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# Functional equations and inequalities in paranormed spaces

Journal of Inequalities and Applications20132013:198

https://doi.org/10.1186/1029-242X-2013-198

• Accepted: 8 March 2013
• Published:

## Abstract

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of an additive functional equation, a quadratic functional equation, a cubic functional equation and a quartic functional equation in paranormed spaces.

Furthermore, we prove the Hyers-Ulam stability of functional inequalities in paranormed spaces by using the fixed point method and the direct method.

MSC:35A17, 47H10, 39B52, 39B72.

## Keywords

• Jordan-von Neumann functional equation
• Hyers-Ulam stability
• paranormed space
• fixed point
• functional equation
• functional inequality

## 1 Introduction and preliminaries

The concept of statistical convergence for sequences of real numbers was introduced by Fast  and Steinhaus  independently, and since then several generalizations and applications of this notion have been investigated by various authors (see ). This notion was defined in normed spaces by Kolk .

We recall some basic facts concerning Fréchet spaces.

Definition 1.1 

Let X be a vector space. A paranorm $P:X\to \left[0,\mathrm{\infty }\right)$ is a function on X such that
1. (1)

$P\left(0\right)=0$;

2. (2)

$P\left(-x\right)=P\left(x\right)$;

3. (3)

$P\left(x+y\right)\le P\left(x\right)+P\left(y\right)$ (triangle inequality)

4. (4)

If $\left\{{t}_{n}\right\}$ is a sequence of scalars with ${t}_{n}\to t$ and $\left\{{x}_{n}\right\}\subset X$ with $P\left({x}_{n}-x\right)\to 0$, then $P\left({t}_{n}{x}_{n}-tx\right)\to 0$ (continuity of multiplication).

The pair $\left(X,P\right)$ is called a paranormed space if P is a paranorm on X.

The paranorm is called total if, in addition, we have
1. (5)

$P\left(x\right)=0$ implies $x=0$.

A Fréchet space is a total and complete paranormed space.

The stability problem of functional equations originated from a question of Ulam  concerning the stability of group homomorphisms. Hyers  gave the first affirmative partial answer to the question of Ulam for Banach spaces. Hyers’ theorem was generalized by Aoki  for additive mappings and by Rassias  for linear mappings by considering an unbounded Cauchy difference. A generalization of the Rassias theorem was obtained by Găvruta  by replacing the unbounded Cauchy difference by a general control function in the spirit of Rassias’ approach.

In 1990, Rassias  during the 27th International Symposium on Functional Equations asked the question whether such a theorem can also be proved for $p\ge 1$. In 1991, Gajda , following the same approach as in Rassias , gave an affirmative solution to this question for $p>1$. It was shown by Gajda , as well as by Rassias and Šemrl , that one cannot prove a Rassias-type theorem when $p=1$ (cf. the books of Czerwik , Hyers, Isac and Rassias ).

The functional equation
$f\left(x+y\right)+f\left(x-y\right)=2f\left(x\right)+2f\left(y\right)$
(1.1)

is called a quadratic functional equation. In particular, every solution of the quadratic functional equation is said to be a quadratic mapping. A Hyers-Ulam stability problem for the quadratic functional equation was proved by Skof  for mappings $f:X\to Y$, where X is a normed space and Y is a Banach space. Cholewa  noticed that the theorem of Skof is still true if the relevant domain X is replaced by an Abelian group. Czerwik  proved the Hyers-Ulam stability of the quadratic functional equation. The stability problems of several functional equations have been extensively investigated by a number of authors, and there are many interesting results concerning this problem (see ).

In , Jun and Kim considered the following cubic functional equation:
$\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)=f\left(x+y\right)+f\left(x-y\right)+6f\left(x\right).$
(1.2)

It is easy to show that the function $f\left(x\right)={x}^{3}$ satisfies the functional equation (1.2), which is called a cubic functional equation, and every solution of the cubic functional equation is said to be a cubic mapping.

In , Lee et al. considered the following quartic functional equation:
$\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)=2f\left(x+y\right)+2f\left(x-y\right)+12f\left(x\right)-3f\left(y\right).$
(1.3)

It is easy to show that the function $f\left(x\right)={x}^{4}$ satisfies the functional equation (1.3), which is called a quartic functional equation, and every solution of the quartic functional equation is said to be a quartic mapping.

In , Gilányi showed that if f satisfies the functional inequality
$\parallel 2f\left(x\right)+2f\left(y\right)-f\left(x{y}^{-1}\right)\parallel \le \parallel f\left(xy\right)\parallel ,$
(1.4)
then f satisfies the Jordan-von Neumann functional equation
$2f\left(x\right)+2f\left(y\right)=f\left(xy\right)+f\left(x{y}^{-1}\right).$

See also . Fechner  and Gilányi  proved the Hyers-Ulam stability of the functional inequality (1.4).

Throughout this paper, assume that $\left(X,P\right)$ is a Fréchet space and that $\left(Y,\parallel \cdot \parallel \right)$ is a Banach space.

In this paper, we prove the Hyers-Ulam stability of the Cauchy additive functional equation, the quadratic functional equation (1.2), the cubic functional equation (1.2) and the quartic functional equation (1.3) in paranormed spaces by using the fixed point method and the direct method.

Furthermore, we prove the Hyers-Ulam stability of the functional inequalities (1.5), (1.6) and (1.7) in paranormed spaces by using the fixed point method and the direct method.

## 2 Hyers-Ulam stability of the Cauchy additive functional equation

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of the Cauchy additive functional equation in paranormed spaces.

Let S be a set. A function $m:S×S\to \left[0,\mathrm{\infty }\right]$ is called a generalized metric on S if m satisfies
1. (1)

$m\left(x,y\right)=0$ if and only if $x=y$;

2. (2)

$m\left(x,y\right)=m\left(y,x\right)$ for all $x,y\in S$;

3. (3)

$m\left(x,z\right)\le m\left(x,y\right)+m\left(y,z\right)$ for all $x,y,z\in S$.

We recall a fundamental result in fixed point theory.

Theorem 2.1 [37, 38]

Let $\left(S,m\right)$ be a complete generalized metric space, and let $J:S\to S$ be a strictly contractive mapping with a Lipschitz constant $\alpha <1$. Then, for each given element $x\in S$, either
$m\left({J}^{n}x,{J}^{n+1}x\right)=\mathrm{\infty }$
for all nonnegative integers n or there exists a positive integer ${n}_{0}$ such that
1. (1)

$m\left({J}^{n}x,{J}^{n+1}x\right)<\mathrm{\infty }$, $\mathrm{\forall }n\ge {n}_{0}$;

2. (2)

the sequence $\left\{{J}^{n}x\right\}$ converges to a fixed point ${y}^{\ast }$ of J;

3. (3)

${y}^{\ast }$ is the unique fixed point of J in the set $W=\left\{y\in S\mid m\left({J}^{{n}_{0}}x,y\right)<\mathrm{\infty }\right\}$;

4. (4)

$m\left(y,{y}^{\ast }\right)\le \frac{1}{1-\alpha }m\left(y,Jy\right)$ for all $y\in W$.

In 1996, Isac and Rassias  were the first to provide applications of stability theory of functional equations for the proof of new fixed point theorems with applications. By using fixed point methods, the stability problems of several functional equations have been extensively investigated by a number of authors (see ).

Note that $P\left(2x\right)\le 2P\left(x\right)$ for all $x\in Y$.

Theorem 2.2 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le 2\alpha \phi \left(\frac{x}{2},\frac{y}{2}\right)$
(2.1)
for all $x,y\in X$. Let $f:X\to Y$ be a mapping such that
$\parallel f\left(x+y\right)-f\left(x\right)-f\left(y\right)\parallel \le \phi \left(x,y\right)$
(2.2)
for all $x,y\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2-2\alpha }\phi \left(x,x\right)$
(2.3)

for all $x\in X$.

Proof Letting $y=x$ in (2.2), we get
$\parallel f\left(2x\right)-2f\left(x\right)\parallel \le \phi \left(x,x\right),$
and so
$\parallel f\left(x\right)-\frac{1}{2}f\left(2x\right)\parallel \le \frac{1}{2}\phi \left(x,x\right)$
(2.4)

for all $x\in X$.

Consider the set
$S:=\left\{h:X\to Y\right\}$
and introduce the generalized metric on S:
$m\left(g,h\right)=inf\left\{\mu \in {\mathbb{R}}_{+}:\parallel g\left(x\right)-h\left(x\right)\parallel \le \mu \phi \left(x,x\right),\phantom{\rule{0.25em}{0ex}}\mathrm{\forall }x\in X\right\},$

where, as usual, $inf\varphi =+\mathrm{\infty }$. It is easy to show that $\left(S,m\right)$ is complete (see [, Lemma 2.1]).

Now we consider the linear mapping $J:S\to S$ such that
$Jh\left(x\right):=\frac{1}{2}h\left(2x\right)$

for all $x\in X$.

Let $g,h\in S$ be given such that $m\left(g,h\right)=\epsilon$. Since
$\parallel Jg\left(x\right)-Jh\left(x\right)\parallel =\parallel \frac{1}{2}g\left(2x\right)-\frac{1}{2}h\left(2x\right)\parallel \le \alpha \phi \left(x,x\right)$
for all $x\in X$, $m\left(g,h\right)=\epsilon$ implies that $m\left(Jg,Jh\right)\le \alpha \epsilon$. This means that
$m\left(Jg,Jh\right)\le \alpha m\left(g,h\right)$

for all $g,h\in S$.

It follows from (2.4) that $m\left(f,Jf\right)\le \frac{1}{2}$.

By Theorem 2.1, there exists a mapping $A:X\to Y$ satisfying the following:
1. (1)
A is a fixed point of J, i.e.,
$A\left(2x\right)=2A\left(x\right)$
(2.5)

for all $x\in X$. The mapping A is a unique fixed point of J in the set
$M=\left\{g\in S:m\left(f,g\right)<\mathrm{\infty }\right\}.$
This implies that A is a unique mapping satisfying (2.5) such that there exists a $\mu \in \left(0,\mathrm{\infty }\right)$ satisfying
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \mu \phi \left(x,x\right)$
for all $x\in X$;
1. (2)
$m\left({J}^{n}f,A\right)\to 0$ as $n\to \mathrm{\infty }$. This implies the equality
$\underset{n\to \mathrm{\infty }}{lim}\frac{1}{{2}^{n}}f\left({2}^{n}x\right)=A\left(x\right)$

for all $x\in X$;
1. (3)
$m\left(f,A\right)\le \frac{1}{1-\alpha }m\left(f,Jf\right)$, which implies the inequality
$m\left(f,A\right)\le \frac{1}{2-2\alpha }.$

This implies that the inequality (2.3) holds true.

It follows from (2.1) and (2.2) that
$\begin{array}{rcl}\parallel A\left(x+y\right)-A\left(x\right)-A\left(y\right)\parallel & =& \underset{n\to \mathrm{\infty }}{lim}\frac{1}{{2}^{n}}\parallel f\left({2}^{n}\left(x+y\right)\right)-f\left({2}^{n}x\right)-f\left({2}^{n}y\right)\parallel \\ \le & \underset{n\to \mathrm{\infty }}{lim}\frac{{2}^{n}{\alpha }^{n}}{{2}^{n}}\phi \left(x,y\right)=0\end{array}$

for all $x,y\in X$. So, $A\left(x+y\right)-A\left(x\right)-A\left(y\right)=0$ for all $x,y\in X$. Thus $A:X\to Y$ is an additive mapping, as desired. □

Corollary 2.3 Let r be a positive real number with $r<1$, and let $f:X\to Y$ be a mapping such that
$\parallel f\left(x+y\right)-f\left(x\right)-f\left(y\right)\parallel \le P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$
for all $x,y\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{2}{2-{2}^{r}}P{\left(x\right)}^{r}$
(2.6)

for all $x\in X$.

Proof Taking $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ and choosing $\alpha ={2}^{r-1}$ in Theorem 2.2, we get the desired result. □

Theorem 2.4 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{2}^{j}}\phi \left({2}^{j}x,{2}^{j}y\right)<\mathrm{\infty }$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping satisfying (2.2). Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2}\mathrm{\Phi }\left(x,x\right)$

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 2.2]. □

Remark 2.5 Let $r<1$. Letting $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ in Theorem 2.4, we obtain the inequality (2.6). The proof is given in [, Theorem 2.2].

Theorem 2.6 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le \frac{\alpha }{2}\phi \left(2x,2y\right)$
(2.7)
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping such that
$P\left(f\left(x+y\right)-f\left(x\right)-f\left(y\right)\right)\le \phi \left(x,y\right)$
(2.8)
for all $x,y\in Y$. Then there exists a unique additive mapping $A:Y\to X$ such that
$P\left(f\left(x\right)-A\left(x\right)\right)\le \frac{\alpha }{2-2\alpha }\phi \left(x,x\right)$
(2.9)

for all $x\in Y$.

Proof Letting $y=x$ in (2.8), we get
$P\left(f\left(2x\right)-2f\left(x\right)\right)\le \phi \left(x,x\right),$
and so
$P\left(f\left(x\right)-2f\left(\frac{x}{2}\right)\right)\le \frac{\alpha }{2}\phi \left(x,x\right)$
(2.10)

for all $x\in Y$.

Consider the set
$S:=\left\{h:Y\to X\right\}$
and introduce the generalized metric on S:
$m\left(g,h\right)=inf\left\{\mu \in {\mathbb{R}}_{+}:P\left(g\left(x\right)-h\left(x\right)\right)\le \mu \phi \left(x,x\right),\phantom{\rule{0.25em}{0ex}}\mathrm{\forall }x\in Y\right\},$

where, as usual, $inf\varphi =+\mathrm{\infty }$. It is easy to show that $\left(S,m\right)$ is complete (see [, Lemma 2.1]).

Now we consider the linear mapping $J:S\to S$ such that
$Jh\left(x\right):=2h\left(\frac{x}{2}\right)$

for all $x\in Y$.

Let $g,h\in S$ be given such that $m\left(g,h\right)=\epsilon$. Since
$P\left(Jg\left(x\right)-Jh\left(x\right)\right)=P\left(2g\left(\frac{x}{2}\right)-2h\left(\frac{x}{2}\right)\right)\le \alpha \phi \left(x,x\right)$
for all $x\in Y$, $m\left(g,h\right)=\epsilon$ implies that $m\left(Jg,Jh\right)\le \alpha \epsilon$. This means that
$m\left(Jg,Jh\right)\le \alpha m\left(g,h\right)$

for all $g,h\in S$.

It follows from (2.10) that $m\left(f,Jf\right)\le \frac{\alpha }{2}$.

By Theorem 2.1, there exists a mapping $A:X\to Y$ satisfying the following:
1. (1)
A is a fixed point of J, i.e.,
$A\left(\frac{x}{2}\right)=\frac{1}{2}A\left(x\right)$
(2.11)

for all $x\in X$. The mapping A is a unique fixed point of J in the set
$M=\left\{g\in S:m\left(f,g\right)<\mathrm{\infty }\right\}.$
This implies that A is a unique mapping satisfying (2.11) such that there exists a $\mu \in \left(0,\mathrm{\infty }\right)$ satisfying
$P\left(f\left(x\right)-A\left(x\right)\right)\le \mu \phi \left(x,x\right)$
for all $x\in Y$;
1. (2)
$m\left({J}^{n}f,A\right)\to 0$ as $n\to \mathrm{\infty }$. This implies the equality
$\underset{n\to \mathrm{\infty }}{lim}{2}^{n}f\left(\frac{x}{{2}^{n}}\right)=A\left(x\right)$

for all $x\in Y$;
1. (3)
$m\left(f,A\right)\le \frac{1}{1-\alpha }m\left(f,Jf\right)$, which implies the inequality
$m\left(f,A\right)\le \frac{\alpha }{2-2\alpha }.$

This implies that the inequality (2.9) holds true.

It follows from (2.7) and (2.8) that
$\begin{array}{rcl}P\left(A\left(x+y\right)-A\left(x\right)-A\left(y\right)\right)& =& \underset{n\to \mathrm{\infty }}{lim}P\left({2}^{n}\left(f\left(\frac{x+y}{{2}^{n}}\right)-f\left(\frac{x}{{2}^{n}}\right)-f\left(\frac{y}{{2}^{n}}\right)\right)\right)\\ \le & \underset{n\to \mathrm{\infty }}{lim}{2}^{n}P\left(\left(f\left(\frac{x+y}{{2}^{n}}\right)-f\left(\frac{x}{{2}^{n}}\right)-f\left(\frac{y}{{2}^{n}}\right)\right)\right)\\ \le & \underset{n\to \mathrm{\infty }}{lim}\frac{{2}^{n}{\alpha }^{n}}{{2}^{n}}\phi \left(x,y\right)=0\end{array}$

for all $x,y\in Y$. So, $A\left(x+y\right)-A\left(x\right)-A\left(y\right)=0$ for all $x,y\in Y$. Thus $A:Y\to X$ is an additive mapping, as desired. □

Corollary 2.7 Let r, θ be positive real numbers with $r>1$, and let $f:Y\to X$ be a mapping such that
$P\left(f\left(x+y\right)-f\left(x\right)-f\left(y\right)\right)\le \theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$
for all $x,y\in Y$. Then there exists a unique Cauchy additive mapping $A:Y\to X$ such that
$P\left(f\left(x\right)-A\left(x\right)\right)\le \frac{2\theta }{{2}^{r}-2}{\parallel x\parallel }^{r}$
(2.12)

for all $x\in Y$.

Proof Taking $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ and choosing $\alpha ={2}^{1-r}$ in Theorem 2.6, we get the desired result. □

Theorem 2.8 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=1}^{\mathrm{\infty }}{2}^{j}\phi \left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{\infty }$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping satisfying (2.8). Then there exists a unique Cauchy additive mapping $A:Y\to X$ such that
$P\left(f\left(x\right)-A\left(x\right)\right)\le \frac{1}{2}\mathrm{\Phi }\left(x,x\right)$

for all $x\in Y$.

Proof The proof is similar to the proof of [, Theorem 2.1]. □

Remark 2.9 Let $r>1$. Letting $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ in Theorem 2.8, we obtain the inequality (2.12). The proof is given in [, Theorem 2.1].

## 3 Hyers-Ulam stability of the quadratic functional equation (1.1)

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of the quadratic functional equation (1.1) in paranormed spaces.

Theorem 3.1 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le 4\alpha \phi \left(\frac{x}{2},\frac{y}{2}\right)$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping satisfying $f\left(0\right)=0$ and
$\parallel f\left(x+y\right)+f\left(x-y\right)-2f\left(x\right)-2f\left(y\right)\parallel \le \phi \left(x,y\right)$
(3.1)
for all $x,y\in X$. Then there exists a unique quadratic mapping ${Q}_{2}:X\to Y$ such that
$\parallel f\left(x\right)-{Q}_{2}\left(x\right)\parallel \le \frac{1}{4-4\alpha }\phi \left(x,x\right)$

for all $x\in X$.

Proof Letting $y=x$ in (3.1), we get
$\parallel f\left(2x\right)-4f\left(x\right)\parallel \le \phi \left(x,x\right),$
and so
$\parallel f\left(x\right)-\frac{1}{4}f\left(2x\right)\parallel \le \frac{1}{4}\phi \left(x,x\right)$

for all $x\in X$.

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 3.2 Let r be a positive real number with $r<2$, and let $f:X\to Y$ be a mapping satisfying $f\left(0\right)=0$ and
$\parallel f\left(x+y\right)+f\left(x-y\right)-2f\left(x\right)-2f\left(y\right)\parallel \le P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$
for all $x,y\in X$. Then there exists a unique quadratic mapping ${Q}_{2}:X\to Y$ such that
$\parallel f\left(x\right)-{Q}_{2}\left(x\right)\parallel \le \frac{2}{4-{2}^{r}}P{\left(x\right)}^{r}$
(3.2)

for all $x\in X$.

Proof Taking $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ and choosing $\alpha ={2}^{r-2}$ in Theorem 3.1, we get the desired result. □

Theorem 3.3 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{4}^{j}}\phi \left({2}^{j}x,{2}^{j}y\right)<\mathrm{\infty }$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping satisfying $f\left(0\right)=0$ and (3.1). Then there exists a unique quadratic mapping ${Q}_{2}:X\to Y$ such that
$\parallel f\left(x\right)-{Q}_{2}\left(x\right)\parallel \le \frac{1}{4}\mathrm{\Phi }\left(x,x\right)$

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 3.2]. □

Remark 3.4 Let $r<2$. Letting $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ in Theorem 3.3, we obtain the inequality (3.2). The proof is given in [, Theorem 3.2].

Theorem 3.5 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le \frac{\alpha }{4}\phi \left(2x,2y\right)$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and
$P\left(f\left(x+y\right)+f\left(x-y\right)-2f\left(x\right)-2f\left(y\right)\right)\le \phi \left(x,y\right)$
(3.3)
for all $x,y\in Y$. Then there exists a unique quadratic mapping ${Q}_{2}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{2}\left(x\right)\right)\le \frac{\alpha }{4-4\alpha }\phi \left(x,x\right)$

for all $x\in Y$.

Proof Letting $y=x$ in (3.3), we get
$P\left(f\left(2x\right)-4f\left(x\right)\right)\le \phi \left(x,x\right),$
and so
$P\left(f\left(x\right)-4f\left(\frac{x}{2}\right)\right)\le \phi \left(\frac{x}{2},\frac{x}{2}\right)\le \frac{\alpha }{4}\phi \left(x,x\right)$

for all $x\in Y$.

The rest of the proof is similar to the proof of Theorem 2.6. □

Corollary 3.6 Let r, θ be positive real numbers with $r>2$, and let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and
$P\left(f\left(x+y\right)+f\left(x-y\right)-2f\left(x\right)-2f\left(y\right)\right)\le \theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$
for all $x,y\in Y$. Then there exists a unique quadratic mapping ${Q}_{2}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{2}\left(x\right)\right)\le \frac{2\theta }{{2}^{r}-4}{\parallel x\parallel }^{r}$
(3.4)

for all $x\in Y$.

Proof Taking $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ and choosing $\alpha ={2}^{2-r}$ in Theorem 3.5, we get the desired result. □

Theorem 3.7 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=1}^{\mathrm{\infty }}{4}^{j}\phi \left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{\infty }$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and (3.3). Then there exists a unique quadratic mapping ${Q}_{2}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{2}\left(x\right)\right)\le \frac{1}{4}\mathrm{\Phi }\left(x,x\right)$

for all $x\in Y$.

Proof The proof is similar to the proof of [, Theorem 3.1]. □

Remark 3.8 Let $r>2$. Letting $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ in Theorem 3.7, we obtain the inequality (3.4). The proof is given in [, Theorem 3.1].

## 4 Hyers-Ulam stability of the cubic functional equation (1.2)

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of the cubic functional equation (1.2) in paranormed spaces.

Theorem 4.1 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le 8\alpha \phi \left(\frac{x}{2},\frac{y}{2}\right)$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping such that
$\parallel \frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-f\left(x+y\right)-f\left(x-y\right)-6f\left(x\right)\parallel \le \phi \left(x,y\right)$
(4.1)
for all $x,y\in X$. Then there exists a unique cubic mapping $C:X\to Y$ such that
$\parallel f\left(x\right)-C\left(x\right)\parallel \le \frac{1}{8-8\alpha }\phi \left(x,0\right)$

for all $x\in X$.

Proof Letting $y=0$ in (4.1), we get
$\parallel f\left(2x\right)-8f\left(x\right)\parallel \le \phi \left(x,0\right),$
and so
$\parallel f\left(x\right)-\frac{1}{8}f\left(2x\right)\parallel \le \frac{1}{8}\phi \left(x,0\right)$

for all $x\in X$.

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 4.2 Let r be a positive real number with $r<3$, and let $f:Y\to X$ be a mapping such that
$\parallel \frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-f\left(x+y\right)-f\left(x-y\right)-6f\left(x\right)\parallel \le P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$
for all $x,y\in X$. Then there exists a unique cubic mapping $C:Y\to X$ such that
$P\left(f\left(x\right)-C\left(x\right)\right)\le \frac{1}{8-{2}^{r}}P{\left(x\right)}^{r}$
(4.2)

for all $x\in Y$.

Proof Taking $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ and choosing $\alpha ={2}^{r-3}$ in Theorem 4.1, we get the desired result. □

Theorem 4.3 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{8}^{j}}\phi \left({2}^{j}x,{2}^{j}y\right)<\mathrm{\infty }$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping satisfying (4.1). Then there exists a unique cubic mapping $C:X\to Y$ such that
$\parallel f\left(x\right)-C\left(x\right)\parallel \le \frac{1}{8}\mathrm{\Phi }\left(x,0\right)$

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 4.2]. □

Remark 4.4 Let $r<3$. Letting $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ in Theorem 4.3, we obtain the inequality (4.2). The proof is given in [, Theorem 4.2].

Theorem 4.5 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le \frac{\alpha }{8}\phi \left(2x,2y\right)$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping such that
$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-f\left(x+y\right)-f\left(x-y\right)-6f\left(x\right)\right)\le \phi \left(x,y\right)$
(4.3)
for all $x,y\in Y$. Then there exists a unique cubic mapping $C:Y\to X$ such that
$P\left(f\left(x\right)-C\left(x\right)\right)\le \frac{\alpha }{8-8\alpha }\phi \left(x,0\right)$

for all $x\in Y$.

Proof Letting $y=0$ in (4.3), we get
$P\left(f\left(2x\right)-8f\left(x\right)\right)\le \phi \left(x,0\right),$
and so
$P\left(f\left(x\right)-8f\left(\frac{x}{2}\right)\right)\le \phi \left(\frac{x}{2},\frac{x}{2}\right)\le \frac{\alpha }{8}\phi \left(x,0\right)$

for all $x\in Y$.

The rest of the proof is similar to the proof of Theorem 2.6. □

Corollary 4.6 Let r, θ be positive real numbers with $r>3$, and let $f:Y\to X$ be a mapping such that
$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-f\left(x+y\right)-f\left(x-y\right)-6f\left(x\right)\right)\le \theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$
for all $x,y\in Y$. Then there exists a unique cubic mapping $C:Y\to X$ such that
$P\left(f\left(x\right)-C\left(x\right)\right)\le \frac{\theta }{{2}^{r}-8}{\parallel x\parallel }^{r}$
(4.4)

for all $x\in Y$.

Proof Taking $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ and choosing $\alpha ={2}^{3-r}$ in Theorem 4.5, we get the desired result. □

Theorem 4.7 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=1}^{\mathrm{\infty }}{8}^{j}\phi \left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{\infty }$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping satisfying (4.3). Then there exists a unique cubic mapping $C:Y\to X$ such that
$P\left(f\left(x\right)-C\left(x\right)\right)\le \frac{1}{8}\mathrm{\Phi }\left(x,0\right)$

for all $x\in Y$.

Proof The proof is similar to the proof of [, Theorem 4.1]. □

Remark 4.8 Let $r>3$. Letting $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ in Theorem 4.7, we obtain the inequality (4.4). The proof is given in [, Theorem 4.1].

## 5 Hyers-Ulam stability of the quartic functional equation (1.3)

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of the quartic functional equation (1.3) in paranormed spaces.

Theorem 5.1 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le 16\alpha \phi \left(\frac{x}{2},\frac{y}{2}\right)$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping satisfying $f\left(0\right)=0$ and
$\parallel \frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-2f\left(x+y\right)-2f\left(x-y\right)-12f\left(x\right)+3f\left(y\right)\parallel \le \phi \left(x,y\right)$
(5.1)
for all $x,y\in X$. Then there exists a unique quartic mapping ${Q}_{4}:X\to Y$ such that
$\parallel f\left(x\right)-{Q}_{4}\left(x\right)\parallel \le \frac{1}{16-16\alpha }\phi \left(x,0\right)$

for all $x\in X$.

Proof Letting $y=0$ in (5.1), we get
$\parallel f\left(2x\right)-16f\left(x\right)\parallel \le \phi \left(x,0\right),$
and so
$\parallel f\left(x\right)-\frac{1}{16}f\left(2x\right)\parallel \le \frac{1}{16}\phi \left(x,0\right)$

for all $x\in X$.

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 5.2 Let r be a positive real number with $r<4$, and let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and
$\parallel \frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-2f\left(x+y\right)-2f\left(x-y\right)-12f\left(x\right)+3f\left(y\right)\parallel \le P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$
for all $x,y\in X$. Then there exists a unique quartic mapping ${Q}_{4}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{4}\left(x\right)\right)\le \frac{1}{16-{2}^{r}}P{\left(x\right)}^{r}$
(5.2)

for all $x\in Y$.

Proof Taking $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ and choosing $\alpha ={2}^{r-4}$ in Theorem 5.1, we get the desired result. □

Theorem 5.3 Let $\phi :{X}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{16}^{j}}\phi \left({2}^{j}x,{2}^{j}y\right)<\mathrm{\infty }$
for all $x,y\in X$. Let $f:X\to Y$ be a mapping satisfying $f\left(0\right)=0$ and (5.1). Then there exists a unique quartic mapping ${Q}_{4}:X\to Y$ such that
$\parallel f\left(x\right)-{Q}_{4}\left(x\right)\parallel \le \frac{1}{16}\mathrm{\Phi }\left(x,0\right)$

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 5.2]. □

Remark 5.4 Let $r<4$. Letting $\phi \left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,y\in X$ in Theorem 5.3, we obtain the inequality (5.2). The proof is given in [, Theorem 5.2].

Theorem 5.5 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y\right)\le \frac{\alpha }{16}\phi \left(2x,2y\right)$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and
$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-2f\left(x+y\right)-2f\left(x-y\right)-12f\left(x\right)+3f\left(y\right)\right)\le \phi \left(x,y\right)$
(5.3)
for all $x,y\in Y$. Then there exists a unique quartic mapping ${Q}_{4}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{4}\left(x\right)\right)\le \frac{\alpha }{16-16\alpha }\phi \left(x,0\right)$

for all $x\in Y$.

Proof Letting $y=0$ in (5.3), we get
$P\left(f\left(2x\right)-16f\left(x\right)\right)\le \phi \left(x,0\right),$
and so
$P\left(f\left(x\right)-16f\left(\frac{x}{2}\right)\right)\le \phi \left(\frac{x}{2},\frac{x}{2}\right)\le \frac{\alpha }{16}\phi \left(x,0\right)$

for all $x\in Y$.

The rest of the proof is similar to the proof of Theorem 2.6. □

Corollary 5.6 Let r, θ be positive real numbers with $r>4$, and let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and
$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2x-y\right)-2f\left(x+y\right)-2f\left(x-y\right)-12f\left(x\right)+3f\left(y\right)\right)\le \theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$
for all $x,y\in Y$. Then there exists a unique quartic mapping ${Q}_{4}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{4}\left(x\right)\right)\le \frac{\theta }{{2}^{r}-16}{\parallel x\parallel }^{r}$
(5.4)

for all $x\in Y$.

Proof Taking $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ and choosing $\alpha ={2}^{4-r}$ in Theorem 5.5, we get the desired result. □

Theorem 5.7 Let $\phi :{Y}^{2}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y\right):=\sum _{j=1}^{\mathrm{\infty }}{16}^{j}\phi \left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{\infty }$
for all $x,y\in Y$. Let $f:Y\to X$ be a mapping satisfying $f\left(0\right)=0$ and (5.3). Then there exists a unique quartic mapping ${Q}_{4}:Y\to X$ such that
$P\left(f\left(x\right)-{Q}_{4}\left(x\right)\right)\le \frac{1}{16}\mathrm{\Phi }\left(x,0\right)$

for all $x\in Y$.

Proof The proof is similar to the proof of [, Theorem 5.1]. □

Remark 5.8 Let $r>4$. Letting $\phi \left(x,y\right)=\theta \left({\parallel x\parallel }^{r}+{\parallel y\parallel }^{r}\right)$ for all $x,y\in X$ in Theorem 5.7, we obtain the inequality (5.4). The proof is given in [, Theorem 5.1].

## 6 Stability of a functional inequality associated with a three-variable Jensen additive functional equation

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of a functional inequality associated with a Jordan-von Neumann type three-variable Jensen additive functional equation in paranormed spaces.

Proposition 6.1 [, Proposition 2.1]

Let $f:X\to Y$ be a mapping such that
$\parallel f\left(x\right)+f\left(y\right)+f\left(z\right)\parallel \le \parallel 2f\left(\frac{x+y+z}{2}\right)\parallel$

for all $x,y,z\in X$. Then f is Cauchy additive.

Theorem 6.2 Let $\phi :{X}^{3}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y,z\right)\le 2\alpha \phi \left(\frac{x}{2},\frac{y}{2},\frac{z}{2}\right)$
(6.1)
for all $x,y,z\in X$. Let $f:X\to Y$ be an odd mapping such that
$\parallel f\left(x\right)+f\left(y\right)+f\left(z\right)\parallel \le \parallel 2f\left(\frac{x+y+z}{2}\right)\parallel +\phi \left(x,y,z\right)$
(6.2)
for all $x,y,z\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2-2\alpha }\phi \left(x,x,-2x\right)$
(6.3)

for all $x\in X$.

Proof Letting $y=x$ and $z=-2x$ in (6.2), we get
$\parallel 2f\left(x\right)-f\left(2x\right)\parallel =\parallel 2f\left(x\right)+f\left(-2x\right)\parallel \le \phi \left(x,x,-2x\right),$
and so
$\parallel f\left(x\right)-\frac{1}{2}f\left(2x\right)\parallel \le \frac{1}{2}\phi \left(x,x,-2x\right)$
(6.4)

for all $x\in X$.

Consider the set
$S:=\left\{h:X\to Y\right\}$
and introduce the generalized metric on S
$m\left(g,h\right)=inf\left\{\mu \in {\mathbb{R}}_{+}:\parallel g\left(x\right)-h\left(x\right)\parallel \le \mu \phi \left(x,x,-2x\right),\phantom{\rule{0.25em}{0ex}}\mathrm{\forall }x\in X\right\},$

where, as usual, $inf\varphi =+\mathrm{\infty }$. It is easy to show that $\left(S,m\right)$ is complete (see [, Lemma 2.1]).

Now we consider the linear mapping $J:S\to S$ such that
$Jh\left(x\right):=\frac{1}{2}h\left(2x\right)$

for all $x\in X$.

Let $g,h\in S$ be given such that $m\left(g,h\right)=\epsilon$. Since
$\parallel Jg\left(x\right)-Jh\left(x\right)\parallel =\parallel \frac{1}{2}g\left(2x\right)-\frac{1}{2}h\left(2x\right)\parallel \le \alpha \phi \left(x,x,-2x\right)$
for all $x\in X$, $m\left(g,h\right)=\epsilon$ implies that $m\left(Jg,Jh\right)\le \alpha \epsilon$. This means that
$m\left(Jg,Jh\right)\le \alpha m\left(g,h\right)$

for all $g,h\in S$.

It follows from (6.4) that $m\left(f,Jf\right)\le \frac{1}{2}$.

By Theorem 2.1, there exists a mapping $A:X\to Y$ satisfying the following:
1. (1)
A is a fixed point of J, i.e.,
$A\left(2x\right)=2A\left(x\right)$
(6.5)

for all $x\in X$. The mapping A is a unique fixed point of J in the set
$M=\left\{g\in S:m\left(f,g\right)<\mathrm{\infty }\right\}.$
This implies that A is a unique mapping satisfying (6.5) such that there exists a $\mu \in \left(0,\mathrm{\infty }\right)$ satisfying
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \mu \phi \left(x,x,-2x\right)$
for all $x\in X$;
1. (2)
$m\left({J}^{n}f,A\right)\to 0$ as $n\to \mathrm{\infty }$. This implies the equality
$\underset{n\to \mathrm{\infty }}{lim}\frac{1}{{2}^{n}}f\left({2}^{n}x\right)=A\left(x\right)$

for all $x\in X$;
1. (3)
$m\left(f,A\right)\le \frac{1}{1-\alpha }m\left(f,Jf\right)$, which implies the inequality
$m\left(f,A\right)\le \frac{1}{2-2\alpha }.$

This implies that the inequality (6.3) holds true.

It follows from (6.1) and (6.2) that
$\begin{array}{rcl}\frac{1}{{2}^{n}}\parallel f\left({2}^{n}x\right)+f\left({2}^{n}y\right)+f\left({2}^{n}z\right)\parallel & \le & \frac{1}{{2}^{n}}\parallel 2f\left(\frac{{2}^{n}\left(x+y+z\right)}{2}\right)\parallel +\frac{1}{{2}^{n}}\phi \left({2}^{n}x,{2}^{n}y,{2}^{n}z\right)\\ \le & \frac{1}{{2}^{n}}\parallel 2f\left(\frac{{2}^{n}\left(x+y+z\right)}{2}\right)\parallel +\frac{{2}^{n}{\alpha }^{n}}{{2}^{n}}\phi \left(x,y,z\right)\end{array}$
(6.6)
for all $x,y,z\in X$. Letting $n\to \mathrm{\infty }$ in (6.6), we get
$\parallel A\left(x\right)+A\left(y\right)+A\left(z\right)\parallel \le \parallel 2A\left(\frac{x+y+z}{2}\right)\parallel$

for all $x,y,z\in X$. By Proposition 6.1, $A:X\to Y$ is Cauchy additive, as desired. □

Corollary 6.3 [, Theorem 2.2]

Let r be a positive real number with $r<1$, and let $f:X\to Y$ be an odd mapping such that
$\parallel f\left(x\right)+f\left(y\right)+f\left(z\right)\parallel \le \parallel 2f\left(\frac{x+y+z}{2}\right)\parallel +P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$
for all $x,y,z\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{2+{2}^{r}}{2-{2}^{r}}P{\left(x\right)}^{r}$
(6.7)

for all $x\in X$.

Proof Taking $\phi \left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y,z\in X$ and choosing $\alpha ={2}^{r-1}$ in Theorem 6.2, we get the desired result. □

Theorem 6.4 Let $\phi :{X}^{3}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y,z\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{2}^{j}}\phi \left({2}^{j}x,{2}^{j}y,{2}^{j}z\right)<\mathrm{\infty }$
for all $x,y,z\in X$. Let $f:X\to Y$ be an odd mapping satisfying (6.2). Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2}\mathrm{\Phi }\left(x,x,-2x\right)$

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 2.2]. □

Remark 6.5 Let $r<1$. Letting $\phi \left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y\in X$ in Theorem 6.4, we obtain the inequality (6.7). The proof is given in [, Theorem 2.2].

## 7 Stability of a functional inequality associated with a three-variable Cauchy additive functional equation

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of a functional inequality associated with a Jordan-von Neumann type three-variable Cauchy additive functional equation in paranormed spaces.

Proposition 7.1 [, Proposition 2.2]

Let $f:X\to Y$ be a mapping such that
$\parallel f\left(x\right)+f\left(y\right)+f\left(z\right)\parallel \le \parallel f\left(x+y+z\right)\parallel$

for all $x,y,z\in X$. Then f is Cauchy additive.

Theorem 7.2 Let $\phi :{X}^{3}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y,z\right)\le 2\alpha \phi \left(\frac{x}{2},\frac{y}{2},\frac{z}{2}\right)$
for all $x,y,z\in X$. Let $f:X\to Y$ be an odd mapping such that
$\parallel f\left(x\right)+f\left(y\right)+f\left(z\right)\parallel \le \parallel f\left(x+y+z\right)\parallel +\phi \left(x,y,z\right)$
(7.1)
for all $x,y,z\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2-2\alpha }\phi \left(x,x,-2x\right)$

for all $x\in X$.

Proof Letting $y=x$ and $z=-2x$ in (7.1), we get
$\parallel 2f\left(x\right)-f\left(2x\right)\parallel =\parallel 2f\left(x\right)+f\left(-2x\right)\parallel \le \phi \left(x,x,-2x\right),$
and so
$\parallel f\left(x\right)-\frac{1}{2}f\left(2x\right)\parallel \le \frac{1}{2}\phi \left(x,x,-2x\right)$
(7.2)

for all $x\in X$.

The rest of the proof is similar to the proof of Theorem 6.2. □

Corollary 7.3 [, Theorem 3.2]

Let r be a positive real number with $r<1$, and let $f:X\to Y$ be an odd mapping such that
$\parallel f\left(x\right)+f\left(y\right)+f\left(z\right)\parallel \le \parallel f\left(x+y+z\right)\parallel +P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$
for all $x,y,z\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{2+{2}^{r}}{2-{2}^{r}}P{\left(x\right)}^{r}$
(7.3)

for all $x\in X$.

Proof Taking $\phi \left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y,z\in X$ and choosing $\alpha ={2}^{r-1}$ in Theorem 7.2, we get the desired result. □

Theorem 7.4 Let $\phi :{X}^{3}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y,z\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{2}^{j}}\phi \left({2}^{j}x,{2}^{j}y,{2}^{j}z\right)<\mathrm{\infty }$
for all $x,y,z\in X$. Let $f:X\to Y$ be an odd mapping satisfying (7.1). Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2}\mathrm{\Phi }\left(x,x,-2x\right)$

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 3.2]. □

Remark 7.5 Let $r<1$. Letting $\phi \left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y\in X$ in Theorem 7.4, we obtain the inequality (7.3). The proof is given in [, Theorem 3.2].

## 8 Stability of a functional inequality associated with the Cauchy-Jensen functional equation

Using the fixed point method and the direct method, we prove the Hyers-Ulam stability of a functional inequality associated with a Jordan-von Neumann type Cauchy-Jensen additive functional equation in paranormed spaces.

Proposition 8.1 [, Proposition 2.3]

Let $f:X\to Y$ be a mapping such that
$\parallel f\left(x\right)+f\left(y\right)+2f\left(z\right)\parallel \le \parallel 2f\left(\frac{x+y}{2}+z\right)\parallel$

for all $x,y,z\in X$. Then f is Cauchy additive.

Theorem 8.2 Let $\phi :{X}^{3}\to \left[0,\mathrm{\infty }\right)$ be a function such that there exists an $\alpha <1$ with
$\phi \left(x,y,z\right)\le 2\alpha \phi \left(\frac{x}{2},\frac{y}{2},\frac{z}{2}\right)$
for all $x,y,z\in X$. Let $f:X\to Y$ be an odd mapping such that
$\parallel f\left(x\right)+f\left(y\right)+2f\left(z\right)\parallel \le \parallel 2f\left(\frac{x+y}{2}+z\right)\parallel +\phi \left(x,y,z\right)$
(8.1)
for all $x,y,z\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2-2\alpha }\phi \left(2x,0,-x\right)$

for all $x\in X$.

Proof Replacing x by 2x and letting $y=0$ and $z=-x$ in (8.1), we get
$\parallel 2f\left(x\right)-f\left(2x\right)\parallel =\parallel 2f\left(x\right)+f\left(-2x\right)\parallel \le \phi \left(2x,0,-x\right)$

for all $x\in X$.

The rest of the proof is the same as in the proof of Theorem 6.2. □

Corollary 8.3 [, Theorem 4.2]

Let r be a positive real number with $r<1$, and let $f:X\to Y$ be an odd mapping such that
$\parallel f\left(x\right)+f\left(y\right)+2f\left(z\right)\parallel \le \parallel 2f\left(\frac{x+y}{2}+z\right)\parallel +P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$
for all $x,y,z\in X$. Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1+{2}^{r}}{2-{2}^{r}}P{\left(x\right)}^{r}$
(8.2)

for all $x\in X$.

Proof Taking $\phi \left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y,z\in X$ and choosing $\alpha ={2}^{r-1}$ in Theorem 8.2, we get the desired result. □

Theorem 8.4 Let $\phi :{X}^{3}\to \left[0,\mathrm{\infty }\right)$ be a function such that
$\mathrm{\Phi }\left(x,y,z\right):=\sum _{j=0}^{\mathrm{\infty }}\frac{1}{{2}^{j}}\phi \left({2}^{j}x,{2}^{j}y,{2}^{j}z\right)<\mathrm{\infty }$
for all $x,y,z\in X$. Let $f:X\to Y$ be an odd mapping satisfying (8.1). Then there exists a unique Cauchy additive mapping $A:X\to Y$ such that
$\parallel f\left(x\right)-A\left(x\right)\parallel \le \frac{1}{2}\mathrm{\Phi }\left(2x,0,-x\right)$
(8.3)

for all $x\in X$.

Proof The proof is similar to the proof of [, Theorem 4.2]. □

Remark 8.5 Let $r<1$. Letting $\phi \left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y\in X$ in Theorem 8.4, we obtain the inequality (8.3). The proof is given in [, Theorem 4.2].

## Declarations

### Acknowledgements

This work was supported by the Daejin University Research Grant in 2013.

## Authors’ Affiliations

(1)
Department of Mathematics, Research Institute for Natural Sciences, Hanyang University, Seoul, 133-791, Korea
(2)
Department of Mathematics, Daejin University, Kyeonggi, 487-711, Korea

## References 