ContentsIntroduction............................................................................................................................................................................... 50. Explanatory notes, definitions and a lemma................................................................................................................. 51. Some fixed point theorems..................................................................................................................................................

In [4], assuming among others subadditivity and submultiplicavity of a function ψ: [0, ∞)→[0, ∞), the authors proved a Hyers-Ulam type stability theorem for “ψ-additive” mappings of a normed space into a normed space. In this note we show that the assumed conditions of the function ψ imply that ψ=0 and, consequently, every “ψ-additive” mapping must be additive

Let (Ω,Σ,μ) be a measure space with two sets A,B ∈ Σ such that 0 < μ (A) < 1 < μ (B) < ∞ and suppose that ϕ and ψ are arbitrary bijections of [0,∞) such that ϕ(0) = ψ(0) = 0. The main result says that if ${\u0283}_{\Omega}xyd\mu \le {\varphi}^{-1}\left({\u0283}_{\Omega}\varphi \circ xd\mu \right){\psi}^{-1}\left({\u0283}_{\Omega}\psi \circ xd\mu \right)$ for all μ-integrable nonnegative step functions x,y then ϕ and ψ must be conjugate power functions. If the measure space (Ω,Σ,μ) has one of the following properties: (a) μ (A) ≤ 1 for every A ∈ Σ of finite measure; (b) μ (A) ≥ 1 for every A ∈ Σ of positive measure, then there exist...

A generalization of the weighted quasi-arithmetic mean generated by continuous and increasing (decreasing) functions $f\u2081,...,{f}_{k}:I\to \mathbb{R}$, k ≥ 2, denoted by ${A}^{[f\u2081,...,{f}_{k}]}$, is considered. Some properties of ${A}^{[f\u2081,...,{f}_{k}]}$, including “associativity” assumed in the Kolmogorov-Nagumo theorem, are shown. Convex and affine functions involving this type of means are considered. Invariance of a quasi-arithmetic mean with respect to a special mean-type mapping built of generalized means is applied in solving a functional equation. For a sequence of...

The functional equation
(F(x)-F(y))/(x-y) = (G(x)+G(y))(H(x)+H(y))
where F,G,H are unknown functions is considered. Some motivations, coming from the equality problem for means, are presented.

An invariance formula in the class of generalized p-variable quasiarithmetic means is provided. An effective form of the limit of the sequence of iterates of mean-type mappings of this type is given. An application to determining functions which are invariant with respect to generalized quasiarithmetic mean-type mappings is presented.

The class of all functions f:(0,∞) → (0,∞) which are continuous at least at one point and affine with respect to the logarithmic mean is determined. Some related results concerning the functions convex with respect to the logarithmic mean are presented.

Let (X, d) be a metric space and T: X → X a continuous map. If the sequence (T n)n∈ℕ of iterates of T is pointwise convergent in X, then for any x ∈ X, the limit $${\mu}_{T}\left(x\right)=\underset{n\to \infty}{lim}{T}^{n}\left(x\right)$$
is a fixed point of T. The problem of determining the form of µT leads to the invariance equation µT ○ T = µT, which is difficult to solve in general if the set of fixed points of T is not a singleton. We consider this problem assuming that X = I p, where I is a real interval, p ≥ 2 a fixed positive integer and T is the mean-type mapping...

For a differentiable function $\mathbf{f}:I\to {\mathbb{R}}^{k},$ where $I$ is a real interval and $k\in \mathbb{N}$, a counterpart of the Lagrange mean-value theorem is presented. Necessary and sufficient conditions for the existence of a mean $M:{I}^{2}\to I$ such that$$\mathbf{f}\left(x\right)-\mathbf{f}\left(y\right)=(x-y){\mathbf{f}}^{\text{'}}\left(M(x,y)\right),\phantom{\rule{1.0em}{0ex}}x,y\in I,$$
are given. Similar considerations for a theorem accompanying the Lagrange mean-value theorem are presented.

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