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# Fixed point theorems for a class of mixed monotone operators with convexity

*Fixed Point Theory and Applications*
**volume 2013**, Article number: 119 (2013)

## Abstract

In this paper, we use partial order theory to study a class of mixed monotone operators with convexity, and we get the existence and uniqueness of fixed points without assuming the operator to be compact or continuous. Our results compliment the theory of mixed monotone operators in ordered Banach spaces.

**MSC:**47H10, 47H07.

## 1 Introduction and preliminaries

It is well known that mixed monotone operators were introduced by Guo and Lakshmikantham [1] in 1987. Thereafter many authors have investigated these kinds of operators in Banach spaces and obtained a lot of interesting and important results (see [1–18] and the references therein). Their study has not only important theoretical meaning but also wide applications in engineering, nuclear physics, biological chemistry technology, *etc.* That is, they are used extensively in nonlinear differential and integral equations. For a small sample of such work, we refer the reader to works [13, 15–21]. However, the mixed monotone operators considered in most of these papers are concave; see [2–4, 7–11, 15, 17, 18], for instance. To our knowledge, the fixed point results on mixed monotone operators with convexity are still very few. So it is worthwhile to investigate this operator. The purpose of this paper is to establish the existence and uniqueness of fixed points for a class of mixed monotone operators with convexity. We will use partial order theory to study the mixed monotone operator with convexity and get the existence and uniqueness of fixed points without assuming the operator to be compact or continuous. Our results compliment the theory of mixed monotone operators in ordered Banach spaces.

For the discussion of the following section, we state here some definitions and notations. For convenience of readers, we suggest that one refers to [1, 2, 17, 21–23] for details.

Suppose that (E,\parallel \cdot \parallel ) is a real Banach space which is partially ordered by a cone P\subset E, *i.e.*, x\le y if and only if y-x\in P. If x\le y and x\ne y, then we denote x<y or y>x. By *θ* we denote the zero element of *E*. Recall that a non-empty closed convex set P\subset E is a cone if it satisfies (i) x\in P, \lambda \ge 0\Rightarrow \lambda x\in P; (ii) x\in P, -x\in P\Rightarrow x=\theta.

Further, *P* is called normal if there exists a constant N>0 such that, for all x,y\in E, \theta \le x\le y implies \parallel x\parallel \le N\parallel y\parallel; in this case *N* is called the normality constant of *P*. If {x}_{1},{x}_{2}\in E, the set [{x}_{1},{x}_{2}]=\{x\in E|{x}_{1}\le x\le {x}_{2}\} is called the order interval between {x}_{1} and {x}_{2}. We say that an operator A:E\to E is increasing if x\le y implies Ax\le Ay.

A:P\times P\to P is said to be a mixed monotone operator if A(x,y) is increasing in *x* and decreasing in *y*, *i.e.*, {u}_{i}, {v}_{i} (i=1,2)\in P, {u}_{1}\le {u}_{2}, {v}_{1}\ge {v}_{2} imply A({u}_{1},{v}_{1})\le A({u}_{2},{v}_{2}). The element x\in P is called a fixed point of *A* if A(x,x)=x.

## 2 Main results

In this section we consider the existence and uniqueness of fixed points for a class of mixed monotone operators in ordered Banach spaces.

**Theorem 2.1** *Let* *E* *be a real Banach space and let* *P* *be a normal cone in* *E*. A:P\times P\to P *is a mixed monotone operator which satisfies the following*:

(H_{1}) *for* t\in (0,1), x,y\in P, *there exists* \alpha (t,x,y)\in (1,+\mathrm{\infty}) *such that*

(H_{2}) *there exist* {u}_{0},{v}_{0}\in P, r\in (0,1) *such that*

*Then* *A* *has a unique fixed point* {u}^{\ast} *in* [{u}_{0},r{v}_{0}]. *Moreover*, *constructing successively the sequences*

*for any initial values* {x}_{0},{y}_{0}\in [{u}_{0},r{v}_{0}], *we have* \parallel {x}_{n}-{u}^{\ast}\parallel \to 0, \parallel {y}_{n}-{u}^{\ast}\parallel \to 0 *as* n\to \mathrm{\infty}.

**Remark 2.1** (i) The operator *A* which satisfies (H_{1}) can be called a mixed monotone operator with convexity; (ii) from (H_{1}), we can get A(\theta ,\theta )=\theta. In fact, for {t}_{0}\in (0,1), A(\theta ,\theta )=A({t}_{0}\theta ,\theta )\le {t}_{0}^{\alpha ({t}_{0},\theta ,\theta )}A(\theta ,\theta ), and thus [1-{t}_{0}^{\alpha ({t}_{0},\theta ,\theta )}]A(\theta ,\theta )\le \theta. It follows from A(\theta ,\theta )\ge \theta that A(\theta ,\theta )=\theta; (iii) from (2.1), we can get

*Proof of Theorem 2.1* Let {w}_{0}=r{v}_{0}, \epsilon ={r}^{\alpha (r,{v}_{0},{u}_{0})-1}. Then {w}_{0}\ge {u}_{0}, \epsilon \in (0,1), and

Construct successively the sequences

From (2.4), (2.5) and the mixed monotonicity of *A*, we have

Next we prove that

From (2.4),

Suppose that when n=k, we have {u}_{0}\le {w}_{k}^{\mathrm{\prime}}\le {w}_{0}. Then, when n=k+1, we obtain

By the induction method, we know that (2.7) holds. On the other hand, from (2.1),

Suppose that when n=k, we have {w}_{k}\le {\epsilon}^{k}{w}_{k}^{\mathrm{\prime}}. Then, when n=k+1, we obtain

By the induction method, we have

By (2.6)-(2.8), we get

Since *P* is normal, we have

Further,

Here *N* is the normality constant.

So, we can claim that \{{u}_{n}\} and \{{w}_{n}\} are Cauchy sequences. Because *E* is complete, there exist {u}^{\ast},{w}^{\ast}\in P such that

By (2.6), we know that {u}_{0}\le {u}_{n}\le {u}^{\ast}\le {w}^{\ast}\le {w}_{n}\le {w}_{0}, and then

Further, \parallel {w}^{\ast}-{u}^{\ast}\parallel \le N{\epsilon}^{n}\parallel {w}_{0}-{u}_{0}\parallel \to 0 (as n\to \mathrm{\infty}), and thus {w}^{\ast}={u}^{\ast}. Then we obtain

Let n\to \mathrm{\infty}, then we get A({u}^{\ast},{u}^{\ast})={u}^{\ast}. That is, {u}^{\ast} is a fixed point of *A* in [{u}_{0},{w}_{0}]=[{u}_{0},r{v}_{0}].

In the following, we prove that {u}^{\ast} is the unique fixed point of *A* in [{u}_{0},{w}_{0}]. Suppose that there is {x}^{\ast}\in [{u}_{0},{w}_{0}] such that A({x}^{\ast},{x}^{\ast})={x}^{\ast}. Then we have {u}_{0}\le {x}^{\ast}\le {w}_{0}. By the induction method and the mixed monotonicity of *A*, we have

Then from the normality of *P*, we have {x}^{\ast}={u}^{\ast}.

Moreover, constructing successively the sequences

for any initial values {x}_{0},{y}_{0}\in [{u}_{0},{w}_{0}], we have {u}_{n}\le {x}_{n},{y}_{n}\le {w}_{n}, n=1,2,\dots . Letting n\to \mathrm{\infty} yields {x}_{n}\to {u}^{\ast}, {y}_{n}\to {u}^{\ast} as n\to \mathrm{\infty}. □

**Remark 2.2** Let \alpha (t,x,y) be a constant \alpha \in (1,+\mathrm{\infty}), then Theorem 2.1 also holds.

**Corollary 2.2** *Let* *E* *be a real Banach space and let* *P* *be a normal cone in* *E*. A:P\times P\to P *is a mixed monotone operator which satisfies* (H_{2}) *and*, *for* t\in (0,1), x,y\in P, *there exists a constant* \alpha \in (1,+\mathrm{\infty}) *such that* A(tx,y)\le {t}^{\alpha}A(x,y). *Then* *A* *has a unique fixed point* {u}^{\ast} *in* [{u}_{0},r{v}_{0}]. *Moreover*, *constructing successively the sequences*

*for any initial values* {x}_{0},{y}_{0}\in [{u}_{0},r{v}_{0}], *we have* \parallel {x}_{n}-{u}^{\ast}\parallel \to 0, \parallel {y}_{n}-{u}^{\ast}\parallel \to 0 *as* n\to \mathrm{\infty}.

From the proof of Theorem 2.1, we can easily obtain the following conclusion.

**Corollary 2.3** *Let* *E* *be a real Banach space and let* *P* *be a normal cone in* *E*. A:P\to P *is an increasing operator which satisfies the following*:

(H_{3}) *for* t\in (0,1), x\in P, *there exists a constant* \alpha \in (1,+\mathrm{\infty}) *such that* A(tx)\le {t}^{\alpha}Ax;

(H_{4}) *there exist* {u}_{0},{v}_{0}\in P, r\in (0,1) *such that* {u}_{0}\le r{v}_{0}, A{u}_{0}\ge {u}_{0}, A{v}_{0}\le {v}_{0}.

*Then* *A* *has a unique fixed point* {u}^{\ast} *in* [{u}_{0},r{v}_{0}]. *Moreover*, *constructing successively the sequence*

*for any initial value* {x}_{0}\in [{u}_{0},r{v}_{0}], *we have* \parallel {x}_{n}-{u}^{\ast}\parallel \to 0 *as* n\to \mathrm{\infty}.

**Remark 2.3** The operator *A* which satisfies (H_{3}) is called *α*-convex. In 1977, A.J.B. Potter introduced the definition of an *α*-convex operator and showed that for \alpha \ge 0, decreasing (−*α*)-convex mappings have contraction ratios less than or equal to *α* and gave the existence of solutions to the nonlinear eigenvalue problem Ax=\lambda x. The method is based upon Hilbert’s projective metric. In [22] Guo studied the existence and uniqueness of fixed points for (−*α*)-convex operators. In [23], we obtained the existence and uniqueness of positive fixed points for *α*-convex operators by means of the properties of cone, concave operators and the monotonicity of set-valued maps. Here, the result on *α*-convex operators is new and the method is also new and different from previous ones.

**Remark 2.4** If A:P\to P is an *α*-convex operator, then *A* is not a decreasing and constant operator. In fact, suppose that *A* is decreasing, then we have Ax\le A(tx)\le {t}^{\alpha}Ax, x\in P, t\in (0,1). Hence, {t}^{\alpha}\ge 1, this is a contradiction. Suppose that *A* is a constant operator, that is, Ax={u}_{0}\in P. Then {u}_{0}=A(tx)\le {t}^{\alpha}Ax={t}^{\alpha}{u}_{0}, and thus {t}^{\alpha}\ge 1, this is also a contradiction.

**Theorem 2.4** *Let* *E* *be a real Banach space and let* *P* *be a normal cone in* *E*. A:P\times P\to P *is a mixed monotone operator which satisfies* (2.1) *and*

(H_{5}) *there exist* {u}_{0},{v}_{0}\in P, R\in (1,+\mathrm{\infty}) *such that*

*Then the operator equation* A(u,u)=bu *has a unique solution* {u}^{\ast} *in* [R{u}_{0},{v}_{0}], *where* b={R}^{\alpha (\frac{1}{R},R{u}_{0},{v}_{0})-1}. *Moreover*, *constructing successively the sequences*

*for any initial values* {x}_{0},{y}_{0}\in [R{u}_{0},{v}_{0}], *we have* \parallel {x}_{n}-{u}^{\ast}\parallel \to 0, \parallel {y}_{n}-{u}^{\ast}\parallel \to 0 *as* n\to \mathrm{\infty}.

*Proof* Let {w}_{0}=R{u}_{0}. Then {w}_{0}\le {v}_{0}. Note that b>1 and from (2.3),

Set B(x,y)={b}^{-1}A(x,y), x,y\in P. Then from the above inequalities, we have

Also, construct successively the sequences

From (2.9) and the mixed monotonicity of *A*, we have

Similar to the proof of Theorem 2.1, we can prove that

Further, by using the same method with the proof of Theorem 2.1, we can get the following conclusions: (i) *B* has a unique fixed point {u}^{\ast} in [{w}_{0},{v}_{0}]; (ii) for any initial values {x}_{0},{y}_{0}\in [{w}_{0},{v}_{0}], constructing successively the sequences

we have \parallel {x}_{n}-{u}^{\ast}\parallel \to 0, \parallel {y}_{n}-{u}^{\ast}\parallel \to 0 as n\to \mathrm{\infty}. Therefore, the operator equation A(u,u)=bu has a unique solution {u}^{\ast} in [R{u}_{0},{v}_{0}]. Moreover, constructing successively the sequences

for any initial values {x}_{0},{y}_{0}\in [R{u}_{0},{v}_{0}], we have \parallel {x}_{n}-{u}^{\ast}\parallel \to 0, \parallel {y}_{n}-{u}^{\ast}\parallel \to 0 as n\to \mathrm{\infty}. □

**Corollary 2.5** *Let* *E* *be a real Banach space and let* *P* *be a normal cone in* *E*. A:P\to P *is an increasing operator which satisfies* (H_{3}) *and*

(H_{6}) *there exist* {u}_{0},{v}_{0}\in P, R\in (1,+\mathrm{\infty}) *such that* A{u}_{0}\ge {u}_{0}, A{v}_{0}\le {v}_{0}, {v}_{0}\ge R{u}_{0}.

*Then the operator equation* Au=bu *has a unique solution* {u}^{\ast} *in* [R{u}_{0},{v}_{0}], *where* b={R}^{\alpha -1}. *Moreover*, *constructing successively the sequence*

*for any initial value* {x}_{0}\in [R{u}_{0},{v}_{0}], *we have* \parallel {x}_{n}-{u}^{\ast}\parallel \to 0 *as* n\to \mathrm{\infty}.

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## Acknowledgements

The author was supported financially by the Youth Science Foundations of China (11201272) and Shanxi Province (2010021002-1).

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Zhai, C. Fixed point theorems for a class of mixed monotone operators with convexity.
*Fixed Point Theory Appl* **2013, **119 (2013). https://doi.org/10.1186/1687-1812-2013-119

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DOI: https://doi.org/10.1186/1687-1812-2013-119

### Keywords

- fixed point
- mixed monotone operator
- normal cone
- convexity
*α*-convex operator