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Abstract In this article, with simple and short proofs without applying fixed point theorems, some hyperstability results corresponding to the functional equations of Cauchy and Jensen are presented in 2-normed spaces. We also obtain some results on hyperstability for the general linear functional equation f ( a x + b y ) = A f ( x ) + B f ( y ) + C $f(ax+by)=Af(x)+Bf(y)+C$ .

Abstract In this paper we give some hyperstability and stability results for the Cauchy and Jensen functional equations on restricted domains. We provide a simple and short proof for Brzdȩk’s result concerning a hyperstability result for the Cauchy equation.

Abstract The main aim of this paper is to establish the Hyers–Ulam stability and hyperstability of a Jensen-type quadratic mapping in 2-Banach spaces. That is, we prove the various types of Hyers–Ulam stability and hyperstability of the Jensen-type quadratic functional equation of the form g ( x + y 2 + z ) + g ( x + y 2 − z ) + g ( x − y 2 + z ) + g ( x − y 2 − z ) = g ( x ) + g ( y ) + 4 g ( z ) , $$ g \biggl( \frac{x+y}{2} + z \biggr) + g \biggl( \frac{x+y}{2} - z \biggr) + g \biggl( \frac{x-y}{2} + z \biggr) + g \biggl( \frac{x-y}{2} - z \biggr) = g(x) + g(y) + 4 g(z), $$ in 2-Banach spaces by using the Hyers direct method.

In this paper, we investigate the Hyers-Ulam stability of additive and Fréchet functional equations on restricted domains. We improve the bounds and thus the results obtained by S. M. Jung and J. M. Rassias. As a consequence, we obtain asymptotic behaviors of functional equations of different types. One of the objectives of this paper is to bring out the involvement of functional equations in various characterizations of inner product spaces.

In this paper, we investigate the Hyers-Ulam stability problem of the following functional equation $ f(x+y)+g(x-y) = h(x)+k(y), $ on an unbounded restricted domain, which generalizes some of the results already obtained by other authors (for example [9,Theorem 2], [11,Theorem 5] and [21,Theorem 2]). Particular cases of this functional equation are Cauchy, Jensen, quadratic and Drygas functional equations. As a consequence, we obtain asymptotic behaviors of this functional equation.

Abstract The aim of this paper is to prove the superstability of the following functional equations: f ( P ( x , y ) ) = g ( x ) h ( y ) , f ( x + y ) = g ( x ) h ( y ) . $$\begin{aligned}& f \bigl(P(x,y) \bigr)= g(x)h(y), \\& f(x+y)=g(x)h(y). \end{aligned}$$

In this paper, we have improved some of the results in [C. Choi and B. Lee, Stability of Mixed Additive-Quadratic and Additive--Drygas Functional Equations. Results Math. 75 no. 1 (2020), Paper No. 38]. Indeed, we investigate the Hyers-Ulam stability problem of the following functional equations\begin{align*} 2\varphi(x + y) + \varphi(x - y) &= 3\varphi(x)+ 3\varphi(y) \\ 2\psi(x + y) + \psi(x - y) &= 3\psi(x) + 2\psi(y) + \psi(-y).\end{align*}We also consider the Pexider type functional equation \[2\psi(x + y) + \psi(x - y) = f(x) + g(y),\] and the additive functional equation\[2\psi(x + y) + \psi(x - y) = 3\psi(x) + \psi(y).\]

Abstract In this paper, we introduce the functional equations f ( 2 x − y ) + f ( x + 2 y ) = 5 [ f ( x ) + f ( y ) ] , f ( 2 x − y ) + f ( x + 2 y ) = 5 f ( x ) + 4 f ( y ) + f ( − y ) , f ( 2 x − y ) + f ( x + 2 y ) = 5 f ( x ) + f ( 2 y ) + f ( − y ) , f ( 2 x − y ) + f ( x + 2 y ) = 4 [ f ( x ) + f ( y ) ] + [ f ( − x ) + f ( − y ) ] . $$\begin{aligned} f(2x-y)+f(x+2y)&=5\bigl[f(x)+f(y)\bigr], \\ f(2x-y)+f(x+2y)&=5f(x)+4f(y)+f(-y), \\ f(2x-y)+f(x+2y)&=5f(x)+f(2y)+f(-y), \\ f(2x-y)+f(x+2y)&=4\bigl[f(x)+f(y)\bigr]+\bigl[f(-x)+f(-y)\bigr]. \end{aligned}$$ We show that these functional equations are quadratic and apply them to characterization of inner product spaces. We also investigate the stability problem on restricted domains. These results are applied to study the asymptotic behaviors of these quadratic functions in complete β-normed spaces.

In this paper, we acquire the general solution of the generalized quadratic functional equation \[ \begin{aligned} \sum_{1 \leq a < b < c \leq m}\varphi\left(r_{a}+r_{b}+r_{c}\right)&=(m-2)\sum_{1\leq a < b\leq m}\varphi\left(r_{a}+r_{b}\right) \\ &\quad-\left(\frac{m^{2}-3m+2}{2}\right)\nonumber \sum_{a=1}^{m}\frac{\varphi\left(r_{a}\right)+\varphi\left(-r_{a}\right)}{2} \end{aligned} \] where $m\geqslant 3$ is an integer. We also investigate Hyers-Ulam stability results by means of using alternative fixed point theorem for this generalized quadratic functional equation.

Abstract First we investigate the Hyers–Ulam stability of the Cauchy functional equation for mappings from bounded (unbounded) intervals into Banach spaces. Then we study the Hyers–Ulam stability of the functional equation f ( x y ) = x g ( y ) + h ( x ) y $f(xy)=xg(y)+h(x)y$ for mappings from bounded (unbounded) intervals into multi-normed spaces.

In this paper, we give some properties of the bi-Jensen functional equation and investigate its Hyers–Ulam stability and hyperstability. We construct a function which is bi-Jensen and is not continuous. Additionally, we investigate the Hyers–Ulam stability of the bi-Jensen functional equation on some restricted unbounded domains. Finally, we apply the obtained results to study some interesting asymptotic behaviors of bi-Jensen functions.

In this paper, we deal with a 2-linear functional equation. The Hyers-Ulam stability of this functional equation is shown on some restricted unbounded domains, and the obtained results are applied to get several hyperstability consequences. Moreover, some asymptotic behaviors of 2-linear functions are investigated. We also study the Hyers-Ulam stability and superstability of the 2-linear functional equation in 2-Banach spaces.

In this paper, we introduce the concept of $g$-dual frames for Hilbert $C^{*}$-modules, and then the properties and stability results of $g$-dual frames are given. A characterization of $g$-dual frames, approximately dual frames and dual frames of a given frame is established. We also give some examples to show that the characterization of $g$-dual frames for Riesz bases in Hilbert spaces is not satisfied in general Hilbert $C^*$-modules.

In this paper, we investigated the asymptotic stability behaviour of the Pexider–Cauchy functional equation in non-Archimedean spaces. We also showed that, under some conditions, if ∥f(x+y)−g(x)−h(y)∥⩽ε, then f,g and h can be approximated by additive mapping in non-Archimedean normed spaces. Finally, we deal with a functional inequality and its asymptotic behaviour.

Let 𝑋 be a normed space and 𝑌 a sequentially complete Hausdorff topological vector space over the field ℚ of rational numbers. Let 𝐷1={(𝑥,𝑦)∈𝑋×𝑋∶‖𝑥‖+‖𝑦‖≥𝑑}, and 𝐷2={(𝑥,𝑦)∈𝑋×𝑋∶‖𝑥‖+‖𝑦‖0. We prove that the Pexiderized Jensen functional equation is stable for functions defined on 𝐷1(𝐷2), and taking values in 𝑌. We consider also the Pexiderized Cauchy functional equation.

Let X,Y be Banach modules over a C∗-algebra and let r1,…,rn∈ℝ be given. We prove the generalized Hyers-Ulam stability of the following functional equation in Banach modules over a unital C∗-algebra: ∑j=1nf(−rjxj+∑1≤i≤n,i≠jrixi)+2∑i=1nrif(xi)=nf(∑i=1nrixi). We show that if ∑i=1nri≠0, ri,rj≠0 for some 1≤i

We use a fixed point method to investigate the stability problem of the quadratic functional equation f(x+y)+f(x−y)=2f(xx∗+yy∗) in C∗-algebras.

Using the fixed point method, we prove the generalized Hyers-Ulam stability of the generalized quadratic functional equation f(rx+sy)=r2f(x)+s2f(y)+(rs/2)[f(x+y)−f(x−y)] in Banach modules, where r,s are nonzero rational numbers with r2+s2≠1.

Let V,W be real vector spaces. It is shown that an odd mapping f:V→W satisfies ∑i−12nf(xi−1/2n∑j=12nxj)=∑i=12nf(xi)−2nf(1/2n∑i=12nxi) for all x1,…,x2n∈V if and only if the odd mapping f:V→W is Cauchy additive. Furthermore, we prove the generalized Hyers-Ulam stability of the above functional equation in real Banach spaces.

In 2006, C. Park proved the stability of homomorphisms in C∗-ternary algebras and of derivations on C∗-ternary algebras for the following generalized Cauchy-Jensen additive mapping: 2f((∑j=1pxj/2)+∑j=1dyj)=∑j=1pf(xj)+2∑j=1df(yj). In this note, we improve and generalize some results concerning this functional equation.

We investigate the stability problem for the following functional inequality ∥αf((x+y)/2α)+βf((y+z)/2β)+γf((z+x)/2γ)∥≤∥f(x+y+z)∥ on restricted domains of Banach modules over a C∗-algebra. As an application we study the asymptotic behavior of a generalized additive mapping.

We will introduce linear operators and obtain their exact norms defined on the function spaces Xλ and Zλ5. These operators are constructed from the Euler-Lagrange type cubic functional equations and their Pexider versions.

Using the Hyers-Ulam-Rassias stability method of functional equations, we investigate homomorphisms in C*-algebras, Lie C*-algebras, and JC*-algebras, and derivations on C*-algebras, Lie C*-algebras, and JC*-algebras associated with the following Apollonius-type additive functional equation f(z−x)+f(z−y)+(1/2)f(x+y)=2f(z−(x+y)/4).