arXiv subjects

Andrzej Wiśnicki

Publications and source records attributed to Andrzej Wiśnicki.

At least 19 recordsLinked to original sources

arXiv2023

On the Karlsson-Nussbaum conjecture for resolvents of nonexpansive mappings

Let $D\subset \mathbb{R}^{n}$ be a bounded convex domain and $F:D\rightarrow D$ a $1$-Lipschitz mapping with respect to the Hilbert metric $d$ on $D$ satisfying condition $d(sx+(1-s)y,sz+(1-s)w)\leq \max \{d(x,z),d(y,w) \}$. We show that if $F$ does not have fixed points, then the convex hull of the accumulation points (in the norm topology) of the family $\{R_{λ}\}_{λ>0}$ of resolvents of $F$ is a subset of $\partial D.$ As a consequence, we show a Wolff-Denjoy type theorem for resolvents of nonexpansive mappings acting on an ellipsoid $D$.

math.FA↗

arXiv2023

A fixed point theorem in $B(H,\ell _{\infty })$

We show that if $X$ is a complete metric space with uniform relative normal structure and $G$ is a subgroup of the isometry group of $X$ with bounded orbits, then there is a point in $X$ fixed by every isometry in $G$. As a corollary, we obtain a theorem of U. Lang (2013) concerning injective metric spaces. A few applications of this theorem are given to the problems of inner derivations. In particular, we show that if $L_{1}(μ)$ is an essential Banach $L_{1}(G)$-bimodule, then any continuous derivation $δ:L_{1}(G)\rightarrow L_{\infty }(μ)$ is inner. This extends a theorem of B. E. Johnson (1991) asserting that the convolution algebra $L_{1}(G)$ is weakly amenable if $G$ is a locally compact group.

math.FA↗

arXiv2021

Wolff-Denjoy theorems in geodesic spaces

We show a Wolff-Denjoy type theorem in complete geodesic spaces in the spirit of Beardon's framework that unifies several results in this area. In particular, it applies to strictly convex bounded domains in $\mathbb{R}^{n}$ or $\mathbb{C}^{n}$ with respect to a large class of metrics including Hilbert's and Kobayashi's metrics. The results are generalized to $1$-Lipschitz compact mappings in infinite-dimesional Banach spaces.

math.FA↗

arXiv2020

Around the nonlinear Ryll-Nardzewski theorem

Suppose that $Q$ is a weak$^{\ast }$ compact convex subset of a dual Banach space with the Radon-Nikodým property. We show that if $(S,Q)$ is a nonexpansive and norm-distal dynamical system, then there is a fixed point of $S$ in $Q$ and the set of fixed points is a nonexpansive retract of $Q.$ As a consequence we obtain a nonlinear extension of the Bader-Gelander-Monod theorem concerning isometries in $L$-embedded Banach spaces. A similar statement is proved for weakly compact convex subsets of a locally convex space, thus giving the nonlinear counterpart of the Ryll-Nardzewski theorem.

math.DS↗

arXiv2019

Amenable semigroups and nonexpansive dynamical systems

We characterize amenability of subspaces of $C(S)$, where $S$ is a semitopological semigroup, in terms of fixed point properties of nonexpansive actions. In particular, we give a complete characterization of a semitopological semigroup with a left invariant mean on WAP(S) that answers a question of A.T.-M. Lau and Y. Zhang in the affirmative. We also propose a new approach to Lau's problem concerning a counterpart of Day-Mitchell's characterization of amenable semigroups and show some partial results, in the case of weak$^{\ast }$ compact convex sets with the Radon-Nikodým property, and in the duals of $M$-embedded Banach spaces.

math.FA↗

arXiv2018

The Knaster-Tarski theorem versus monotone nonexpansive mappings

Let $X$ be a partially ordered set with the property that each family of order intervals of the form $[a,b],[a,\rightarrow )$ with the finite intersection property has a nonempty intersection. We show that every directed subset of $X$ has a supremum. Then we apply the above result to prove that if $X$ is a topological space with a partial order $\preceq $ for which the order intervals are compact, $\mathcal{F}$ a nonempty commutative family of monotone maps from $X$ into $X$ and there exists $c\in X$ such that $c\preceq Tc$ for every $T\in \mathcal{F}$, then the set of common fixed points of $\mathcal{F}$ is nonempty and has a maximal element. The result, specialized to the case of Banach spaces gives a general fixed point theorem that drops almost all assumptions from the recent results in this area. An application to the theory of integral equations of Urysohn's type is also given.

math.GN↗

arXiv2017

On some results on the stability of Markov operators

We formulate a criterion for the existence of an invariant measure for a Feller semigroup defined on a metric space with the e-property for bounded continuous functions and use it to prove the asymptotic stability of a semigroup satisfying a lower bound condition. Our results complement those of A. Lasota and J. A. Yorke in proper metric spaces and of T. Szarek in Polish spaces.

math.PR↗

arXiv2016

Amenable semigroups of nonexpansive mappings on weakly compact convex sets

We show a few fixed point theorems for semigroups acting on weakly compact convex subsets of Banach spaces when $LUC(S), AP(S), WAP(S)$ or $WAP(S)\cap LUC(S)$ have a left invariant mean. In particular, we give a characterization of semitopological semigroups that have a left invariant mean on the space of weakly almost periodic functions in terms of a fixed point property for nonexpansive mappings. It answers, in the case of Banach spaces, Question 4 of [A.T.-M. Lau, Y. Zhang, J. Funct. Anal. 263 (2012), 2949--2977] in affirmative. We also extend the fixed point theorem of R. Hsu from left reversible discrete semigroups to left amenable semitopological semigroups in Banach spaces.

math.FA↗

arXiv2016

Applications of uniform asymptotic regularity to fixed point theorems

We show that there are no nontrivial surjective uniformly asymptotically regular mappings acting on a metric space and derive some consequences of this fact. In particular, we prove that a jointly continuous left amenable or left reversible semigroup generated by firmly nonexpansive mappings on a bounded $τ$-compact subset of a Banach space has a common fixed point, and give a qualitative complement to the Markov-Kakutani theorem.

math.FA↗

arXiv2015

Uniformly Lipschitzian group actions on hyperconvex spaces

Suppose that $\{T_{a}:a\in G\}$ is a group of uniformly $L$-Lipschitzian mappings with bounded orbits $\left\{T_{a}x:a\in G\right\}$ acting on a hyperconvex metric space $M$. We show that if $L<\sqrt{2}$, then the set of common fixed points $Fix \, G$ is a nonempty Hölder continuous retract of $M$. As a consequence, it follows that all surjective isometries acting on a bounded hyperconvex space have a common fixed point. A fixed point theorem for $L$-Lipschitzian involutions and some generalizations to the case of $λ$-hyperconvex spaces are also given.

math.FA↗

arXiv2014

A common fixed point theorem for a commuting family of weak$^{\ast }$ continuous nonexpansive mappings

It is shown that if $S$ is a commuting family of weak$^{\ast }$ continuous nonexpansive mappings acting on a weak$^{\ast }$ compact convex subset $C$ of the dual Banach space $E$, then the set of common fixed points of $S$ is a nonempty nonexpansive retract of $C$. This partially solves a long-standing open problem in metric fixed point theory in the case of commutative semigroups.

math.FA↗

arXiv2014

Hyper-extensions in metric fixed point theory

We apply a modern axiomatic system of nonstandard analysis in metric fixed point theory. In particular, we formulate a nonstandard iteration scheme for nonexpansive mappings and present a nonstandard approach to fixed-point problems in direct sums of Banach spaces.

math.FA↗

arXiv2013

The minimal displacement and extremal spaces

We show that both separable preduals of $L_{1}$ and non-type I $C^*$-algebras are strictly extremal with respect to the minimal displacement of $k$-Lipschitz mappings acting on the unit ball of a Banach space. In particular, every separable $C(K)$ space is strictly extremal.

math.FA↗

arXiv2013

Hölder continuous retractions and amenable semigroups of uniformly Lipschitzian mappings in Hilbert spaces

Suppose that S is a left amenable semitopological semigroup. We prove that if ${T_{t}: t \in S}$ is a uniformly k-Lipschitzian semigroup on a bounded closed and convex subset C of a Hilbert space and $k<\sqrt{2}$, then the set of fixed points of this semigroup is a Hölder continuous retract of C. This gives a qualitative complement to the Ishihara-Takahashi fixed point existence theorem.

math.FA↗

arXiv2012

The fixed point property in direct sums and modulus R(a,X)

We show that the direct sum of Banach spaces $X_{1},..., X_{r}$ with a strictly monotone norm has the weak fixed point property for nonexpansive mappings whenever $M(X_{i})>1$ for each $i=1,...,r$. In particular, $(X_{1} \oplus ... \oplus X_{r})_ψ$ enjoys the fixed point property if Banach spaces $X_{i}$ are uniformly nonsquare. This combined with the earlier results gives a definitive answer for r=2: the direct sum of uniformly nonsquare spaces $X_{1}, X_{2}$ with any monotone norm has FPP. Our results are extended for asymptotically nonexpansive mappings in the intermediate sense.

math.FA↗

arXiv2012

On the Suzuki nonexpansive-type mappings

It is shown that if $C$ is a nonempty convex and weakly compact subset of a Banach space $X$ with $M(X)>1$ and $T:C\rightarrow C$ satisfies condition $(C)$ or is continuous and satisfies condition $(C_λ)$ for some $λ\in (0,1)$, then $T$ has a fixed point. In particular, our theorem holds for uniformly nonsquare Banach spaces. A similar statement is proved for nearly uniformly noncreasy spaces.

math.FA↗

arXiv2012

The super fixed point property for asymptotically nonexpansive mappings

We show that the super fixed point property for nonexpansive mappings and for asymptotically nonexpansive mappings in the intermediate sense are equivalent. As a consequence, we obtain fixed point theorems for asymptotically nonexpansive mappings in uniformly nonsquare and uniformly noncreasy Banach spaces. The results are generalized for commuting families of asymptotically nonexpansive mappings.

math.FA↗

arXiv2012

On the structure of fixed-point sets of asymptotically regular semigroups

We extend a few recent results of Górnicki (2011) asserting that the set of fixed points of an asymptotically regular mapping is a retract of its domain. In particular, we prove that in some cases the resulting retraction is Hölder continuous. We also characterise Bynum's coefficients and the Opial modulus in terms of nets.

math.FA↗