Open Access Research

Fixed point theorems for decreasing operators in ordered Banach spaces with lattice structure and their applications

Xingchang Li* and Zhihao Wang

Author Affiliations

Center for Economic Research, Harbin University of Commerce, Harbin, 150028, China

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Fixed Point Theory and Applications 2013, 2013:18  doi:10.1186/1687-1812-2013-18


The electronic version of this article is the complete one and can be found online at: http://www.fixedpointtheoryandapplications.com/content/2013/1/18


Received:2 December 2012
Accepted:14 January 2013
Published:30 January 2013

© 2013 Li and Wang; licensee Springer

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

This paper presents some theorems of the fixed point for decreasing operators in Banach spaces with lattice structure. The results are applied to nonlinear second-order elliptic equations.

MSC: 47H10, 34B15.

Keywords:
decreasing operators; lattice structure; nonlinear; elliptic equations

1 Introduction and preliminaries

The fixed point theory for monotone operators in ordered Banach spaces has been investigated extensively in the past 30 years [1-8]. Many new fixed point theorems have been proved under the nonlinear contractive condition by using the theorem of cone and monotone iterative technique. These results have been applied to study the ordinary differential equations, partial differential equations, and integral equations.

In this paper, we investigate decreasing operators in ordered Banach spaces with lattice structure. The theoretical results of fixed points are extended by using the famous Schauder fixed point theorem for the operators. We weaken the conditions of the Schauder fixed point theorem. The results of this paper have no need for the closed bounded and convex property of domains for the operators. To demonstrate the applicability of our results, we apply them to study a problem of nonlinear second-order elliptic equations in the final section of the paper, and the existence of solution is obtained.

Let E be a Banach space and P be a cone of E. We define a partial ordering ≤ with respect to P by <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M1">View MathML</a> if only if <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M2','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M2">View MathML</a>. A cone <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3">View MathML</a> is called normal if there is a constant <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M4','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M4">View MathML</a> such that <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M5','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M5">View MathML</a> implies <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M6','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M6">View MathML</a> for all <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M7">View MathML</a>. The least positive constant N satisfying the above inequality is called the normal constant of P.

Let E be a partially ordered set. We call E a lattice in the partial ordering ≤. For arbitrary <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M7','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M7">View MathML</a>, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M9','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M9">View MathML</a> and <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M10','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M10">View MathML</a> exist. One can see [7] for the definition and the properties of the lattice.

Let <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>, the operator <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M12">View MathML</a> is said to be an increasing operator if <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M13">View MathML</a>, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M1">View MathML</a>, implies <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M15','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M15">View MathML</a>; the operator <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M12','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M12">View MathML</a> is said to be a decreasing operator if <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M13','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M13">View MathML</a>, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M1','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M1">View MathML</a>, implies <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M19','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M19">View MathML</a>.

Lemma 1.1[9]

LetEbe a real Banach space, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>be nonempty, closed bounded convex, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M21">View MathML</a>be condensing. ThenAhas a fixed point inD.

Lemma 1.2[10]

LetEbe a real Banach space, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>be nonempty, closed bounded convex, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M21">View MathML</a>be completely continuous. ThenAhas a fixed point inD.

Lemma 1.3[11]

LetEbe a real Banach space, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>be nonempty, closed bounded convex, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M21','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M21">View MathML</a>be strict-set-contraction mappings. ThenAhas a fixed point inD.

Remark 1 Lemma 1.1 is the famous Sadovskii fixed point theorem; Lemma 1.2 is the famous Schauder fixed point theorem; Lemma 1.3 is the famous Darbo fixed point theorem.

2 Main results

Theorem 2.1LetEbe an ordered Banach space with lattice structure, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>be bounded, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M27">View MathML</a>be a decreasing and condensing operator. Then the operatorAhas a fixed point inD.

Proof For any <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M28','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M28">View MathML</a>, since <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M29','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M29">View MathML</a>, we have <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M30','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M30">View MathML</a>.

Since E is a Banach space with lattice structure and <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a> is bounded, there exists <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M32','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M32">View MathML</a> such that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M33','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M33">View MathML</a>

That is,

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M34','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M34">View MathML</a>

(2.1)

Since A is a decreasing operator, we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M35','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M35">View MathML</a>

(2.2)

(2.1) and (2.2) show that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M36','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M36">View MathML</a>

(2.3)

Similar to the proof of (2.3), there exists <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M37','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M37">View MathML</a> such that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M38','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M38">View MathML</a>

That is,

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M39','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M39">View MathML</a>

(2.4)

Since A is a decreasing operator, we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M40','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M40">View MathML</a>

(2.5)

(2.4) and (2.5) show that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M41','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M41">View MathML</a>

(2.6)

(2.3) and (2.6) together with <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M42">View MathML</a> show that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M43','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M43">View MathML</a>

(2.7)

For any <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M44">View MathML</a>, since A is a decreasing operator, we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M45','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M45">View MathML</a>

By (2.7), we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M46','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M46">View MathML</a>

It is easy to know that <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47">View MathML</a> is a closed convex set. Since <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a> is bounded, we have <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47">View MathML</a> is bounded. Hence, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47">View MathML</a> is a closed bounded convex set. Thus, Lemma 1.1 implies that the operator A has a fixed point in D. □

Theorem 2.2LetEbe an ordered Banach space with lattice structure, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3">View MathML</a>be a normal cone, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52">View MathML</a>be a decreasing and condensing operator. Then the operatorAhas a fixed point inE.

Proof For any <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M53','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M53">View MathML</a>, since <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52">View MathML</a>, we have <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M55','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M55">View MathML</a>.

Since E is a Banach space with lattice structure, there exists <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M56','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M56">View MathML</a> such that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M57','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M57">View MathML</a>

That is,

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M58','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M58">View MathML</a>

(2.8)

Since A is a decreasing operator, we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M59','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M59">View MathML</a>

(2.9)

(2.8) and (2.9) show that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M60','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M60">View MathML</a>

(2.10)

Similar to the proof of (2.10), there exist <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M61','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M61">View MathML</a> such that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M62','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M62">View MathML</a>

That is,

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M63','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M63">View MathML</a>

(2.11)

Since A is a decreasing operator, we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M64','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M64">View MathML</a>

(2.12)

(2.11) and (2.12) show that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M65','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M65">View MathML</a>

(2.13)

(2.10) and (2.13) together with <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M42','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M42">View MathML</a> show that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M67','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M67">View MathML</a>

(2.14)

For any <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M44','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M44">View MathML</a>, since A is a decreasing operator, we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M69','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M69">View MathML</a>

By (2.14), we have

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M70','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M70">View MathML</a>

It is easy to know that <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47">View MathML</a> is a closed convex set. Since P is a normal cone of E, we have <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47">View MathML</a> is bounded. Hence, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M47">View MathML</a> is a closed bounded convex set. Thus, Lemma 1.1 implies that the operator A has a fixed point in D. □

3 Corollaries and relative results

Similar to the proof of Theorem 2.1, by Lemma 1.2 and Lemma 1.3, we can get the following corollaries and relative results.

Corollary 3.1LetEbe an ordered Banach space with lattice structure, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>be bounded, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M27">View MathML</a>be a decreasing and completely continuous operator. Then the operatorAhas a fixed point inD.

Corollary 3.2LetEbe an ordered Banach space with lattice structure, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3">View MathML</a>be a normal cone, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52">View MathML</a>be a decreasing and completely continuous operator. Then the operatorAhas a fixed point inE.

Corollary 3.3LetEbe an ordered Banach space with lattice structure, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M11">View MathML</a>be bounded, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M27','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M27">View MathML</a>be a decreasing and strict-set-contraction mapping. Then the mappingAhas a fixed point inD.

Corollary 3.4LetEbe an ordered Banach space with lattice structure, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M3">View MathML</a>be a normal cone, and<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M52">View MathML</a>be a decreasing and strict-set-contraction mapping. Then the mappingAhas a fixed point inE.

4 Applications

In this section, we use Theorem 2.1 to show the existence of a solution for the uniformly elliptic differential problem. Let Ω be a bounded convex domain in <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M82','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M82">View MathML</a> (<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M83','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M83">View MathML</a>) whose boundary Ω is assumed to be sufficiently smooth. Consider a uniformly elliptic differential operator on <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M84','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M84">View MathML</a>

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M85','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M85">View MathML</a>

i.e., there exists a positive constant <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M86','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M86">View MathML</a> such that <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M87','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M87">View MathML</a> for any <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M88','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M88">View MathML</a> and <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M89','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M89">View MathML</a>, where <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M90','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M90">View MathML</a>, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M91','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M91">View MathML</a>. For the sake of simplicity, we will assume that all functions <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M92','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M92">View MathML</a>, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M93','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M93">View MathML</a>, <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M94','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M94">View MathML</a> are sufficiently smooth.

Considering the Dirichlet problem

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M95','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M95">View MathML</a>

(4.1)

we have the following conclusions.

Theorem 4.1Suppose that<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M96','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M96">View MathML</a>, which is decreasing onu, then the problem (4.1) has a positive solution.

Proof It is easy to know that <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M97','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M97">View MathML</a> is a Banach space with a maximum norm <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M98','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M98">View MathML</a> and it is also a lattice. Let <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M99','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M99">View MathML</a> and P be a normal cone in E. It is well known (see [1,10]) that the solution of the Dirichlet problem (4.1) is equivalent to the fixed point of the integral operator A

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M100','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M100">View MathML</a>

where <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M101">View MathML</a> denotes the Green function of a differential operator L with boundary condition <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M102','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M102">View MathML</a>. It is also well known that <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M101','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M101">View MathML</a> satisfies the following inequality:

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M104','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M104">View MathML</a>

Hence, the linear integral operator

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M105','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M105">View MathML</a>

is a completely continuous operator from E into E. Clearly, the superposition operator <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M106','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M106">View MathML</a> that maps P into P is continuous and bounded. Therefore, the operator <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M107','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M107">View MathML</a> that maps P into P is completely continuous, and thus A is condensing.

Moreover, the mapping A is decreasing in u. In fact, by hypotheses, for <a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M108','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M108">View MathML</a>,

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M109','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M109">View MathML</a>

implies that

<a onClick="popup('http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M110','MathML',630,470);return false;" target="_blank" href="http://www.fixedpointtheoryandapplications.com/content/2013/1/18/mathml/M110">View MathML</a>

so A is decreasing.

So, the condition of Theorem 2.1 holds, Theorem 4.1 is proved. □

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

XL carried out the the main theorem and the main conclusion. ZW carried out the application of the main theorem. All authors read and approved the final manuscript.

Acknowledgements

The first author was supported financially by the NSFC (71240007, 11001151), NSFSP (ZR2010AM005).

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