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Zsolt Páles

Publications and source records attributed to Zsolt Páles.

87 records · Page 5Linked to original sources
arXiv2015

Characterizations of higher-order convexity properties with respect to Chebyshev systems

In this paper various notions of convexity of real functions with respect to Chebyshev systems defined over arbitrary subsets of the real line are introduced. As an auxiliary notion, a concept of a relevant divided difference and also a related lower Dinghas type derivative are also defined. The main results of the paper offer various characterizations of the convexity notions in terms of the nonnegativity of a generalized divided difference and the corresponding lower Dinghas type derivative.

math.CA↗
arXiv2015

Characterization of generalized quasi-arithmetic means

In this paper we characterize generalized quasi-arithmetic means, that is means of the form $M(x_1,...,x_n):=(f_1+...+f_n)^{-1}(f_1(x_1)+...+f_n(x_n))$, where $f_1,...,f_n:I\to\mathbb{R}$ are strictly increasing and continuous functions. Our characterization involves the Gauss composition of the cyclic mean-type mapping induced by $M$ and a generalized bisymmetry equation.

math.CA↗
arXiv2014

Additive solvability and linear independence of the solutions of a system of functional equations

The aim of this paper is twofold. On one hand, the additive solvability of the system of functional equations \[d_{k}(xy)=\sum_{i=0}^{k}Γ(i,k-i) d_{i}(x)d_{k-i}(y) \qquad (x,y\in \R,\,k\in\{0,\ldots,n\}) \] is studied, where $Δ_n:=\big\{(i,j)\in\Z\times\Z\mid 0\leq i,j\mbox{and}i+j\leq n\big\}$ and $Γ\colonΔ_n\to\R$ is a symmetric function such that $Γ(i,j)=1$ whenever $i\cdot j=0$. On the other hand, the linear dependence and independence of the additive solutions $d_{0},d_{1},\dots,d_{n}\colon \R\to\R$ of the above system of equations is characterized. As a consequence of the main result, for any nonzero real derivation $d\colon\R\to\R$, the iterates $d^0,d^1,\dots,d^n$ of $d$ are shown to be linearly independent, and the graph of the mapping $x\mapsto (x,d^1(x),\dots,d^n(x))$ to be dense in $\R^{n+1}$.

math.AC↗
arXiv2014

Bernstein--Doetsch type theorems for set-valued maps of strongly and approximately convex and concave type

In this paper, we investigate properties of set-valued mappings that establish connection between the values of this map at two arbitrary points of the domain and the value at their midpoint. Such properties are, for instance, Jensen convexity/concavity, $K$-Jensen convexity/concavity (where $K$ is the set of nonnegative elements of an ordered vector space), and approximate/strong $K$-Jensen convexity/concavity. Assuming weak but natural regularity assumptions on the set-valued map, our main purpose is to deduce the convexity/concavity consequences of these properties in the appropriate sense. Our two main theorems will generalize most of the known results in this field, in particular the celebrated Bernstein--Doetsch Theorem from 1915, and thus they offer a unified view of these theories.

math.CA↗
arXiv2014

A contraction principle in semimetric spaces

A branch of generalizations of the Banach Fixed Point Theorem replaces contractivity by a weaker but still effective property. The aim of the present note is to extend the contraction principle in this spirit for such complete semimetric spaces that fulfill an extra regularity property. The stability of fixed points is also investigated in this setting. As applications, fixed point results are presented for several important generalizations of metric spaces.

math.FA↗
arXiv2012

Characterization of segment and convexity preserving maps

In this note functions that transform open segments of a linear space into open segments of another linear space are studied and characterized. Assuming that the range is non-collinear, it is proved that such a map can always be expressed as the ratio of two affine functions.

math.CA↗
arXiv2012

Computer aided solution of the invariance equation for two-variable Stolarsky means

We solve the so-called invariance equation in the class of two-variable Stolarsky means ${S_{p,q}:p,q\in\R}$, i.e., we find necessary and sufficient conditions on the 6 parameters $a,b,c,d,p,q$ such that the identity [S_{p,q}\big(S_{a,b}(x,y),S_{c,d}(x,y)\big)=S_{p,q}(x,y) \qquad (x,y \in \R_+)] be valid. We recall that, for $pq(p-q)\neq 0$ and $x\neq y$, the Stolarsky mean $S_{p,q}$ is defined by [S_{p,q}(x,y):=(\dfrac{q(x^p-y^p)}{p(x^q-y^q)})^{\frac1{p-q}}.] In the proof first we approximate the Stolarsky mean and we use the computer algebra system Maple V Release 9 to compute the Taylor expansion of the approximation up to 12th order, which enables us to describe all the cases of the equality.

math.CA↗
arXiv2012

On a generalization of the plank problem

Let two concentric circles $k$ and $K$ be given on the plane with radii $r$ and $R$, $r<R$. Assume that the disc bounded by $k$ is covered by angular domains whose vertices are within $K$. Then the sum of the angles of these angular domains is greater than or equal to the view angle of $k$ from an arbitrary point of $K$.

math.CA↗
arXiv2012

Computer aided solution of the invariance equation for two-variable Gini means

Our aim is to solve the so-called invariance equation in the class of two-variable Gini means ${G_{p,q}:p,q\in\R}$, i.e., to find necessary and sufficient conditions on the 6 parameters $a,b,c,d,p,q$ such that the identity [G_{p,q}\big(G_{a,b}(x,y),G_{c,d}(x,y)\big)=G_{p,q}(x,y) \qquad (x,y \in \R_+)] be valid. We recall that, for $p\neq q$, the Gini mean $G_{p,q}$ is defined by [G_{p,q}(x,y):=(\dfrac{x^p+y^p}{x^q+y^q})^{\frac1{p-q}}\qquad (x,y \in \R_+).] The proof uses the computer algebra system Maple V Release 9 to compute a Taylor expansion up to 12th order, which enables us to describe all the cases of the equality.

math.CA↗
arXiv2012

Implications between approximate convexity properties and approximate Hermite-Hadamard inequalities

In this paper, the connection between the functional inequalities $$ f\Big(\frac{x+y}{2}\Big)\leq\frac{f(x)+f(y)}{2}+α_J(x-y) \qquad (x,y\in D)$$ and $$ \int_0^1f\big(tx+(1-t)y\big)ρ(t)dt \leqλf(x)+(1-λ)f(y) +α_H(x-y) \qquad (x,y\in D)$$ is investigated, where $D$ is a convex subset of a linear space, $f:D\to\R$, $α_H,α_J:D-D\to\R$ are even functions, $λ\in[0,1]$, and $ρ:[0,1]\to\R_+$ is an integrable nonnegative function with $\int_0^1ρ(t)dt=1$.

math.CA↗
arXiv2012

On strong $(α,\F)$-convexity

In this paper, strongly $(α,T)$-convex functions, i.e., functions $f:D\to \R$ satisfying the functional inequality $$ f(tx+(1-t)y)\leq tf(x)+(1-t)f(y)-tα\big((1-t)(x-y)\big)-(1-t)α\big(t(y-x)\big)$$ for $x,y\in D$ and $t\in T\cap[0,1]$ are investigated. Here $D$ is a convex set in a linear space, $α$ is a nonnegative function on $D-D$, and $T\subseteq\R$ is a nonempty set. The main results provide various characterizations of strong $(α,T)$-convexity in the case when $T$ is a subfield of $\R$.

math.CA↗
arXiv2012

On $φ$-convexity

In this paper, approximate convexity and approximate midconvexity properties, called $φ$-convexity and $φ$-midconvexity, of real valued function are investigated. Various characterizations of $φ$-convex and $φ$-midconvex functions are obtained. Furthermore, the relationship between $φ$-midconvexity and $φ$-convexity is established.

math.CA↗
arXiv2012

Wigner's theorem revisited

In this paper, we give the general solution of the functional equation $$\big\{\|f(x)+f(y)\|,\|f(x)-f(y)\|\big\}=\big\{\|x+y\|,\|x-y\|\big\}\qquad(x,y\in X)$$ where $f:X\to Y$ and $X,Y$ are inner product spaces. Related equations are also considered. Our main tool is a real version of Wigner's unitary--antiunitary theorem.

math.CA↗
arXiv2006

Generalized Jacobian for Functions with Infinite Dimensional Range and Domain

In this paper, locally Lipschitz functions acting between infinite dimensional normed spaces are considered. When the range is a dual space and satisfies the Radon--Nikodým property, Clarke's generalized Jacobian will be extended to this setting. Characterization and fundamental properties of the extended generalized Jacobian are established including the nonemptiness, the $β$-compactness, the $β$-upper semicontinuity, and a mean-value theorem. A connection with known notions is provided and chain rules are proved using key results developed. This included the vectorization and restriction theorem, and the extension theorem. Therefore, the generalized Jacobian introduced in this paper is proved to enjoy all the properties required of a derivative like-set.

math.FA↗