# Functional equations and inequalities in paranormed spaces

## 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.

## 1 Introduction and preliminaries

The concept of statistical convergence for sequences of real numbers was introduced by Fast [1] and Steinhaus [2] independently, and since then several generalizations and applications of this notion have been investigated by various authors (see [3â€“7]). This notion was defined in normed spaces by Kolk [8].

We recall some basic facts concerning FrÃ©chet spaces.

Definition 1.1 [9]

Let X be a vector space. A paranorm $P:Xâ†’\left[0,\mathrm{âˆž}\right)$ is a function on X such that

1. (1)

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

2. (2)

$P\left(âˆ’x\right)=P\left(x\right)$;

3. (3)

$P\left(x+y\right)â‰¤P\left(x\right)+P\left(y\right)$ (triangle inequality)

4. (4)

If $\left\{{t}_{n}\right\}$ is a sequence of scalars with ${t}_{n}â†’t$ and $\left\{{x}_{n}\right\}âŠ‚X$ with $P\left({x}_{n}âˆ’x\right)â†’0$, then $P\left({t}_{n}{x}_{n}âˆ’tx\right)â†’0$ (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 FrÃ©chet space is a total and complete paranormed space.

The stability problem of functional equations originated from a question of Ulam [10] concerning the stability of group homomorphisms. Hyers [11] gave the first affirmative partial answer to the question of Ulam for Banach spaces. Hyersâ€™ theorem was generalized by Aoki [12] for additive mappings and by Rassias [13] for linear mappings by considering an unbounded Cauchy difference. A generalization of the Rassias theorem was obtained by GÄƒvruta [14] by replacing the unbounded Cauchy difference by a general control function in the spirit of Rassiasâ€™ approach.

In 1990, Rassias [15] during the 27th International Symposium on Functional Equations asked the question whether such a theorem can also be proved for $pâ‰¥1$. In 1991, Gajda [16], following the same approach as in Rassias [13], gave an affirmative solution to this question for $p>1$. It was shown by Gajda [16], as well as by Rassias and Å emrl [17], that one cannot prove a Rassias-type theorem when $p=1$ (cf. the books of Czerwik [18], Hyers, Isac and Rassias [19]).

The functional equation

$f\left(x+y\right)+f\left(xâˆ’y\right)=2f\left(x\right)+2f\left(y\right)$
(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 [20] for mappings $f:Xâ†’Y$, where X is a normed space and Y is a Banach space. Cholewa [21] noticed that the theorem of Skof is still true if the relevant domain X is replaced by an Abelian group. Czerwik [22] 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 [23â€“29]).

In [30], Jun and Kim considered the following cubic functional equation:

$\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)=f\left(x+y\right)+f\left(xâˆ’y\right)+6f\left(x\right).$
(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 [31], Lee et al. considered the following quartic functional equation:

$\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)=2f\left(x+y\right)+2f\left(xâˆ’y\right)+12f\left(x\right)âˆ’3f\left(y\right).$
(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 [32], GilÃ¡nyi showed that if f satisfies the functional inequality

$âˆ¥2f\left(x\right)+2f\left(y\right)âˆ’f\left(x{y}^{âˆ’1}\right)âˆ¥â‰¤âˆ¥f\left(xy\right)âˆ¥,$
(1.4)

then f satisfies the Jordan-von Neumann functional equation

$2f\left(x\right)+2f\left(y\right)=f\left(xy\right)+f\left(x{y}^{âˆ’1}\right).$

See also [33]. Fechner [34] and GilÃ¡nyi [35] proved the Hyers-Ulam stability of the functional inequality (1.4).

Park, Cho and Han [36] proved the Hyers-Ulam stability of the following functional inequalities:

(1.5)
(1.6)
(1.7)

Throughout this paper, assume that $\left(X,P\right)$ is a FrÃ©chet space and that $\left(Y,âˆ¥â‹\dots âˆ¥\right)$ 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 $m:SÃ—Sâ†’\left[0,\mathrm{âˆž}\right]$ 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 $x,yâˆˆS$;

3. (3)

$m\left(x,z\right)â‰¤m\left(x,y\right)+m\left(y,z\right)$ for all $x,y,zâˆˆS$.

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 $J:Sâ†’S$ be a strictly contractive mapping with a Lipschitz constant $\mathrm{Î±}<1$. Then, for each given element $xâˆˆS$, either

$m\left({J}^{n}x,{J}^{n+1}x\right)=\mathrm{âˆž}$

for all nonnegative integers n or there exists a positive integer ${n}_{0}$ such that

1. (1)

$m\left({J}^{n}x,{J}^{n+1}x\right)<\mathrm{âˆž}$, $\mathrm{âˆ€}nâ‰¥{n}_{0}$;

2. (2)

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

3. (3)

${y}^{âˆ—}$ is the unique fixed point of J in the set $W=\left\{yâˆˆSâˆ£m\left({J}^{{n}_{0}}x,y\right)<\mathrm{âˆž}\right\}$;

4. (4)

$m\left(y,{y}^{âˆ—}\right)â‰¤\frac{1}{1âˆ’\mathrm{Î±}}m\left(y,Jy\right)$ for all $yâˆˆW$.

In 1996, Isac and Rassias [39] 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 [40â€“44]).

Note that $P\left(2x\right)â‰¤2P\left(x\right)$ for all $xâˆˆY$.

Theorem 2.2 Let $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤2\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2}\right)$
(2.1)

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping such that

$âˆ¥f\left(x+y\right)âˆ’f\left(x\right)âˆ’f\left(y\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,y\right)$
(2.2)

for all $x,yâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2âˆ’2\mathrm{Î±}}\mathrm{Ï†}\left(x,x\right)$
(2.3)

for all $xâˆˆX$.

Proof Letting $y=x$ in (2.2), we get

$âˆ¥f\left(2x\right)âˆ’2f\left(x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,x\right),$

and so

$âˆ¥f\left(x\right)âˆ’\frac{1}{2}f\left(2x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Ï†}\left(x,x\right)$
(2.4)

for all $xâˆˆX$.

Consider the set

$S:=\left\{h:Xâ†’Y\right\}$

and introduce the generalized metric on S:

$m\left(g,h\right)=inf\left\{\mathrm{Î¼}âˆˆ{\mathbb{R}}_{+}:âˆ¥g\left(x\right)âˆ’h\left(x\right)âˆ¥â‰¤\mathrm{Î¼}\mathrm{Ï†}\left(x,x\right),\phantom{\rule{0.25em}{0ex}}\mathrm{âˆ€}xâˆˆX\right\},$

where, as usual, $inf\mathrm{Ï•}=+\mathrm{âˆž}$. It is easy to show that $\left(S,m\right)$ is complete (see [[45], Lemma 2.1]).

Now we consider the linear mapping $J:Sâ†’S$ such that

$Jh\left(x\right):=\frac{1}{2}h\left(2x\right)$

for all $xâˆˆX$.

Let $g,hâˆˆS$ be given such that $m\left(g,h\right)=\mathrm{Îµ}$. Since

$âˆ¥Jg\left(x\right)âˆ’Jh\left(x\right)âˆ¥=âˆ¥\frac{1}{2}g\left(2x\right)âˆ’\frac{1}{2}h\left(2x\right)âˆ¥â‰¤\mathrm{Î±}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆX$, $m\left(g,h\right)=\mathrm{Îµ}$ implies that $m\left(Jg,Jh\right)â‰¤\mathrm{Î±}\mathrm{Îµ}$. This means that

$m\left(Jg,Jh\right)â‰¤\mathrm{Î±}m\left(g,h\right)$

for all $g,hâˆˆS$.

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

By Theorem 2.1, there exists a mapping $A:Xâ†’Y$ 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 $xâˆˆX$. The mapping A is a unique fixed point of J in the set

$M=\left\{gâˆˆS:m\left(f,g\right)<\mathrm{âˆž}\right\}.$

This implies that A is a unique mapping satisfying (2.5) such that there exists a $\mathrm{Î¼}âˆˆ\left(0,\mathrm{âˆž}\right)$ satisfying

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\mathrm{Î¼}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆX$;

1. (2)

$m\left({J}^{n}f,A\right)â†’0$ as $nâ†’\mathrm{âˆž}$. This implies the equality

$\underset{nâ†’\mathrm{âˆž}}{lim}\frac{1}{{2}^{n}}f\left({2}^{n}x\right)=A\left(x\right)$

for all $xâˆˆX$;

1. (3)

$m\left(f,A\right)â‰¤\frac{1}{1âˆ’\mathrm{Î±}}m\left(f,Jf\right)$, which implies the inequality

$m\left(f,A\right)â‰¤\frac{1}{2âˆ’2\mathrm{Î±}}.$

This implies that the inequality (2.3) holds true.

It follows from (2.1) and (2.2) that

$\begin{array}{rcl}âˆ¥A\left(x+y\right)âˆ’A\left(x\right)âˆ’A\left(y\right)âˆ¥& =& \underset{nâ†’\mathrm{âˆž}}{lim}\frac{1}{{2}^{n}}âˆ¥f\left({2}^{n}\left(x+y\right)\right)âˆ’f\left({2}^{n}x\right)âˆ’f\left({2}^{n}y\right)âˆ¥\\ â‰¤& \underset{nâ†’\mathrm{âˆž}}{lim}\frac{{2}^{n}{\mathrm{Î±}}^{n}}{{2}^{n}}\mathrm{Ï†}\left(x,y\right)=0\end{array}$

for all $x,yâˆˆX$. So, $A\left(x+y\right)âˆ’A\left(x\right)âˆ’A\left(y\right)=0$ for all $x,yâˆˆX$. Thus $A:Xâ†’Y$ is an additive mapping, as desired.â€ƒâ–¡

Corollary 2.3 Let r be a positive real number with $r<1$, and let $f:Xâ†’Y$ be a mapping such that

$âˆ¥f\left(x+y\right)âˆ’f\left(x\right)âˆ’f\left(y\right)âˆ¥â‰¤P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$

for all $x,yâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{2}{2âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(2.6)

for all $xâˆˆX$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’1}$ in Theorem 2.2, we get the desired result.â€ƒâ–¡

Theorem 2.4 Let $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{2}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y\right)<\mathrm{âˆž}$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping satisfying (2.2). Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Î¦}\left(x,x\right)$

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[46], Theorem 2.2].â€ƒâ–¡

Remark 2.5 Let $r<1$. Letting $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 2.4, we obtain the inequality (2.6). The proof is given in [[46], Theorem 2.2].

Theorem 2.6 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤\frac{\mathrm{Î±}}{2}\mathrm{Ï†}\left(2x,2y\right)$
(2.7)

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping such that

$P\left(f\left(x+y\right)âˆ’f\left(x\right)âˆ’f\left(y\right)\right)â‰¤\mathrm{Ï†}\left(x,y\right)$
(2.8)

for all $x,yâˆˆY$. Then there exists a unique additive mapping $A:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’A\left(x\right)\right)â‰¤\frac{\mathrm{Î±}}{2âˆ’2\mathrm{Î±}}\mathrm{Ï†}\left(x,x\right)$
(2.9)

for all $xâˆˆY$.

Proof Letting $y=x$ in (2.8), we get

$P\left(f\left(2x\right)âˆ’2f\left(x\right)\right)â‰¤\mathrm{Ï†}\left(x,x\right),$

and so

$P\left(f\left(x\right)âˆ’2f\left(\frac{x}{2}\right)\right)â‰¤\frac{\mathrm{Î±}}{2}\mathrm{Ï†}\left(x,x\right)$
(2.10)

for all $xâˆˆY$.

Consider the set

$S:=\left\{h:Yâ†’X\right\}$

and introduce the generalized metric on S:

$m\left(g,h\right)=inf\left\{\mathrm{Î¼}âˆˆ{\mathbb{R}}_{+}:P\left(g\left(x\right)âˆ’h\left(x\right)\right)â‰¤\mathrm{Î¼}\mathrm{Ï†}\left(x,x\right),\phantom{\rule{0.25em}{0ex}}\mathrm{âˆ€}xâˆˆY\right\},$

where, as usual, $inf\mathrm{Ï•}=+\mathrm{âˆž}$. It is easy to show that $\left(S,m\right)$ is complete (see [[45], Lemma 2.1]).

Now we consider the linear mapping $J:Sâ†’S$ such that

$Jh\left(x\right):=2h\left(\frac{x}{2}\right)$

for all $xâˆˆY$.

Let $g,hâˆˆS$ be given such that $m\left(g,h\right)=\mathrm{Îµ}$. Since

$P\left(Jg\left(x\right)âˆ’Jh\left(x\right)\right)=P\left(2g\left(\frac{x}{2}\right)âˆ’2h\left(\frac{x}{2}\right)\right)â‰¤\mathrm{Î±}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆY$, $m\left(g,h\right)=\mathrm{Îµ}$ implies that $m\left(Jg,Jh\right)â‰¤\mathrm{Î±}\mathrm{Îµ}$. This means that

$m\left(Jg,Jh\right)â‰¤\mathrm{Î±}m\left(g,h\right)$

for all $g,hâˆˆS$.

It follows from (2.10) that $m\left(f,Jf\right)â‰¤\frac{\mathrm{Î±}}{2}$.

By Theorem 2.1, there exists a mapping $A:Xâ†’Y$ 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 $xâˆˆX$. The mapping A is a unique fixed point of J in the set

$M=\left\{gâˆˆS:m\left(f,g\right)<\mathrm{âˆž}\right\}.$

This implies that A is a unique mapping satisfying (2.11) such that there exists a $\mathrm{Î¼}âˆˆ\left(0,\mathrm{âˆž}\right)$ satisfying

$P\left(f\left(x\right)âˆ’A\left(x\right)\right)â‰¤\mathrm{Î¼}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆY$;

1. (2)

$m\left({J}^{n}f,A\right)â†’0$ as $nâ†’\mathrm{âˆž}$. This implies the equality

$\underset{nâ†’\mathrm{âˆž}}{lim}{2}^{n}f\left(\frac{x}{{2}^{n}}\right)=A\left(x\right)$

for all $xâˆˆY$;

1. (3)

$m\left(f,A\right)â‰¤\frac{1}{1âˆ’\mathrm{Î±}}m\left(f,Jf\right)$, which implies the inequality

$m\left(f,A\right)â‰¤\frac{\mathrm{Î±}}{2âˆ’2\mathrm{Î±}}.$

This implies that the inequality (2.9) holds true.

It follows from (2.7) and (2.8) that

$\begin{array}{rcl}P\left(A\left(x+y\right)âˆ’A\left(x\right)âˆ’A\left(y\right)\right)& =& \underset{nâ†’\mathrm{âˆž}}{lim}P\left({2}^{n}\left(f\left(\frac{x+y}{{2}^{n}}\right)âˆ’f\left(\frac{x}{{2}^{n}}\right)âˆ’f\left(\frac{y}{{2}^{n}}\right)\right)\right)\\ â‰¤& \underset{nâ†’\mathrm{âˆž}}{lim}{2}^{n}P\left(\left(f\left(\frac{x+y}{{2}^{n}}\right)âˆ’f\left(\frac{x}{{2}^{n}}\right)âˆ’f\left(\frac{y}{{2}^{n}}\right)\right)\right)\\ â‰¤& \underset{nâ†’\mathrm{âˆž}}{lim}\frac{{2}^{n}{\mathrm{Î±}}^{n}}{{2}^{n}}\mathrm{Ï†}\left(x,y\right)=0\end{array}$

for all $x,yâˆˆY$. So, $A\left(x+y\right)âˆ’A\left(x\right)âˆ’A\left(y\right)=0$ for all $x,yâˆˆY$. Thus $A:Yâ†’X$ is an additive mapping, as desired.â€ƒâ–¡

Corollary 2.7 Let r, Î¸ be positive real numbers with $r>1$, and let $f:Yâ†’X$ be a mapping such that

$P\left(f\left(x+y\right)âˆ’f\left(x\right)âˆ’f\left(y\right)\right)â‰¤\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$

for all $x,yâˆˆY$. Then there exists a unique Cauchy additive mapping $A:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’A\left(x\right)\right)â‰¤\frac{2\mathrm{Î¸}}{{2}^{r}âˆ’2}{âˆ¥xâˆ¥}^{r}$
(2.12)

for all $xâˆˆY$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{1âˆ’r}$ in Theorem 2.6, we get the desired result.â€ƒâ–¡

Theorem 2.8 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=1}{\overset{\mathrm{âˆž}}{âˆ‘}}{2}^{j}\mathrm{Ï†}\left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{âˆž}$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping satisfying (2.8). Then there exists a unique Cauchy additive mapping $A:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’A\left(x\right)\right)â‰¤\frac{1}{2}\mathrm{Î¦}\left(x,x\right)$

for all $xâˆˆY$.

Proof The proof is similar to the proof of [[46], Theorem 2.1].â€ƒâ–¡

Remark 2.9 Let $r>1$. Letting $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ in Theorem 2.8, we obtain the inequality (2.12). The proof is given in [[46], 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 $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤4\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2}\right)$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping satisfying $f\left(0\right)=0$ and

$âˆ¥f\left(x+y\right)+f\left(xâˆ’y\right)âˆ’2f\left(x\right)âˆ’2f\left(y\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,y\right)$
(3.1)

for all $x,yâˆˆX$. Then there exists a unique quadratic mapping ${Q}_{2}:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’{Q}_{2}\left(x\right)âˆ¥â‰¤\frac{1}{4âˆ’4\mathrm{Î±}}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆX$.

Proof Letting $y=x$ in (3.1), we get

$âˆ¥f\left(2x\right)âˆ’4f\left(x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,x\right),$

and so

$âˆ¥f\left(x\right)âˆ’\frac{1}{4}f\left(2x\right)âˆ¥â‰¤\frac{1}{4}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆX$.

The rest of the proof is similar to the proof of Theorem 2.2.â€ƒâ–¡

Corollary 3.2 Let r be a positive real number with $r<2$, and let $f:Xâ†’Y$ be a mapping satisfying $f\left(0\right)=0$ and

$âˆ¥f\left(x+y\right)+f\left(xâˆ’y\right)âˆ’2f\left(x\right)âˆ’2f\left(y\right)âˆ¥â‰¤P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$

for all $x,yâˆˆX$. Then there exists a unique quadratic mapping ${Q}_{2}:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’{Q}_{2}\left(x\right)âˆ¥â‰¤\frac{2}{4âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(3.2)

for all $xâˆˆX$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’2}$ in Theorem 3.1, we get the desired result.â€ƒâ–¡

Theorem 3.3 Let $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{4}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y\right)<\mathrm{âˆž}$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping satisfying $f\left(0\right)=0$ and (3.1). Then there exists a unique quadratic mapping ${Q}_{2}:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’{Q}_{2}\left(x\right)âˆ¥â‰¤\frac{1}{4}\mathrm{Î¦}\left(x,x\right)$

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[46], Theorem 3.2].â€ƒâ–¡

Remark 3.4 Let $r<2$. Letting $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 3.3, we obtain the inequality (3.2). The proof is given in [[46], Theorem 3.2].

Theorem 3.5 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤\frac{\mathrm{Î±}}{4}\mathrm{Ï†}\left(2x,2y\right)$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and

$P\left(f\left(x+y\right)+f\left(xâˆ’y\right)âˆ’2f\left(x\right)âˆ’2f\left(y\right)\right)â‰¤\mathrm{Ï†}\left(x,y\right)$
(3.3)

for all $x,yâˆˆY$. Then there exists a unique quadratic mapping ${Q}_{2}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{2}\left(x\right)\right)â‰¤\frac{\mathrm{Î±}}{4âˆ’4\mathrm{Î±}}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆY$.

Proof Letting $y=x$ in (3.3), we get

$P\left(f\left(2x\right)âˆ’4f\left(x\right)\right)â‰¤\mathrm{Ï†}\left(x,x\right),$

and so

$P\left(f\left(x\right)âˆ’4f\left(\frac{x}{2}\right)\right)â‰¤\mathrm{Ï†}\left(\frac{x}{2},\frac{x}{2}\right)â‰¤\frac{\mathrm{Î±}}{4}\mathrm{Ï†}\left(x,x\right)$

for all $xâˆˆY$.

The rest of the proof is similar to the proof of Theorem 2.6.â€ƒâ–¡

Corollary 3.6 Let r, Î¸ be positive real numbers with $r>2$, and let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and

$P\left(f\left(x+y\right)+f\left(xâˆ’y\right)âˆ’2f\left(x\right)âˆ’2f\left(y\right)\right)â‰¤\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$

for all $x,yâˆˆY$. Then there exists a unique quadratic mapping ${Q}_{2}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{2}\left(x\right)\right)â‰¤\frac{2\mathrm{Î¸}}{{2}^{r}âˆ’4}{âˆ¥xâˆ¥}^{r}$
(3.4)

for all $xâˆˆY$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{2âˆ’r}$ in Theorem 3.5, we get the desired result.â€ƒâ–¡

Theorem 3.7 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=1}{\overset{\mathrm{âˆž}}{âˆ‘}}{4}^{j}\mathrm{Ï†}\left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{âˆž}$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and (3.3). Then there exists a unique quadratic mapping ${Q}_{2}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{2}\left(x\right)\right)â‰¤\frac{1}{4}\mathrm{Î¦}\left(x,x\right)$

for all $xâˆˆY$.

Proof The proof is similar to the proof of [[46], Theorem 3.1].â€ƒâ–¡

Remark 3.8 Let $r>2$. Letting $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ in Theorem 3.7, we obtain the inequality (3.4). The proof is given in [[46], 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 $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤8\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2}\right)$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping such that

$âˆ¥\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’f\left(x+y\right)âˆ’f\left(xâˆ’y\right)âˆ’6f\left(x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,y\right)$
(4.1)

for all $x,yâˆˆX$. Then there exists a unique cubic mapping $C:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’C\left(x\right)âˆ¥â‰¤\frac{1}{8âˆ’8\mathrm{Î±}}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆX$.

Proof Letting $y=0$ in (4.1), we get

$âˆ¥f\left(2x\right)âˆ’8f\left(x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,0\right),$

and so

$âˆ¥f\left(x\right)âˆ’\frac{1}{8}f\left(2x\right)âˆ¥â‰¤\frac{1}{8}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆX$.

The rest of the proof is similar to the proof of Theorem 2.2.â€ƒâ–¡

Corollary 4.2 Let r be a positive real number with $r<3$, and let $f:Yâ†’X$ be a mapping such that

$âˆ¥\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’f\left(x+y\right)âˆ’f\left(xâˆ’y\right)âˆ’6f\left(x\right)âˆ¥â‰¤P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$

for all $x,yâˆˆX$. Then there exists a unique cubic mapping $C:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’C\left(x\right)\right)â‰¤\frac{1}{8âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(4.2)

for all $xâˆˆY$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’3}$ in Theorem 4.1, we get the desired result.â€ƒâ–¡

Theorem 4.3 Let $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{8}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y\right)<\mathrm{âˆž}$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping satisfying (4.1). Then there exists a unique cubic mapping $C:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’C\left(x\right)âˆ¥â‰¤\frac{1}{8}\mathrm{Î¦}\left(x,0\right)$

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[46], Theorem 4.2].â€ƒâ–¡

Remark 4.4 Let $r<3$. Letting $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 4.3, we obtain the inequality (4.2). The proof is given in [[46], Theorem 4.2].

Theorem 4.5 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤\frac{\mathrm{Î±}}{8}\mathrm{Ï†}\left(2x,2y\right)$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping such that

$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’f\left(x+y\right)âˆ’f\left(xâˆ’y\right)âˆ’6f\left(x\right)\right)â‰¤\mathrm{Ï†}\left(x,y\right)$
(4.3)

for all $x,yâˆˆY$. Then there exists a unique cubic mapping $C:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’C\left(x\right)\right)â‰¤\frac{\mathrm{Î±}}{8âˆ’8\mathrm{Î±}}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆY$.

Proof Letting $y=0$ in (4.3), we get

$P\left(f\left(2x\right)âˆ’8f\left(x\right)\right)â‰¤\mathrm{Ï†}\left(x,0\right),$

and so

$P\left(f\left(x\right)âˆ’8f\left(\frac{x}{2}\right)\right)â‰¤\mathrm{Ï†}\left(\frac{x}{2},\frac{x}{2}\right)â‰¤\frac{\mathrm{Î±}}{8}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆY$.

The rest of the proof is similar to the proof of Theorem 2.6.â€ƒâ–¡

Corollary 4.6 Let r, Î¸ be positive real numbers with $r>3$, and let $f:Yâ†’X$ be a mapping such that

$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’f\left(x+y\right)âˆ’f\left(xâˆ’y\right)âˆ’6f\left(x\right)\right)â‰¤\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$

for all $x,yâˆˆY$. Then there exists a unique cubic mapping $C:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’C\left(x\right)\right)â‰¤\frac{\mathrm{Î¸}}{{2}^{r}âˆ’8}{âˆ¥xâˆ¥}^{r}$
(4.4)

for all $xâˆˆY$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{3âˆ’r}$ in Theorem 4.5, we get the desired result.â€ƒâ–¡

Theorem 4.7 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=1}{\overset{\mathrm{âˆž}}{âˆ‘}}{8}^{j}\mathrm{Ï†}\left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{âˆž}$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping satisfying (4.3). Then there exists a unique cubic mapping $C:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’C\left(x\right)\right)â‰¤\frac{1}{8}\mathrm{Î¦}\left(x,0\right)$

for all $xâˆˆY$.

Proof The proof is similar to the proof of [[46], Theorem 4.1].â€ƒâ–¡

Remark 4.8 Let $r>3$. Letting $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ in Theorem 4.7, we obtain the inequality (4.4). The proof is given in [[46], 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 $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤16\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2}\right)$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping satisfying $f\left(0\right)=0$ and

$âˆ¥\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’2f\left(x+y\right)âˆ’2f\left(xâˆ’y\right)âˆ’12f\left(x\right)+3f\left(y\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,y\right)$
(5.1)

for all $x,yâˆˆX$. Then there exists a unique quartic mapping ${Q}_{4}:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’{Q}_{4}\left(x\right)âˆ¥â‰¤\frac{1}{16âˆ’16\mathrm{Î±}}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆX$.

Proof Letting $y=0$ in (5.1), we get

$âˆ¥f\left(2x\right)âˆ’16f\left(x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,0\right),$

and so

$âˆ¥f\left(x\right)âˆ’\frac{1}{16}f\left(2x\right)âˆ¥â‰¤\frac{1}{16}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆX$.

The rest of the proof is similar to the proof of Theorem 2.2.â€ƒâ–¡

Corollary 5.2 Let r be a positive real number with $r<4$, and let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and

$âˆ¥\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’2f\left(x+y\right)âˆ’2f\left(xâˆ’y\right)âˆ’12f\left(x\right)+3f\left(y\right)âˆ¥â‰¤P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$

for all $x,yâˆˆX$. Then there exists a unique quartic mapping ${Q}_{4}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{4}\left(x\right)\right)â‰¤\frac{1}{16âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(5.2)

for all $xâˆˆY$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’4}$ in Theorem 5.1, we get the desired result.â€ƒâ–¡

Theorem 5.3 Let $\mathrm{Ï†}:{X}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{16}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y\right)<\mathrm{âˆž}$

for all $x,yâˆˆX$. Let $f:Xâ†’Y$ be a mapping satisfying $f\left(0\right)=0$ and (5.1). Then there exists a unique quartic mapping ${Q}_{4}:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’{Q}_{4}\left(x\right)âˆ¥â‰¤\frac{1}{16}\mathrm{Î¦}\left(x,0\right)$

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[46], Theorem 5.2].â€ƒâ–¡

Remark 5.4 Let $r<4$. Letting $\mathrm{Ï†}\left(x,y\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 5.3, we obtain the inequality (5.2). The proof is given in [[46], Theorem 5.2].

Theorem 5.5 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y\right)â‰¤\frac{\mathrm{Î±}}{16}\mathrm{Ï†}\left(2x,2y\right)$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and

$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’2f\left(x+y\right)âˆ’2f\left(xâˆ’y\right)âˆ’12f\left(x\right)+3f\left(y\right)\right)â‰¤\mathrm{Ï†}\left(x,y\right)$
(5.3)

for all $x,yâˆˆY$. Then there exists a unique quartic mapping ${Q}_{4}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{4}\left(x\right)\right)â‰¤\frac{\mathrm{Î±}}{16âˆ’16\mathrm{Î±}}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆY$.

Proof Letting $y=0$ in (5.3), we get

$P\left(f\left(2x\right)âˆ’16f\left(x\right)\right)â‰¤\mathrm{Ï†}\left(x,0\right),$

and so

$P\left(f\left(x\right)âˆ’16f\left(\frac{x}{2}\right)\right)â‰¤\mathrm{Ï†}\left(\frac{x}{2},\frac{x}{2}\right)â‰¤\frac{\mathrm{Î±}}{16}\mathrm{Ï†}\left(x,0\right)$

for all $xâˆˆY$.

The rest of the proof is similar to the proof of Theorem 2.6.â€ƒâ–¡

Corollary 5.6 Let r, Î¸ be positive real numbers with $r>4$, and let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and

$P\left(\frac{1}{2}f\left(2x+y\right)+\frac{1}{2}f\left(2xâˆ’y\right)âˆ’2f\left(x+y\right)âˆ’2f\left(xâˆ’y\right)âˆ’12f\left(x\right)+3f\left(y\right)\right)â‰¤\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$

for all $x,yâˆˆY$. Then there exists a unique quartic mapping ${Q}_{4}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{4}\left(x\right)\right)â‰¤\frac{\mathrm{Î¸}}{{2}^{r}âˆ’16}{âˆ¥xâˆ¥}^{r}$
(5.4)

for all $xâˆˆY$.

Proof Taking $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ and choosing $\mathrm{Î±}={2}^{4âˆ’r}$ in Theorem 5.5, we get the desired result.â€ƒâ–¡

Theorem 5.7 Let $\mathrm{Ï†}:{Y}^{2}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y\right):=\underset{j=1}{\overset{\mathrm{âˆž}}{âˆ‘}}{16}^{j}\mathrm{Ï†}\left(\frac{x}{{2}^{j}},\frac{y}{{2}^{j}}\right)<\mathrm{âˆž}$

for all $x,yâˆˆY$. Let $f:Yâ†’X$ be a mapping satisfying $f\left(0\right)=0$ and (5.3). Then there exists a unique quartic mapping ${Q}_{4}:Yâ†’X$ such that

$P\left(f\left(x\right)âˆ’{Q}_{4}\left(x\right)\right)â‰¤\frac{1}{16}\mathrm{Î¦}\left(x,0\right)$

for all $xâˆˆY$.

Proof The proof is similar to the proof of [[46], Theorem 5.1].â€ƒâ–¡

Remark 5.8 Let $r>4$. Letting $\mathrm{Ï†}\left(x,y\right)=\mathrm{Î¸}\left({âˆ¥xâˆ¥}^{r}+{âˆ¥yâˆ¥}^{r}\right)$ for all $x,yâˆˆX$ in Theorem 5.7, we obtain the inequality (5.4). The proof is given in [[46], 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 [[36], Proposition 2.1]

Let $f:Xâ†’Y$ be a mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+f\left(z\right)âˆ¥â‰¤âˆ¥2f\left(\frac{x+y+z}{2}\right)âˆ¥$

for all $x,y,zâˆˆX$. Then f is Cauchy additive.

Theorem 6.2 Let $\mathrm{Ï†}:{X}^{3}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y,z\right)â‰¤2\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2},\frac{z}{2}\right)$
(6.1)

for all $x,y,zâˆˆX$. Let $f:Xâ†’Y$ be an odd mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+f\left(z\right)âˆ¥â‰¤âˆ¥2f\left(\frac{x+y+z}{2}\right)âˆ¥+\mathrm{Ï†}\left(x,y,z\right)$
(6.2)

for all $x,y,zâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2âˆ’2\mathrm{Î±}}\mathrm{Ï†}\left(x,x,âˆ’2x\right)$
(6.3)

for all $xâˆˆX$.

Proof Letting $y=x$ and $z=âˆ’2x$ in (6.2), we get

$âˆ¥2f\left(x\right)âˆ’f\left(2x\right)âˆ¥=âˆ¥2f\left(x\right)+f\left(âˆ’2x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,x,âˆ’2x\right),$

and so

$âˆ¥f\left(x\right)âˆ’\frac{1}{2}f\left(2x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Ï†}\left(x,x,âˆ’2x\right)$
(6.4)

for all $xâˆˆX$.

Consider the set

$S:=\left\{h:Xâ†’Y\right\}$

and introduce the generalized metric on S

$m\left(g,h\right)=inf\left\{\mathrm{Î¼}âˆˆ{\mathbb{R}}_{+}:âˆ¥g\left(x\right)âˆ’h\left(x\right)âˆ¥â‰¤\mathrm{Î¼}\mathrm{Ï†}\left(x,x,âˆ’2x\right),\phantom{\rule{0.25em}{0ex}}\mathrm{âˆ€}xâˆˆX\right\},$

where, as usual, $inf\mathrm{Ï•}=+\mathrm{âˆž}$. It is easy to show that $\left(S,m\right)$ is complete (see [[45], Lemma 2.1]).

Now we consider the linear mapping $J:Sâ†’S$ such that

$Jh\left(x\right):=\frac{1}{2}h\left(2x\right)$

for all $xâˆˆX$.

Let $g,hâˆˆS$ be given such that $m\left(g,h\right)=\mathrm{Îµ}$. Since

$âˆ¥Jg\left(x\right)âˆ’Jh\left(x\right)âˆ¥=âˆ¥\frac{1}{2}g\left(2x\right)âˆ’\frac{1}{2}h\left(2x\right)âˆ¥â‰¤\mathrm{Î±}\mathrm{Ï†}\left(x,x,âˆ’2x\right)$

for all $xâˆˆX$, $m\left(g,h\right)=\mathrm{Îµ}$ implies that $m\left(Jg,Jh\right)â‰¤\mathrm{Î±}\mathrm{Îµ}$. This means that

$m\left(Jg,Jh\right)â‰¤\mathrm{Î±}m\left(g,h\right)$

for all $g,hâˆˆS$.

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

By Theorem 2.1, there exists a mapping $A:Xâ†’Y$ 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 $xâˆˆX$. The mapping A is a unique fixed point of J in the set

$M=\left\{gâˆˆS:m\left(f,g\right)<\mathrm{âˆž}\right\}.$

This implies that A is a unique mapping satisfying (6.5) such that there exists a $\mathrm{Î¼}âˆˆ\left(0,\mathrm{âˆž}\right)$ satisfying

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\mathrm{Î¼}\mathrm{Ï†}\left(x,x,âˆ’2x\right)$

for all $xâˆˆX$;

1. (2)

$m\left({J}^{n}f,A\right)â†’0$ as $nâ†’\mathrm{âˆž}$. This implies the equality

$\underset{nâ†’\mathrm{âˆž}}{lim}\frac{1}{{2}^{n}}f\left({2}^{n}x\right)=A\left(x\right)$

for all $xâˆˆX$;

1. (3)

$m\left(f,A\right)â‰¤\frac{1}{1âˆ’\mathrm{Î±}}m\left(f,Jf\right)$, which implies the inequality

$m\left(f,A\right)â‰¤\frac{1}{2âˆ’2\mathrm{Î±}}.$

This implies that the inequality (6.3) holds true.

It follows from (6.1) and (6.2) that

$\begin{array}{rcl}\frac{1}{{2}^{n}}âˆ¥f\left({2}^{n}x\right)+f\left({2}^{n}y\right)+f\left({2}^{n}z\right)âˆ¥& â‰¤& \frac{1}{{2}^{n}}âˆ¥2f\left(\frac{{2}^{n}\left(x+y+z\right)}{2}\right)âˆ¥+\frac{1}{{2}^{n}}\mathrm{Ï†}\left({2}^{n}x,{2}^{n}y,{2}^{n}z\right)\\ â‰¤& \frac{1}{{2}^{n}}âˆ¥2f\left(\frac{{2}^{n}\left(x+y+z\right)}{2}\right)âˆ¥+\frac{{2}^{n}{\mathrm{Î±}}^{n}}{{2}^{n}}\mathrm{Ï†}\left(x,y,z\right)\end{array}$
(6.6)

for all $x,y,zâˆˆX$. Letting $nâ†’\mathrm{âˆž}$ in (6.6), we get

$âˆ¥A\left(x\right)+A\left(y\right)+A\left(z\right)âˆ¥â‰¤âˆ¥2A\left(\frac{x+y+z}{2}\right)âˆ¥$

for all $x,y,zâˆˆX$. By Proposition 6.1, $A:Xâ†’Y$ is Cauchy additive, as desired.â€ƒâ–¡

Corollary 6.3 [[47], Theorem 2.2]

Let r be a positive real number with $r<1$, and let $f:Xâ†’Y$ be an odd mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+f\left(z\right)âˆ¥â‰¤âˆ¥2f\left(\frac{x+y+z}{2}\right)âˆ¥+P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$

for all $x,y,zâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{2+{2}^{r}}{2âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(6.7)

for all $xâˆˆX$.

Proof Taking $\mathrm{Ï†}\left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y,zâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’1}$ in Theorem 6.2, we get the desired result.â€ƒâ–¡

Theorem 6.4 Let $\mathrm{Ï†}:{X}^{3}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y,z\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{2}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y,{2}^{j}z\right)<\mathrm{âˆž}$

for all $x,y,zâˆˆX$. Let $f:Xâ†’Y$ be an odd mapping satisfying (6.2). Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Î¦}\left(x,x,âˆ’2x\right)$

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[47], Theorem 2.2].â€ƒâ–¡

Remark 6.5 Let $r<1$. Letting $\mathrm{Ï†}\left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 6.4, we obtain the inequality (6.7). The proof is given in [[47], 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 [[36], Proposition 2.2]

Let $f:Xâ†’Y$ be a mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+f\left(z\right)âˆ¥â‰¤âˆ¥f\left(x+y+z\right)âˆ¥$

for all $x,y,zâˆˆX$. Then f is Cauchy additive.

Theorem 7.2 Let $\mathrm{Ï†}:{X}^{3}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y,z\right)â‰¤2\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2},\frac{z}{2}\right)$

for all $x,y,zâˆˆX$. Let $f:Xâ†’Y$ be an odd mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+f\left(z\right)âˆ¥â‰¤âˆ¥f\left(x+y+z\right)âˆ¥+\mathrm{Ï†}\left(x,y,z\right)$
(7.1)

for all $x,y,zâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2âˆ’2\mathrm{Î±}}\mathrm{Ï†}\left(x,x,âˆ’2x\right)$

for all $xâˆˆX$.

Proof Letting $y=x$ and $z=âˆ’2x$ in (7.1), we get

$âˆ¥2f\left(x\right)âˆ’f\left(2x\right)âˆ¥=âˆ¥2f\left(x\right)+f\left(âˆ’2x\right)âˆ¥â‰¤\mathrm{Ï†}\left(x,x,âˆ’2x\right),$

and so

$âˆ¥f\left(x\right)âˆ’\frac{1}{2}f\left(2x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Ï†}\left(x,x,âˆ’2x\right)$
(7.2)

for all $xâˆˆX$.

The rest of the proof is similar to the proof of Theorem 6.2.â€ƒâ–¡

Corollary 7.3 [[47], Theorem 3.2]

Let r be a positive real number with $r<1$, and let $f:Xâ†’Y$ be an odd mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+f\left(z\right)âˆ¥â‰¤âˆ¥f\left(x+y+z\right)âˆ¥+P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$

for all $x,y,zâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{2+{2}^{r}}{2âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(7.3)

for all $xâˆˆX$.

Proof Taking $\mathrm{Ï†}\left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y,zâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’1}$ in Theorem 7.2, we get the desired result.â€ƒâ–¡

Theorem 7.4 Let $\mathrm{Ï†}:{X}^{3}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y,z\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{2}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y,{2}^{j}z\right)<\mathrm{âˆž}$

for all $x,y,zâˆˆX$. Let $f:Xâ†’Y$ be an odd mapping satisfying (7.1). Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Î¦}\left(x,x,âˆ’2x\right)$

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[47], Theorem 3.2].â€ƒâ–¡

Remark 7.5 Let $r<1$. Letting $\mathrm{Ï†}\left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 7.4, we obtain the inequality (7.3). The proof is given in [[47], 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 [[36], Proposition 2.3]

Let $f:Xâ†’Y$ be a mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+2f\left(z\right)âˆ¥â‰¤âˆ¥2f\left(\frac{x+y}{2}+z\right)âˆ¥$

for all $x,y,zâˆˆX$. Then f is Cauchy additive.

Theorem 8.2 Let $\mathrm{Ï†}:{X}^{3}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that there exists an $\mathrm{Î±}<1$ with

$\mathrm{Ï†}\left(x,y,z\right)â‰¤2\mathrm{Î±}\mathrm{Ï†}\left(\frac{x}{2},\frac{y}{2},\frac{z}{2}\right)$

for all $x,y,zâˆˆX$. Let $f:Xâ†’Y$ be an odd mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+2f\left(z\right)âˆ¥â‰¤âˆ¥2f\left(\frac{x+y}{2}+z\right)âˆ¥+\mathrm{Ï†}\left(x,y,z\right)$
(8.1)

for all $x,y,zâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2âˆ’2\mathrm{Î±}}\mathrm{Ï†}\left(2x,0,âˆ’x\right)$

for all $xâˆˆX$.

Proof Replacing x by 2x and letting $y=0$ and $z=âˆ’x$ in (8.1), we get

$âˆ¥2f\left(x\right)âˆ’f\left(2x\right)âˆ¥=âˆ¥2f\left(x\right)+f\left(âˆ’2x\right)âˆ¥â‰¤\mathrm{Ï†}\left(2x,0,âˆ’x\right)$

for all $xâˆˆX$.

The rest of the proof is the same as in the proof of Theorem 6.2.â€ƒâ–¡

Corollary 8.3 [[47], Theorem 4.2]

Let r be a positive real number with $r<1$, and let $f:Xâ†’Y$ be an odd mapping such that

$âˆ¥f\left(x\right)+f\left(y\right)+2f\left(z\right)âˆ¥â‰¤âˆ¥2f\left(\frac{x+y}{2}+z\right)âˆ¥+P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$

for all $x,y,zâˆˆX$. Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1+{2}^{r}}{2âˆ’{2}^{r}}P{\left(x\right)}^{r}$
(8.2)

for all $xâˆˆX$.

Proof Taking $\mathrm{Ï†}\left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,y,zâˆˆX$ and choosing $\mathrm{Î±}={2}^{râˆ’1}$ in Theorem 8.2, we get the desired result.â€ƒâ–¡

Theorem 8.4 Let $\mathrm{Ï†}:{X}^{3}â†’\left[0,\mathrm{âˆž}\right)$ be a function such that

$\mathrm{Î¦}\left(x,y,z\right):=\underset{j=0}{\overset{\mathrm{âˆž}}{âˆ‘}}\frac{1}{{2}^{j}}\mathrm{Ï†}\left({2}^{j}x,{2}^{j}y,{2}^{j}z\right)<\mathrm{âˆž}$

for all $x,y,zâˆˆX$. Let $f:Xâ†’Y$ be an odd mapping satisfying (8.1). Then there exists a unique Cauchy additive mapping $A:Xâ†’Y$ such that

$âˆ¥f\left(x\right)âˆ’A\left(x\right)âˆ¥â‰¤\frac{1}{2}\mathrm{Î¦}\left(2x,0,âˆ’x\right)$
(8.3)

for all $xâˆˆX$.

Proof The proof is similar to the proof of [[47], Theorem 4.2].â€ƒâ–¡

Remark 8.5 Let $r<1$. Letting $\mathrm{Ï†}\left(x,y,z\right)=P{\left(x\right)}^{r}+P{\left(y\right)}^{r}+P{\left(z\right)}^{r}$ for all $x,yâˆˆX$ in Theorem 8.4, we obtain the inequality (8.3). The proof is given in [[47], Theorem 4.2].

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## Acknowledgements

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

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Correspondence to Jung Rye Lee.

### Competing interests

The authors declare that they have no competing interests.

### Authorsâ€™ contributions

Both authors conceived of the study, participated in its design and coordination, drafted the manuscript, participated in the sequence alignment, and read and approved the final manuscript.

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Park, C., Lee, J.R. Functional equations and inequalities in paranormed spaces. J Inequal Appl 2013, 198 (2013). https://doi.org/10.1186/1029-242X-2013-198