to \mathrm{\infty}}m(\lambda )=\mathrm{\infty}$, *where* $m(\lambda )=inf\{\parallel C(\lambda ,x)\parallel :x\in \overline{\mathrm{\Omega}}\}$.

*Then the following statements hold*:

- (a)
*For each*$\epsilon >0$,*there exists a point*$({\lambda}_{\epsilon},{x}_{\epsilon})$*in*$(0,\mathrm{\infty})\times \partial \mathrm{\Omega}$*such that*$T{x}_{\epsilon}+C({\lambda}_{\epsilon},{x}_{\epsilon})+\epsilon J{x}_{\epsilon}=0.$ - (b)
*If*$0\notin T(\partial \mathrm{\Omega})$*and**T**satisfies condition*(${S}_{q}$)*on**∂*Ω,*then the implicit eigenvalue problem*$Tx+C(\lambda ,x)=0$

*has a solution* $({\lambda}_{0},{x}_{0})$ *in* $(0,\mathrm{\infty})\times \partial \mathrm{\Omega}$.

*Proof*(a) For our aim, we use the Browder degree ${d}_{B}$ given in [6]. Let

*ε*be any positive number. We first prove that there is a number $\mathrm{\Lambda}\in (0,\mathrm{\infty})$ such that

Assume the contrary. For a sequence $\{{\mathrm{\Lambda}}_{n}\}$ in $(0,\mathrm{\infty})$ with ${\mathrm{\Lambda}}_{n}\to \mathrm{\infty}$, the following occurs:

*n*. Since the sequence $\{{\parallel C({\mathrm{\Lambda}}_{n},{x}_{n})\parallel}^{-1}C({\mathrm{\Lambda}}_{n},{x}_{n})\}$ is bounded in the reflexive Banach space ${X}^{\ast}$, we may suppose that ${\parallel C({\mathrm{\Lambda}}_{n},{x}_{n})\parallel}^{-1}C({\mathrm{\Lambda}}_{n},{x}_{n})$ converges weakly to some ${h}_{0}\in {X}^{\ast}$. Using (c1), it is clear that ${h}_{0}\ne 0$. It follows from (2.2) that

However, $\parallel C({\mathrm{\Lambda}}_{n},{x}_{n})\parallel \to \mathrm{\infty}$ by (c2) implies $\parallel T{x}_{n}\parallel \to \mathrm{\infty}$, which is a contradiction to condition (${T}_{\mathrm{\infty}}^{(0)}$). Hence assertion (2.1) holds.

*H*is of class (${S}_{+}$). To prove this, let $\{{u}_{j}\}$ be any sequence in $\overline{\mathrm{\Omega}}$ with ${u}_{j}\rightharpoonup {u}_{0}$ and $\{{t}_{j}\}$ be any sequence in $[0,1]$ with ${t}_{j}\to {t}_{0}$ such that

*T*and

*J*are monotone, it follows from

*J*satisfies condition (${S}_{+}$), we obtain

*T*is demicontinuous on $\overline{\mathrm{\Omega}}$. This means that $H({t}_{j},{u}_{j})\rightharpoonup H(0,{u}_{0})$. If ${t}_{0}>0$, we have

Since *C* satisfies condition (${S}_{+}$), we get ${u}_{j}\to {u}_{0}$ from which $T{u}_{j}\rightharpoonup T{u}_{0}$, $C({t}_{j}\mathrm{\Lambda},{u}_{j})\rightharpoonup C({t}_{0}\mathrm{\Lambda},{u}_{0})$, and $J{u}_{j}\to J{u}_{0}$. Consequently, $H({t}_{j},{u}_{j})\rightharpoonup H({t}_{0},{u}_{0})$. We have just shown that the mapping *H* is of class (${S}_{+}$).

- (b)Let $\{{\epsilon}_{n}\}$ be a sequence in $(0,\mathrm{\infty})$ such that ${\epsilon}_{n}\to 0$. According to statement (a), there exists a sequence $\{({\lambda}_{{\epsilon}_{n}},{x}_{{\epsilon}_{n}})\}$ in $(0,\mathrm{\infty})\times \partial \mathrm{\Omega}$ such that$T{x}_{{\epsilon}_{n}}+C({\lambda}_{{\epsilon}_{n}},{x}_{{\epsilon}_{n}})+{\epsilon}_{n}J{x}_{{\epsilon}_{n}}=0.$

where ${\lambda}_{0}\in [0,\mathrm{\infty}]$, ${x}_{0}\in X$, and ${c}^{\ast}\in {X}^{\ast}$. Note that ${\lambda}_{0}$ belongs to $(0,\mathrm{\infty})$. In fact, if ${\lambda}_{0}=0$, then $C({\lambda}_{n},{x}_{n})\to 0$ implies $T{x}_{n}\to 0$. Since *T* satisfies condition (${S}_{q}$) on *∂* Ω, we obtain that ${x}_{n}\to {x}_{0}\in \partial \mathrm{\Omega}$ and therefore $T{x}_{0}=0$, which contradicts the hypothesis that $0\notin T(\partial \mathrm{\Omega})$. If ${\lambda}_{0}=\mathrm{\infty}$, then (c2) implies $\parallel C({\lambda}_{n},{x}_{n})\parallel \to \mathrm{\infty}$ and so $\parallel T{x}_{n}\parallel \to \mathrm{\infty}$. As in (2.3), a similar argument proves that ${\parallel T{x}_{n}\parallel}^{-1}T{x}_{n}$ converges weakly to some nonzero vector, which contradicts condition (${T}_{\mathrm{\infty}}^{(0)}$). Thus we have shown that ${\lambda}_{0}\in (0,\mathrm{\infty})$.

*T*

By the maximal monotonicity of *T*, we have ${x}_{0}\in D(T)$ and $T{x}_{0}=-{c}^{\ast}$. Letting $x={x}_{0}$ in (2.13), we get a contradiction. Therefore, (2.11) is true.

Since *C* satisfies condition (${S}_{+}$) and ${\lambda}_{0}\in (0,\mathrm{\infty})$, we have ${x}_{n}\to {x}_{0}\in \partial \mathrm{\Omega}$, which implies $C({\lambda}_{n},{x}_{n})\rightharpoonup C({\lambda}_{0},{x}_{0})$. Hence we obtain from (2.9) that $T{x}_{n}\rightharpoonup -C({\lambda}_{0},{x}_{0})$. By Lemma 1.1, we conclude that $T{x}_{0}+C({\lambda}_{0},{x}_{0})=0$. This completes the proof. □

**Remark 2.2** In the proof of Theorem 2.1, the demicontinuity of *T* on $\overline{\mathrm{\Omega}}$ is needed to show that *H* is of class (${S}_{+}$). This is guaranteed under an additional condition $\overline{\mathrm{\Omega}}\subset intD(T)$. Actually, local boundedness of *T* on $intD(T)$ implies the demicontinuity of *T*; see, *e.g.*, [5].

## 3 Eigenvalue problem with condition (${S}_{+}$)

In this section, we study a multiplicative eigenvalue problem as a special case of the implicit eigenvalue problem in the previous section. As a key tool, we employ the Browder degree for nonlinear operators of the form $T+f$ with *T* maximal monotone and *f* bounded with condition (${S}_{+}$).

We give a variant of Corollary 1 in [4] under normalized conditions.

**Theorem 3.1** *Let* Ω *be a bounded open set in* *X* *with* $0\in \mathrm{\Omega}$. *Let* $T:D(T)\subset X\to {X}^{\ast}$ *be a maximal monotone operator with* $0\in D(T)$ *and* $T(0)=0$ *such that* *T* *satisfies condition* (${T}_{\mathrm{\infty}}^{(0)}$) *on* $D(T)\cap \overline{\mathrm{\Omega}}$. *Assume that* $C:\overline{\mathrm{\Omega}}\to {X}^{\ast}$ *is a demicontinuous bounded operator which satisfies condition* (${S}_{+}$) *and the two additional conditions*:

*There is a positive number*$\mathcal{N}$

*such that the weak sequential closure of the set*

*does not contain zero vector*,

*where*

(c2) $inf\{\parallel Cx\parallel :x\in \overline{\mathrm{\Omega}}\}$ *is not equal to* 0.

*Then we have the following properties*:

- (a)
*For each*$\epsilon >0$,*there exists*$({\lambda}_{\epsilon},{x}_{\epsilon})\in (0,\mathrm{\infty})\times (D(T)\cap \partial \mathrm{\Omega})$*such that*$T{x}_{\epsilon}+{\lambda}_{\epsilon}C{x}_{\epsilon}+\epsilon J{x}_{\epsilon}=0.$ - (b)
*If*$0\notin T(D(T)\cap \partial \mathrm{\Omega})$*and**T**satisfies condition*(${S}_{q}$)*on*$D(T)\cap \partial \mathrm{\Omega}$,*then the eigenvalue problem*$Tx+\lambda Cx=0$

*has a solution* $({\lambda}_{0},{x}_{0})$ *in* $(0,\mathrm{\infty})\times (D(T)\cap \partial \mathrm{\Omega})$.

*Proof*(a) Fix $\epsilon >0$. Let ${d}_{B}$ denote the Browder degree in the sense of [7]. We first claim that there exists a number Λ in $(0,\mathrm{\infty})$ such that

*n*. By the boundedness of the sequence $\{{\parallel C{x}_{n}\parallel}^{-1}C{x}_{n}\}$ in ${X}^{\ast}$, we may suppose that ${\parallel C{x}_{n}\parallel}^{-1}C{x}_{n}\rightharpoonup {h}_{0}$ for some ${h}_{0}\in {X}^{\ast}$. It follows from (c1) and (3.2) that ${h}_{0}\ne 0$ and

But (c2) implies $\parallel T{x}_{n}\parallel \to \mathrm{\infty}$, which contradicts condition (${T}_{\mathrm{\infty}}^{(0)}$). Hence assertion (3.1) holds.

*e.g.*, Theorem 12 in [7], we have

- (b)Let $\{{\epsilon}_{n}\}$ be a sequence in $(0,\mathrm{\infty})$ such that ${\epsilon}_{n}\to 0$. By (a), there exists a sequence $\{({\lambda}_{n},{x}_{n})\}$ in $(0,\mathrm{\infty})\times (D(T)\cap \partial \mathrm{\Omega})$ such that$T{x}_{n}+{\lambda}_{n}C{x}_{n}+{\epsilon}_{n}J{x}_{n}=0.$(3.3)

where ${\lambda}_{0}\in [0,\mathrm{\infty}]$, ${x}_{0}\in X$, and ${c}^{\ast}\in {X}^{\ast}$. Note that ${\lambda}_{0}$ belongs to $(0,\mathrm{\infty})$. Indeed, if ${\lambda}_{0}=0$, then it follows from $T{x}_{n}\to 0$ and condition (${S}_{q}$) that ${x}_{n}\to {x}_{0}\in \partial \mathrm{\Omega}$ and therefore ${x}_{0}\in D(T)$ and $T{x}_{0}=0$, which contradicts the hypothesis that $0\notin T(D(T)\cap \partial \mathrm{\Omega})$. If ${\lambda}_{0}=\mathrm{\infty}$, then (c2) implies $\parallel {\lambda}_{n}C{x}_{n}\parallel \to \mathrm{\infty}$ and so $\parallel T{x}_{n}\parallel \to \mathrm{\infty}$. But we can show as above that ${\parallel T{x}_{n}\parallel}^{-1}T{x}_{n}$ converges weakly to some nonzero vector, which contradicts condition (${T}_{\mathrm{\infty}}^{(0)}$).

*T*

By the maximal monotonicity of *T*, we get a contradiction. Therefore, (3.5) holds.

Since *C* satisfies condition (${S}_{+}$) and is demicontinuous, we have ${x}_{n}\to {x}_{0}\in \partial \mathrm{\Omega}$, which implies $C{x}_{n}\rightharpoonup C{x}_{0}$. Hence we obtain from (3.3) that $T{x}_{n}\rightharpoonup -{\lambda}_{0}C{x}_{0}$. By Lemma 1.1, we conclude that ${x}_{0}\in D(T)\cap \partial \mathrm{\Omega}$ and $T{x}_{0}+{\lambda}_{0}C{x}_{0}=0$, what we wanted to prove. □

**Remark 3.2** We point out that in Theorem 3.1, the condition $\overline{\mathrm{\Omega}}\subset intD(T)$ is not necessary to be assumed.

## 4 Densely defined perturbations

This section is devoted to the eigenvalue problem for densely defined quasibounded perturbations of maximal monotone operators in reflexive Banach spaces. To do this, we apply the Kartsatos-Skrypnik degree for densely defined (${\tilde{S}}_{+}$)-perturbations of maximal monotone operators developed in [8].

Recall that an operator $C:D(C)\subset X\to {X}^{\ast}$ is *quasibounded* if for every $S>0$ there exists a constant $K(S)>0$ such that for all $u\in D(C)$ with $\parallel u\parallel \le S$ and $\u3008Cu,u\u3009\le 0$, we have $\parallel Cu\parallel \le K(S)$.

As in Section 3, we employ a normalization method to obtain an eigenvalue result for generalized pseudomonotone operators.

**Theorem 4.1** *Let* Ω *be a bounded open set in* *X* *with* $0\in \mathrm{\Omega}$ *and* *L* *be a dense subspace of X*. *Let* $T:D(T)\subset X\to {X}^{\ast}$ *be a maximal monotone operator with* $0\in D(T)$ *and* $T(0)=0$ *which satisfies condition* (${T}_{\mathrm{\infty}}^{(0)}$) *on* $D(T)\cap \overline{\mathrm{\Omega}}$. *Assume that* $C:D(C)\subset X\to {X}^{\ast}$ *is a generalized pseudomonotone quasibounded operator with* $L\subset D(C)$. *Furthermore*, *assume that*

*There exists a positive number*$\mathcal{N}$

*such that the weak sequential closure of the set*

*does not contain zero vector*,

*where*

(h2) $inf\{\parallel Cx\parallel :x\in D(C)\cap \overline{\mathrm{\Omega}}\}$ *is not equal to* 0.

(h3) *For every* $F\in \mathcal{F}(L)$ *and* $v\in L$, *the function* $c(F,v):F\to \mathbb{R}$, $c(F,v)(u)=\u3008Cu,v\u3009$, *is continuous on* *F*, *where* $\mathcal{F}(L)$ *denotes the set of all finite*-*dimensional subspaces of* *L*.

*Then the following statements hold*:

- (a)
*For each*$\epsilon >0$,*there exists a point*$({\lambda}_{\epsilon},{x}_{\epsilon})$*in*$(0,\mathrm{\infty})\times (D(T+C)\cap \partial \mathrm{\Omega})$*such that*$T{x}_{\epsilon}+{\lambda}_{\epsilon}C{x}_{\epsilon}+\epsilon {J}_{\psi}{x}_{\epsilon}=0.$

*Here*$D(T+C)$

*denotes the intersection of*$D(T)$

*and*$D(C)$.

- (b)
*If*$0\notin T(D(T)\cap \partial \mathrm{\Omega})$*and**T**satisfies condition*(${S}_{0}$)*on*$D(T)\cap \partial \mathrm{\Omega}$,*then the eigenvalue problem*$Tx+\lambda Cx=0$

*has a solution* $({\lambda}_{0},{x}_{0})$ *in* $(0,\mathrm{\infty})\times (D(T+C)\cap \partial \mathrm{\Omega})$.

*Proof*(a) We will apply the Kartsatos-Skrypnik degree ${d}_{S}$ given in [8]. Let

*ε*be an arbitrary positive number. Since

*T*satisfies condition (${T}_{\mathrm{\infty}}^{(0)}$) on $D(T)\cap \overline{\mathrm{\Omega}}$ and ${J}_{\psi}$ is bounded, we can prove as in Theorem 3.1 that under hypotheses (h1) and (h2), there is a positive number Λ such that

For $t\in [0,1]$, we set ${T}^{t}:=T$ and ${C}^{t}:=t\mathrm{\Lambda}C+\epsilon {J}_{\psi}$, where $D({T}^{t})$ and $D({C}^{t})$ denote the domain of ${T}^{t}$ and ${C}^{t}$, respectively. In this case, $D({T}^{t})=D(T)$ for $t\in [0,1]$, $D({C}^{0})=X$ for $t=0$ and $D({C}^{t})=D(C)$ for $t\in (0,1]$. Notice that the operators ${C}^{0}=\epsilon {J}_{\psi}$ and ${C}^{1}=\mathrm{\Lambda}C+\epsilon {J}_{\psi}$ satisfy condition (${\tilde{S}}_{+}$), based on the facts that *C* is generalized pseudomonotone and ${J}_{\psi}$ is bounded and satisfies condition (${S}_{+}$).

In the sense of Definition 4.2 in [8], we check the following conditions on two families $\{{T}^{t}\}$ and $\{{C}^{t}\}$. In fact, conditions on $\{{T}^{t}\}$ are obviously satisfied, with ${T}^{t}$ independent of *t*, due to maximal monotonicity of *T*, $0\in D(T)$, and $T(0)=0$.

(${c}_{1}^{t}$) Since ${J}_{\psi}$ is monotone and bounded, it follows from the quasiboundedness of *C* that $\{{C}^{t}\}$ is uniformly quasibounded.

*L*such that ${t}_{n}\to {t}_{0}$, ${u}_{n}\rightharpoonup {u}_{0}$, ${C}^{{t}_{n}}{u}_{n}\rightharpoonup {h}^{\ast}$ and

*C*is generalized pseudomonotone, we obtain from (4.3) that ${u}_{0}\in D(C)$, $C{u}_{0}={h}_{1}^{\ast}$, and $\u3008C{u}_{n},{u}_{n}\u3009\to \u3008C{u}_{0},{u}_{0}\u3009$. Thus,

Since ${J}_{\psi}$ satisfies condition (${S}_{+}$), we have ${u}_{n}\to {u}_{0}$ and so ${J}_{\psi}{u}_{n}\to {J}_{\psi}{u}_{0}$. Consequently, ${u}_{0}\in D({C}^{{t}_{0}})$ and ${C}^{{t}_{0}}{u}_{0}={t}_{0}\mathrm{\Lambda}{h}_{1}^{\ast}+\epsilon {h}_{2}^{\ast}={h}^{\ast}$. Therefore, condition (${c}_{2}^{t}$) is satisfied.

(${c}_{3}^{t}$) For every $F\in \mathcal{F}(L)$ and $v\in L$, the function $\tilde{c}(F,v):[0,1]\times F\to \mathbb{R}$, $\tilde{c}(F,v)(t,u)=\u3008{C}^{t}u,v\u3009$, is continuous on $[0,1]\times F$ because $c(F,v)$ is continuous on *F* and ${J}_{\psi}$ is continuous on *X*.

- (b)Let $\{{\epsilon}_{n}\}$ be a sequence in $(0,\mathrm{\infty})$ such that ${\epsilon}_{n}\to 0$. According to (a), there are sequences $\{{\lambda}_{n}\}$ in $(0,\mathrm{\infty})$ and $\{{x}_{n}\}$ in $D(T+C)\cap \partial \mathrm{\Omega}$ such that$T{x}_{n}+{\lambda}_{n}C{x}_{n}+{\epsilon}_{n}{J}_{\psi}{x}_{n}=0.$(4.4)

*T*with $T(0)=0$ that

*C*implies that $\{C{x}_{n}\}$ is bounded. Without loss of generality, we may suppose that

where ${\lambda}_{0}\in [0,\mathrm{\infty}]$, ${x}_{0}\in X$, and ${c}^{\ast}\in {X}^{\ast}$. Since T satisfies conditions (${S}_{q}$) and (${T}_{\mathrm{\infty}}^{(0)}$) and since $0\notin T(D(T)\cap \partial \mathrm{\Omega})$, it is easily verified that ${\lambda}_{0}$ belongs to $(0,\mathrm{\infty})$.

*C*is generalized pseudomonotone, we have by (4.5)

Since *T* satisfies condition (${S}_{0}$) on $D(T)\cap \partial \mathrm{\Omega}$, we have ${x}_{n}\to {x}_{0}$. By the maximal monotonicity of *T*, we conclude that ${x}_{0}\in D(T+C)\cap \partial \mathrm{\Omega}$ and $T{x}_{0}+{\lambda}_{0}C{x}_{0}=0$. This completes the proof. □

As a consequence of Theorem 4.1, we get another eigenvalue result for densely defined operators satisfying condition (${\tilde{S}}_{+}$) in comparison with Theorem 4 in [4].

**Corollary 4.2**

*Let*

*T*, Ω,

*and*

*L*

*be as in Theorem*4.1.

*Assume that*$C:D(C)\subset X\to {X}^{\ast}$

*is a quasibounded operator with*$L\subset D(C)$

*and satisfies condition*(${\tilde{S}}_{+}$).

*Furthermore*,

*assume that conditions*(h1), (h2),

*and*(h3)

*in Theorem*4.1

*are satisfied*.

*Then*:

- (a)
*For each*$\epsilon >0$,*there exists*$({\lambda}_{\epsilon},{x}_{\epsilon})\in (0,\mathrm{\infty})\times (D(T+C)\cap \partial \mathrm{\Omega})$*such that*$T{x}_{\epsilon}+{\lambda}_{\epsilon}C{x}_{\epsilon}+\epsilon {J}_{\psi}{x}_{\epsilon}=0$. - (b)
*If*$0\notin T(D(T)\cap \partial \mathrm{\Omega})$*and**T**satisfies condition*(${S}_{q}$)*on*$D(T)\cap \partial \mathrm{\Omega}$,*then there exists*$({\lambda}_{0},{x}_{0})\in (0,\mathrm{\infty})\times (D(T+C)\cap \partial \mathrm{\Omega})$*such that*$T{x}_{0}+{\lambda}_{0}C{x}_{0}=0$.

*Proof*Statement (a) follows from part (a) of Theorem 4.1 by noting that if

*C*satisfies condition (${\tilde{S}}_{+}$), then

*C*is generalized pseudomonotone.

- (b)Let $\{{\epsilon}_{n}\}$ be a sequence in $(0,\mathrm{\infty})$ such that ${\epsilon}_{n}\to 0$. In view of (a), we can choose a sequence $\{({\lambda}_{n},{x}_{n})\}$ in $(0,\mathrm{\infty})\times (D(T+C)\cap \partial \mathrm{\Omega})$ such that$T{x}_{n}+{\lambda}_{n}C{x}_{n}+{\epsilon}_{n}{J}_{\psi}{x}_{n}=0.$

*C*satisfies condition (${\tilde{S}}_{+}$), we have

Combining this with $T{x}_{n}\rightharpoonup -{\lambda}_{0}C{x}_{0}$, we obtain that ${x}_{0}\in D(T+C)\cap \partial \mathrm{\Omega}$ and $T{x}_{0}+{\lambda}_{0}C{x}_{0}=0$ as required. □

**Remark 4.3** When *C* satisfies condition (${\tilde{S}}_{+}$), condition (${S}_{0}$) on *T* appearing in Theorem 4.1 may be replaced by weaker condition (${S}_{q}$); see the proof of Theorem 4.1.

## Declarations

### Acknowledgements

The authors would like to thank anonymous referees for valuable comments and suggestions. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2011-0021-829).

## Authors’ Affiliations

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