# Some notes on the paper "The equivalence of cone metric spaces and metric spaces"

- Mehdi Asadi
^{1}Email author, - Billy E Rhoades
^{2}and - Hossein Soleimani
^{3}

**2012**:87

**DOI: **10.1186/1687-1812-2012-87

© Asadi et al; licensee Springer. 2012

**Received: **9 May 2011

**Accepted: **21 May 2012

**Published: **21 May 2012

## Abstract

In this article, we shall show that the metrics defined by Feng and Mao, and Du are equivalent. We also provide some examples for one of the metrics.

## 1 Introduction and preliminary

*E*be a topological vector space (t.v.s.) with zero vector

*θ*. A nonempty subset

*K*of

*E*is called a convex cone if

*K*+

*K*⊆

*K*and λ

*K*⊆

*K*for each λ

*≥*0. A convex cone

*K*is said to be pointed if

*K*∩ -

*K*= {

*θ*}. For a given cone

*K*⊆

*E*, we can define a partial ordering ≼ with respect to

*K*by

*x < y*will stand for

*x*≼ y and

*x*≠

*y*while

*x*≺≺

*y*stands for

*y − x*∈

*K°*, where

*K°*denotes the interior of

*K*. In the following, we shall always assume that

*Y*is a locally convex Hausdorff t.v.s. with zero vector

*θ, K*is a proper, closed, and convex pointed cone in

*Y*with

*K°*≠ ∅,

*e*∈

*K°*and ≼ a partial ordering with respect to

*K*. The nonlinear scalarization function ${\xi}_{e}:Y\to \mathbb{R}$ is defined by

for all *y* ∈ *Y*.

We will use *P* instead of *K* when *E* is a real Banach spaces.

**Lemma 1.1** [1] *For each r* ∈ *R and y* ∈ *Y, the following statements are satisfied:*

*(i) ξ*_{
e
}(*y*) *≤ r* ⇔ *y* ∈ *re − K*.

*(ii) ξ*_{
e
}(*y*) *> r* ⇔ *y* ∉ *re − K*.

*(iii) ξ*_{
e
}(*y*) *≥ r* ⇔ *y* ∉ *re − K°*.

*(iv) ξ*_{
e
}(*y*) *< r* ⇔ *y* ∈ *re − K°*.

*(v) ξ*_{
e
}(.) *is positively homogeneous and continuous on Y* .

*(vi) y*_{1} ∈ *y*_{2} + *K* ⇒ *ξ*_{
e
}(*y*_{2}) *≤ ξ*_{
e
}(*y*_{1})

*(vii) ξ*_{
e
}(*y*_{1} + *y*_{2}) *≤ ξ*_{
e
}(*y*_{1}) + *ξ*_{
e
}(*y*_{2}) *for all y*_{1}, *y*_{2} ∈ *Y*.

**Definition 1.2** [1] *Let X be a nonempty set. A vector-valued function d* : *X × X* → *Y is said to be a TVS-cone metric, if the following conditions hold:*

*(C1) θ* ≼ *d*(*x, y*) *for all x, y* ∈ *X and d*(*x, y*) = *θ iff x* = *y*

*(C2) d*(*x, y*) = *d*(*y, x*) *for all x, y* ∈ *X*

*(C3)d*(*x, y*) ≼ (*x, z*) + *d*(*z, y*) *for all x, y, z* ∈ *X*.

*The pair* (*X, d*) *is then called a TVS-cone metric space*.

Huang and Zhang [2] discuss the case in which *Y* is a real Banach space and call a vector-valued function *d* : *X × X* → *Y* a cone metric if *d* satisfies (C1)-(C3). Clearly, a cone metric space, in the sense of Huang and Zhang, is a special case of a TVS-cone metric space.

In the following, some conclusions are listed.

**Lemma 1.3**[3]

*Let*(

*X, D*)

*be a cone metric space. Then*

*is a metric on X*.

**Theorem 1.4** [3] *The metric space* (*X, d*) *is complete if and only if the cone metric space* (*X, D*) *is complete* .

**Theorem 1.5** [1] *Let* (*X, D*) *be a TVS-cone metric space. Then d*_{2} : *X × X* → [0, ∞) *defined by d*_{2}(*x, y*) = *ξ*_{
e
}(*D*(*x, y*)) *is a metric*.

## 2 Main results

We first show that the metrics introduced the Lemma 1.3 and the Theorem 1.5 are equivalent. Then, we provide some examples involving the metric defined in Lemma 1.3.

**Theorem 2.1** *For every cone metric D* : *X × X* → *E there exists a metric* $d:X\times X\to {\mathbb{R}}^{+}$ *which is equivalent to D on X*.

**Proof**. Define

*d*(

*x, y*) = inf {||

*u*||:

*D*(

*x, y*) ≼

*u*}. By the Lemma 1.3

*d*is a metric. We shall now show that each sequence {

*x*

_{ n }} ⊆

*X*which converges to a point

*x*∈

*X*in the (

*X, d*) metric also converges to

*x*in the (

*X, D*) metric, and conversely. We have

Put *v*_{
n
} := *u*_{
nn
} then $\left|\right|{v}_{n}\left|\right|\phantom{\rule{2.77695pt}{0ex}}<\phantom{\rule{2.77695pt}{0ex}}d\left({x}_{n},\phantom{\rule{2.77695pt}{0ex}}x\right)+\frac{1}{n}$ and *D*(*x*_{
n
}*, x*) ≼ *v*_{
n
}. Now if *x*_{
n
} → *x* in (*X, d*) then *d*(*x*_{
n
}*, x*) → 0 and so *v*_{
n
} → 0 too, therefore for all *c* ≻≻ 0 there exists $N\in \mathbb{N}$ such that *v*_{
n
} ≺≺ *c* for all *n ≥ N*. This implies that *D*(*x*_{
n
}*, x*) ≺≺ *c* for all *n ≥ N*. Namely *x*_{
n
} → *x* in (*X, D*).

Conversely, for every real *ε >* 0, choose *c* ∈ *E* with *c* ≻≻ 0 and ||*c*|| *< ε*. Then there exists $N\in \mathbb{N}$ such that *D*(*x*_{
n
}*, x*) ≺≺ *c* for all *n ≥ N*. This means that for all *ε >* 0 there exists $N\in \mathbb{N}$ such that *d*(*x*_{
n
}*, x*) *≤* ||*c*|| *< ε* for all *n ≥ N*. Therefore *d*(*x*_{
n
}*, x*) → 0 as *n* → *∞* so *x*_{
n
} → *x* in (*X, d*).

□

**Theorem 2.2** *If d*_{1}(*x, y*) = inf {||*u*||: *D*(*x, y*) ≼ *u*} *and d*_{2}(*x, y*) = *ξ*_{
e
}(*D*(*x, y*)) *where D is a cone metric on X. Then d*_{1} *is equivalent with d*_{2}.

**Proof**. Let ${x}_{n}\underrightarrow{{d}_{1}}x$then ${d}_{1}\left({x}_{n},x\right)\underrightarrow{\mathbb{R}}0$so by Theorem 2.1 in ${x}_{n}\underrightarrow{D}x$ so

and or *εe − D*(*x*_{
n
}*, x*) ∈ *K°* for all *n ≥ N*. So *D*(*x*_{
n
}*, x*) ∈ *e* - *K°* for *n ≥ N*. Now by [[1], Lemma 1.1 (iv)] *ξ*_{
e
}(*D*(*x*_{
n
}*, x*)) *< ε* for all *n ≥ N*. Namely *d*_{2}(*x*_{
n
}*, x*) *< ε* for all *n ≥ N* therefore ${d}_{2}\left({x}_{n},x\right)\underrightarrow{\mathbb{R}}0$ or ${x}_{n}\stackrel{{d}_{2}}{\to}x$.

So *D*(*x*_{
n
}*, x*) ∈ *εe−K°* for *n ≥ N* by [[1], Lemma 1.1 (iv)]. Hence, *D*(*x*_{
n
}*, x*) = *εe−k* for some *k* ∈ *K°*, so *D*(*x*_{
n
}*, x*) ≺≺ *εe* for *n ≥ N* this implies that ${x}_{n}\stackrel{D}{\to}x$ and again by Theorem 2.1 ${x}_{n}\stackrel{{d}_{1}}{\to}x$. □

In the following examples, we use the metric of Lemma 1.3.

**Example 2.3**

*Let*$0\ne a\in P\subseteq {\mathbb{R}}^{n}$

*with*||

*a*|| = 1

*and for every*$x,y\in {\mathbb{R}}^{n}$

*define*

*Then D is a cone metric on*${\mathbb{R}}^{n}$

*and its equivalent metric d is*

*which is a discrete metric*.

**Example 2.4**

*Let a, b ≥*0

*and consider the cone metric*$D:\mathbb{R}\times \mathbb{R}\to {\mathbb{R}}^{2}$

*with*

*where d*

_{1},

*d*

_{2}

*are metrics on*.

*Then its equivalent metric is*

*In particular if d*

_{1}(

*x, y*):=

*|x − y| and d*

_{2}(

*x, y*):=

*α|x − y|, where α ≥*0

*then D is the same famous cone metric which has been introduced in [*[2]

*, Example 1] and its equivalent metric is*

**Example 2.5**

*For q >*0,

*b >*1,

*E*=

*l*

^{ q }

*, P*= {{

*x*

_{ n }}

_{ n }

_{≥1}:

*x*

_{ n }

*≥*0,

*for all n*}

*and*(

*X, ρ*)

*a metric space, define D*:

*X × X*→

*E which is the same cone metric as [*[4]

*, Example 1.3] by*

*Then its equivalent metric on × is*

## Declarations

### Acknowledgements

This research was supported by the Zanjan Branch, Islamic Azad University, Zanjan, Iran. Mehdi Asadi would like to acknowledge this support. The first and third authors would like proudly to dedicate this paper to *Professor Billy E. Rhoades* in recognition of his the valuable works in mathematics. The authors would also like to thank *Professor S. Mansour Vaezpour* for his helpful advise which led them to present this article. They also express their deep gratitude to the referee for his/her valuable comments and suggestions.

## Authors’ Affiliations

## References

- Du WS: A note on cone metric fixed point theory and its equivalence.
*Nonlinear Anal*2010, 72: 2259–2261. 10.1016/j.na.2009.10.026MATHMathSciNetView ArticleGoogle Scholar - Huang LG, Zhang X: Cone metric spaces and fixed point theorems of contractive mapping.
*J Math Anal Appl*2007, 322(2):1468–1476.View ArticleGoogle Scholar - Feng Y, Mao W: Equivalence of cone metric spaces and metric spaces.
*Fixed Point Theory*2010, 11(2):259–264.MATHMathSciNetGoogle Scholar - Haghi RH, Rezapour Sh: Fixed points of multifunctions on regular cone metric spaces.
*Expositiones Mathematicae*2010, 28(1):71–77. 10.1016/j.exmath.2009.04.001MATHMathSciNetView ArticleGoogle Scholar

## Copyright

This article is published under license to BioMed Central Ltd. 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.