\documentclass[12pt, reqno]{amsart} \usepackage{amsmath, amsthm, amscd, amsfonts, amssymb, graphicx, color} \usepackage[bookmarksnumbered, colorlinks, plainpages]{hyperref} \renewcommand{\baselinestretch}{1.1} \setlength{\textwidth}{16 cm} \setlength{\textheight}{8.75in} \setlength{\evensidemargin}{0.15in} \setlength{\oddsidemargin}{0.15in} \newcommand{\R}{\mbox{$\mathbb{R}$}} \newcommand{\N}{\mbox{$\mathbb{N}$}} \newcommand{\Q}{\mbox{$\mathbb{Q}$}} \newcommand{\C}{\mbox{$\mathbb{C}$}} \newcommand{\Z}{\mbox{$\mathbb{Z}$}} \newcommand{\K}{\mbox{$\mathbb{K}$}} \newcommand{\nwl}{\left (} \newcommand{\nwp}{\right )} \newcommand{\wzor}[1]{{\rm (\ref{#1})}} \newcommand{\ckd}{\hspace{0.5cm} $\square$\medskip\par\noindent} \newcommand{\dowod}{{\it Proof:}\ \ } \newcommand{\id}{{\mathcal J}} \newcommand{\ip}[2]{\left\langle#1|#2\right\rangle} \newcommand{\kropka}{\hspace{-1.3ex}.\ } \newcommand{\eps}{\varepsilon} \markboth {$~$ \hfill \footnotesize {\rm H. Azadi Kenary, Kh. Shafaat and H. Keshavarz } \hfill $~$} {$~$ \hfill \footnotesize {\rm Approximate $m$-Lie homomorphisms ...} \hfill$~$} \begin{document} \thispagestyle{empty} \setcounter{page}{1} \begin{center} {\large\bf Hyers-Ulam-Rassias Approximation on $m$-Lie Algebras} \vskip.35in {\bf H. Azadi Kenary , Kh. Shafaat and H. Keshavarz} \\[2mm] {\footnotesize Department of Mathematics, College of Sciences, Yasouj University, Yasouj 75914-353, Iran } \end{center} \vskip 3mm \noindent{\footnotesize{\bf Abstract.} Using the fixed point method, we establish the stability of $m-$Lie homomorphisms and Jordan $m-$Lie homomorphisms on $m-$Lie algebras associated to the following additive functional equation $$ 2\mu f\left(\sum_{i=1}^m m x_i\right)=\sum_{i=1}^mf\left(\mu \left(m x_i + \sum_{j=1~,i\neq j}^m x_j\right)\right)+f\left(\sum_{i=1}^m \mu x_i\right)$$ where $m$ be an integer greater than 2 and all $\mu\in \Bbb T_{\frac{1}{n_0}} :=\left\{e^{i\theta} \ ;\ 0\leq\theta\leq \frac{2\pi}{n_0}\right\}$. \vskip.15in \footnotetext { \textbf{2000 Mathematics Subject Classification}:17A42, 39B82, 39B52.} \footnotetext { \textbf{Keywords}: $m-$Lie algebra; homomorphism; Jordan homomorphism; Stability; Fixed point approach; functional equation } \footnotetext{\textbf{Corresponding author}{\tt : h.azadikenary@gmail.com}} \baselineskip=16pt %\theoremstyle{definition} \newtheorem{df}{Definition}[section] \newtheorem{rk}[df]{Remark} %\theoremstyle{plain} \newtheorem{lem}[df]{Lemma} \newtheorem{thm}[df]{Theorem} \newtheorem{pro}[df]{Proposition} \newtheorem{cor}[df]{Corollary} \newtheorem{ex}[df]{Example} \setcounter{section}{0} \numberwithin{equation}{section} \vskip .2in %-------------------------------------------------------------------------------------------------------------- \section{\bf Introduction} Let $n$ be a natural number greater or equal to 3. The notion of an $n-$Lie algebra was introduced by V.T. Filippov in 1985 \cite{1}. The Lie product is taken between $n$ elements of the algebra instead of two. This new bracket is $n-$linear, anti--symmetric and satisfies a generalization of the Jacobi identity. \\ An $n-$Lie algebra is a natural generalization of a Lie algebra. Namely: A vector space $V$ together with a multi--linear, antisymmetric $n-$ary operation $[~~~~ ] : \Lambda^nV \rightarrow V $ is called an $n-$Lie algebra, $n \geq 3$, if the $n-$ary bracket is a derivation with respect to itself, i.e, \begin{equation}\label{00} [[x_1,...,x_n],x_{n+1},...,x_{2n-1}]=\sum_{i=1}^n [x_1,...x_{i-1}[x_i, x_{n+1},...,x_{2n-1}],...,x_n] \end{equation} where $x_1,x_2,...,x_{2n-1} \in V$. Equation $(\ref{00})$ is called the generalized Jacobi identity. The meaning of this identity is similar to that of the usual Jacobi identity for a Lie algebra (which is a $2-$Lie algebra).\\ From now on, we only consider $n-$Lie algebras over the field of complex numbers. An $n-$Lie algebra $A$ is a normed $n-$Lie algebra if there exists a norm $||\ ||$ on $A$ such that $||[x_1,x_2,...,x_n]||\leq ||x_1|| ||x_2||... ||x_n||$ for all $x_1,x_2,...,x_n \in A$. A normed $n-$Lie algebra $A$ is called a Banach $n-$Lie algebra, if $(A,||\ ||)$ is a Banach space. Let $(A,[~~]_A)$ and $(B,[~~]_B)$ be two Banach $n-$Lie algebras. A $\Bbb C-$linear mapping $H:(A,[~]_A)\rightarrow(B,[~]_B)$ is called an $n-$Lie homomorphism if $H([x_1x_2...x_n]_A)=[H(x_1)H(x_2)...H(x_n)]_B$ for all $x_1,x_2,...,x_n \in A.$ A $\Bbb C-$linear mapping $H:(A,[~]_A)\rightarrow(B,[~]_B)$ is called a Jordan $n-$Lie homomorphism if $H([xx...x]_A)=[H(x)H(x)...H(x)]_B$ for all $x \in A.$ The study of stability problems had been formulated by Ulam \cite{10} during a talk in 1940: Under what condition does there exist a homomorphism near an approximate homomorphism? In the following year, Hyers \cite{11} was answered affirmatively the question of Ulam for Banach spaces, which states that if $\varepsilon > 0$ and $f :X\rightarrow¨ Y$ is a map with $X$ a normed space, $Y$ a Banach spaces such that \begin{equation}\label{E0} \|f(x + y)-f(x)-f(y)\|\leq \varepsilon \end{equation} for all $x, y \in X,$ then there exists a unique additive map $T : X \rightarrow Y$ such that $\|f(x)-T(x)\|\leq \varepsilon $ for all $x \in X.$ A generalized version of the theorem of Hyers for approximately linear mappings was presented by Rassias \cite{12} in 1978 by considering the case when inequality $(\ref{E0})$ is unbounded. Due to that fact, the additive functional equation $f(x + y)=f(x)+f(y)$ is said to have the generalized Hyers--Ulam--Rassias stability property. A large list of references concerning the stability of functional equations can be found in \cite{az}--\cite{azc}.\\ In this paper, by using the fixed point method, we establish the stability of $m-$Lie homomorphisms and Jordan $m-$Lie homomorphisms on $m-$Lie Banach algebras associated to the following generalized Jensen type functional equation \begin{equation}\label{e11} 2\mu f\left(\sum_{i=1}^m m x_i\right)-\sum_{i=1}^mf\left(\mu \left(m x_i + \sum_{j=1~,i\neq j}^m x_j\right)\right)-f\left(\sum_{i=1}^m \mu x_i\right)=0 \end{equation} for all $\mu\in \Bbb T_{\frac{1}{n_o}} :=\left\{e^{i\theta} \ ;\ 0\leq\theta\leq \frac{2\pi}{n_o}\right\},$ where $m\geq2.$ Throughout this paper, assume that $(A,[~]_A),(B,[~]_B)$ are two $m$-Lie Banach algebras. \vskip .2in %-------------------------------------------------------------------------------------------------------------- \section{\bf Main Results} Before proceeding to the main results, we recall a fundamental result in fixed point theory. \begin{thm}\label{t1.2}\cite{12} Let $(\Omega,d)$ be a complete generalized metric space and $T:\Omega\rightarrow\Omega$ be a strictly contractive function with Lipschitz constant $L$. Then for each given $x\in\Omega$, either $d(T^m x, T^{m+1} x)=\infty~$ for all $m\geq0,$ or other exists a natural number $m_{0}$ such that: $(i)$ $d(T^m x, T^{m+1} x)<\infty ~$for all $m \geq m_{0};$ $\bullet$ the sequence $\{T^m x\}$ is convergent to a fixed point $y^*$ of $~T$; $\bullet$ $y^*$ is the unique fixed point of $~T$ in $~\Lambda=\{y\in\Omega:d(T^{m_{0}} x, y)<\infty\};$ $\bullet$ $d(y,y^*)\leq\frac{1}{1-L}d(y, Ty)$ for all $~y\in\Lambda.$ \end{thm} \begin{thm}\cite{azc} Let $V$ and $W$ be real vector spaces. A mapping $f:V\rightarrow W$ satisfies the following functional equation $$2 f\left(\sum_{i=1}^m m x_i\right)=\sum_{i=1}^mf\left(m x_i + \sum_{j=1~,i\neq j}^m x_j\right)+f\left(\sum_{i=1}^m x_i\right)$$ if and only if $f$ is additive. \end{thm} We start our work with the main theorem of our paper. \begin{thm}\label{t2.1} Let $n_0 \in \Bbb N$ be a fixed positive integer. Let $f:A\rightarrow B$ be a mapping for which there exists a function $\varphi:A^{m}\rightarrow [0,\infty)$ such that \begin{eqnarray}\label{e21} \begin{split} \left\|2\mu f\left(\sum_{i=1}^m m x_i\right)-\sum_{i=1}^mf\left(\mu \left(m x_i + \sum_{j=1~,i\neq j}^m x_j\right)\right)-f\left(\sum_{i=1}^m \mu x_i\right)\right\|\leq\varphi(x_1,x_2,\cdots,x_m) \end{split} \end{eqnarray} for all $\mu\in \Bbb T_{\frac{1}{n_o}} :=\left\{e^{i\theta} \ ;\ 0\leq\theta\leq \frac{2\pi}{n_0}\right\}$ and all $x_1,\cdots,x_m \in A,$ and that \begin{eqnarray}\label{e211} \begin{split} \|f([x_1x_2 \cdots x_n]_A)-[f(x_1)f(x_2)\cdots f(x_m)]_B\|_B\leq\varphi(x_1,x_2,\cdots,x_m) \end{split} \end{eqnarray} for all $x_1,\cdots,x_m \in A.$ If there exists an $L< 1$ such that \begin{equation}\label{e22} \varphi(x_1,x_2,\cdots,x_m)\leq m L\varphi\left(\frac{x_1}{m},\frac{x_2}{m},\cdots,\frac{x_m}{m}\right) \end{equation} for all $x_1,\cdots,x_m \in A,$ then there exists a unique $m-$Lie homomorphism $H:A\rightarrow B$ such that \begin{equation}\label{e23} \|f(x)-H(x)\|\leq \frac{\varphi(x,0,0,\cdots,0)}{m-mL} \end{equation} for all $x \in A.$ \end{thm} \begin{proof} Let $\Omega$ be the set of all functions from $A$ into $B$ and let $d(g,h):=\inf\{C \in \Bbb R^+ :\|g(x)-h(x)\|_B\leq C\varphi(x,0,\cdots,0), \forall x\in A\}.$ It is easy to show that $(\Omega,d)$ is a generalized complete metric space \cite{41}. Now we define the mapping $J:\Omega\rightarrow \Omega$ by $J(h)(x)=\frac{1}{m}h(mx)$ for all $x \in A.$ Note that for all $g, h \in \Omega,$ \begin{align*} d (g,h)1.$ Suppose that a function $f:A\rightarrow B$ satisfying (\ref{xxx}) and (\ref{yyy}). Then there exists a unique $m-$Lie homomorphism $H:A\rightarrow B$ such that \begin{equation}\label{e2213x} \|f(x)-H(x)\| \leq \frac{m\theta\|x\|_{A}^{p}}{m^{p+1}-m^2} \end{equation} for all $x \in A.$ \end{cor} \begin{proof} Put $\varphi(x_1,x_2,\cdots,x_m):=\theta \sum^{m}_{i=1}(\|x_{i}\|_{A}^{p})$ for all $x_1,\cdots,x_n \in A$ in Theorem \ref{t2.1x}. Then $(\ref{e23x})$ holds for $p ~<1$, and $(\ref{e2213x})$ holds when $L=m^{(1-p)}.$ \end{proof} \begin{thm}\label{t2.2} Let $n_0 \in \Bbb N$ be a fixed positive integer. Let $f:A\rightarrow B$ be a mapping for which there exists a function $\varphi:A^{m}\rightarrow [0,\infty)$ such that \begin{eqnarray}\label{e221} \begin{split} \left\|2\mu f\left(\sum_{i=1}^m m x_i\right)-\sum_{i=1}^m f \left(\mu \left(m x_i + \sum_{j=1~,i\neq j}^m x_j\right)\right)-f\left(\sum_{i=1}^m \mu x_i\right)\right\| \leq\varphi(x_1,x_2,\cdots,x_m) \end{split} \end{eqnarray} for all $\mu\in \Bbb T_{\frac{1}{n_o}} :=\left\{e^{i\theta} \ ;\ 0\leq\theta\leq \frac{2\pi}{n_0}\right\}$ and all $x_1,\cdots,x_m \in A,$ and that \begin{eqnarray}\label{e2211} \begin{split}\label{jk} \|f([xx \cdots x]_A)-[f(x)f(x)\cdots f(x)]_B\|_B\leq\varphi(x,x,\cdots,x) \end{split} \end{eqnarray} for all $x \in A.$ If there exists an $L< 1$ such that \begin{equation}\label{e222} \varphi(x_1,x_2,\cdots,x_m)\leq m L\varphi\left(\frac{x_1}{m},\frac{x_2}{m},\cdots,\frac{x_m}{m}\right) \end{equation} for all $x_1,\cdots,x_m \in A,$ then there exists a unique Jordan $m-$Lie homomorphism $H:A\rightarrow B$ such that \begin{equation}\label{e223} \|f(x)-H(x)\|\leq \frac{\varphi(x,0,\cdots,0)}{m-mL} \end{equation} for all $x \in A.$ \end{thm} \begin{proof} By the same reasoning as in the proof of Theorem \ref{t2.1}, we can define the mapping $H(x)= \lim_{k \to \infty}\frac{1}{m^{ k}}f(m^{ k}x)$ for all $x \in A.$ Moreover, we can show that $H$ is $\Bbb C$--linear. It follows from $(\ref{e2211})$ that \begin{align*} \|&H([xx \cdots x]_A)-[H(x)H(x)\cdots H(x)]_B\|\\&=\lim_{k \to \infty}\left\|\frac{H([m^k x \cdots m^k x]_A)}{m^{mk}}-\frac{[H(m^k x)H(m^k x) \cdots H(m^k x)]_B}{m^{mk}}\right\|\leq \lim_{k \to \infty}\frac{1}{m^{mk}}\varphi(m^k x,m^k x,...,m^k x)=0 \end{align*} for all $x \in A.$ So $H([xx \cdots x]_A)=[H(x)H(x)\cdots H(x)]_B$ for all $x \in A.$ Hence $H:A\rightarrow B$ is a Jordan $m-Lie$ homomorphism satisfying $(\ref{e223}).$ \end{proof} \begin{cor}\label{c22} Let $\theta $ and $p$ be non--negative real numbers such that $p <1.$ Suppose that a function $f:A\rightarrow B$ satisfies \begin{align*} \left\|2\mu f\left(\sum_{i=1}^m m x_i\right)-\sum_{i=1}^m f \left(\mu \left(m x_i + \sum_{j=1~,i\neq j}^m x_j\right)\right)-f\left(\sum_{i=1}^m \mu x_i\right)\right\| \leq \theta\sum^{n}_{i=1}(\|x_{i}\|_{A}^{p}) \end{align*} for all $\mu\in \Bbb T_{\frac{1}{n_o}} $ and all $x_1,\cdots,x_m \in A$ and $ \|f([xx \cdots x]_A)-[f(x)f(x)\cdots f(x)]_B\| \leq n \theta(\|x\|_{A}^{p}) $ for all $x\in A.$ Then there exists a unique Jordan $m-$Lie homomorphism $H:A\rightarrow B$ such that \begin{equation}\label{e213} \|f(x)-H(x)\|_B \leq \frac{\theta\|x\|_{A}^{p}}{m-m^p} \end{equation} for all $x \in A.$ \end{cor} \begin{proof} It follows from Theorem \ref{t2.2} by putting $\varphi(x_1,x_2,\cdots,x_m):=\theta \sum^{m}_{i=1}(\|x_{i}\|_{A}^{p})$ for all $x_1,\cdots,x_m \in A$ and $L=m^{(p-1)}.$ \end{proof} Similarly, we have the following and we will omit the proof. \begin{thm}\label{t2.1xxx} Let $n_0 \in \Bbb N$ be a fixed positive integer. Let $f:A\rightarrow B$ be a mapping for which there exists a function $\varphi:A^{m}\rightarrow [0,\infty)$ satisfying (\ref{e21}) for all $\mu\in \Bbb T_{\frac{1}{n_o}} :=\left\{e^{i\theta} \ ;\ 0\leq\theta\leq \frac{2\pi}{n_o}\right\}$ and (\ref{e211}). If there exists an $L< 1$ such that $ \varphi\left(\frac{x_1}{m},\frac{x_2}{m},\cdots,\frac{x_m}{m}\right)\leq \frac{L}{m}\varphi(x_1,x_2,\cdots,x_m) $ for all $x_1,\cdots,x_m \in A,$ then there exists a unique Jordan $m-$Lie homomorphism $H:A\rightarrow B$ such that \begin{equation}\label{e23xx} \|f(x)-H(x)\|\leq \frac{L\varphi(x,0,0,\cdots,0)}{m-mL} \end{equation} for all $x \in A.$ \end{thm} \begin{cor} Let $\theta $ and $p$ be non--negative real numbers such that $p >1.$ Suppose that a function $f:A\rightarrow B$ satisfying (\ref{xxx}) and (\ref{jk}). Then there exists a unique Jordan $m-$Lie homomorphism $H:A\rightarrow B$ such that \begin{equation}\label{e2213xx} \|f(x)-H(x)\|_B \leq \frac{\theta\|x\|_{A}^{p}}{m^{p}-m} \end{equation} for all $x \in A.$ \end{cor} \begin{proof} Put $\varphi(x_1,x_2,\cdots,x_m):=\theta \sum^{m}_{i=1}(\|x_{i}\|_{A}^{p})$ for all $x_1,\cdots,x_n \in A$ in Theorem \ref{t2.1xxx}. Then $(\ref{e23xx})$ holds for $p ~>1$, and $(\ref{e2213xx})$ holds when $L=m^{(1-p)}.$ \end{proof} \vskip .2in %------------------------------------------------------------- \begin{thebibliography}{99} \bibitem{az}H. Azadi Kenary, {\it Random approximation of an additive functional equation of m-Apollonius type}, Acta Mathematica Scientia Volume 32, Issue 5, September 2012, Pages 1813-1825. \bibitem{azc} 17. H. Azadi Kenary and Y. J. Cho, {\it Stability of mixed additive-quadratic Jensen type functional equation in various spaces}, Computer and Mathematics with Applications, Vol. 61, Issue 9, 2704-2724, 2011. \bibitem{11} D.H. Hyers, {\it On the stability of the linear functional equation}, Proc. Natl. Acad. Sci. 27, 222--224 (1941). \bibitem {12} Th.M. Rassias, {\it On the stability of the linear mapping in Banach spaces}, Proc. Amer. Math. Soc. 72 297--300 (1978). \bibitem {10} S.M. Ulam, Chapter VI, science ed., Wiley, New York, 1940. \end{thebibliography} } \end{document}