Questions tagged [fixed-point-theorems]

A fixed-point theorem is a result saying that a function $F$ will have at least one fixed point (a point $x$ for which $F(x) = x$), under some conditions on $F$ that can be stated in general terms.

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Convex sets via fixed point equations

I have an equation of the general form $$ X = S \cup T X $$ where $S \subset \mathbb R^n$ is a convex polytope (given by its bounding hyperplanes), $T\colon \mathbb R^n \to \mathbb R^n$ is a linear ...
rimu's user avatar
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Relationship between fixed points and inversions in permutations

Inversions in a permutation $Y$ are defined as pairs where $Y_a < Y_b$ but $a > b$, while fixed points in $Y$ are defined as elements where $Y_a = a$ (i.e., 1-cycles). Let $S_\alpha$ be the set ...
virtuolie's user avatar
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Is it known whether a homeomorphism close to the identity of a compact manifold with nonzero Euler characteristic necessarily has a fixed point?

I recently saw in Kirby's list of open problems that it isn't known if two commuting homeomorphisms of a compact manifold close to the identity necessarily share a common fixed point, when the ...
Ahmad Rafiqi's user avatar
3 votes
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Is there a fixed point theorem that applies to $f: \sum_k x_k 10^k \mapsto \sum_k x_k!$?

Let $f:\mathbb{Z}\rightarrow \mathbb{Z}$, $f:x=\sum_k x_k 10^k \mapsto \sum_k x_k!$ where $x_k$ is the $k$-th digit of $x$ in base ten. This function came up in a Project Euler problem. The question ...
Jackson Walters's user avatar
6 votes
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Are bounded groups of thin operators on Hilbert space similar to groups of unitaries?

QUESTION. Let $G$ be a group of bounded operators on $\ell^2$, satisfying $\sup_{x\in G} \lVert x\rVert <\infty$, whose elements are all of the form "identity+compact" (sometimes called &...
Yemon Choi's user avatar
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Is the Poincare Birkhoff theorem valid if we change the volume form of the annulus region?

Is the Poincare-Birkhoff theorem valid if we change the volume form of the annulus region? Note: A possible approach could be the following: Is it true to say that the answer is affirmative ...
Ali Taghavi's user avatar
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Extension of averaged nonexpansiveness for mappings that are not self maps

Let $\mathcal{H}$ be a Hilbert space and let $\alpha \in (0,1)$. We say that an operator $f:\mathcal{H} \rightarrow \mathcal{H}$ is Nonexpansive if $\|f(x)-f(y)\|_{\mathcal{H}} \le \|x - y\|_{\...
mlbj's user avatar
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A contraction mapping theorem

How to use the contraction mapping theorem to prove the following result: Let $X$ and $Y$ be Banach spaces, let $a>0$, and let $$B_a=B_a\left(z_0\right)=\left\{z \in X:\left\|z-z_0\right\| \leq a\...
Davidi Cone's user avatar
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Convergence of stochastic linear recurrences

Suppose that $\zeta_t$ is a univariate, stationary stochastic process ($t\in\mathbb{N}^+$). Let $x_0\in\mathbb{R}^n$, and let $f:\mathbb{R}\rightarrow\mathbb{R}^{n\times n}$ be a continuously ...
cfp's user avatar
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Generalization of Kakutani-Ky Fan Theorem without convexity assumptions

Crossposted at Mathematics SE I'm wondering if there exists some extension of the Kakutani-Ky Fan Theorem Theorem. Let $K$ be a nonempty, compact and convex subset of a locally convex space $X$. ...
Redeldio's user avatar
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Does a contractible locally connected continuum have an fixed point property?

I'm surprised that I can't find any research on this topic. Maybe it's too obvious? Kinoshita proved that contractible continuum do not have FPP, but his example is not locally connected. Maybe if we ...
LoliDeveloper's user avatar
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1 answer
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The ultrapower of the direct sum is the direct sum of ultrapowers

Currently, I'm reading the paper "Towards the fixed point property for superreflexive spaces" by Andrzej Wiśnicki. In this article, given $X_1,\dots,X_n$ Banach spaces, he defines $(X_1\...
JaviLark01's user avatar
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Implementable numerical scheme for the equation $a=\text{Erf}\big(z/\sqrt{2N_{a}}\big)$

Let $z>0$ be fixed and $A$ be the set of non-increasing functions from $\mathbb R_+$ to $[0,1]$ with norm $\|\cdot\|:=\|\cdot\|_\infty$. Define by $F$ the operator on $A$ by \begin{equation*} F(...
Fawen90's user avatar
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6 votes
1 answer
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Invariant subspaces for matrices via fixed points on Grassmannians

Let $A$ be an $n \times n$ invertible complex matrix. Let $Gr(k)=Gr(k,\mathbb{C}^n)$ be the complex $k$-Grassmannian, $1\leq k \leq n$. Since $A$ is invertible, it maps a $k$-dimensional subspace to a ...
Amudhan's user avatar
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12 votes
3 answers
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Fixed point theorem for the uncountable power of an interval

Does the Brouwer fixed point theorem holds for the uncountable power $[0,1]^\kappa$ of the interval, $\kappa\geq\aleph_1$ ? That is, does every continuous endomorphism $[0,1]^\kappa\to [0,1]^\kappa$ ...
user494312's user avatar
1 vote
1 answer
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Fixed points of rational continuous piecewise affine maps

Say that a compact convex polytope is rational if is the intersection of half-spaces whose bounding hyperplanes are the zero-sets of affine functions of the coordinates with rational coefficients. Say ...
James Propp's user avatar
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3 votes
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Is every tree a deformation retract of the disk?

I apologise if this question is not suitable for MathOverflow. We define a graph here to mean a disjoint union of points and copies of $[0,1]$ quotiented so that the endpoints of any interval lie on a ...
Isky Mathews's user avatar
2 votes
1 answer
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Differentiability of the fixed points of a family of contraction maps

Given a general Banach space $B$ and a one-parameter family of contractions $F_t:B\to B$ which is defined for all $t \in (a,b)$. $F_t$ depends continuously on $t$ (in the sense $\lim_{\varepsilon\to 0}...
toaster's user avatar
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Converse of Knaster-Tarski's theorem as choice principle

Knaster-Tarski's theorem States that if $(A,\le)$ is a complete lattice, then every monotone function $f :A \to A$ has a fixed point. The proof is carried out in $\mathsf{ZF}$. By $\mathsf{KTC}$ we ...
Average-user's user avatar
2 votes
1 answer
203 views

Fixed points cohomology via Lannes T-functor

Is there any reference to the proof of following: let $T$ denote the Lannes functor. Then (see the link above for more details) for any finite $E$-complex $X$ (where $E$ is finite-dimensional $\mathbb ...
Vanya Karpov's user avatar
4 votes
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Geometrical meaning of spaces that possess the weak* uniform Kadec-Klee property

What is the geometric meaning or interpretation of spaces that possess the weak* uniform Kadec-Klee property? I am writing the last part of my undergraduate thesis and I would like to add a comment in ...
Tomás Pérez Fernández's user avatar
9 votes
1 answer
659 views

Has Goedel's Second Incompleteness Theorem been proven using Lawvere's Fixed Point Theorem?

This question is a request for assistance in surveying the existing literature on applications of Lawvere's Fixed Point Theorem (LFPT). Yanofsky [0] has demonstrated several applications of LFPT to ...
jpt4's user avatar
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11 votes
1 answer
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Infinite vertex-transitive graph where every automorphism has a fixed vertex

This is a follow-up to the question Connected vertex-transitive graph with the fixed-point property. In particular, it is based on a comment by user bof. Let $G = (V,E)$ be a graph with $V$ infinite. ...
Sam Hopkins's user avatar
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5 votes
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Connected vertex-transitive graph with the fixed-point property

Many connected vertex-transitive graphs $G=(V,E)$ have the property that some of their automorphisms other than the identity have fixed points. To point out two simple examples: If $G = K_3$ then the ...
Dominic van der Zypen's user avatar
1 vote
0 answers
85 views

Reference for unique classical solution to quasilinear uniformly parabolic PDEs

In this post, the author mentioned that "we know there is a unique classical solution (see the references below, for example)". I have tried to read the two references the author provided, ...
beyond's user avatar
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1 answer
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Two maps into $[0,1]$ are equal at some point

In the paper below, there appears the following theorem: whose proof is left to the reader. It's not immediately obvious how I would prove this. How about the special case $X=Y=[0,1]$? It seems to be ...
D.S. Lipham's user avatar
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Banach's fixed point theorem for quasilinear parabolic PDEs

I have recently started reading into PDE theory, and came across the following question. Consider the PDE $$ \begin{cases} \partial_t \rho = \Delta (\rho + \rho^2) & \text{ on } (0,T) \times \...
Peter Koepernik's user avatar
2 votes
1 answer
353 views

The Existence of PDE by Banach vs Leray-Schauder fixed point

Regarding their proof, I deem the Banach fixed point theorem to be more analytical while Leray-Schauder more topological in nature. Owing to this, I am more inclined to use Banach method first, but ...
Math The Novice's user avatar
3 votes
0 answers
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A foliation version of S.Husseini counter example in fixed point theory

In this question we are indirectly inspired by an example by S.Husseini Amer. J.Math 1977 "The Products of Manifolds with the f.p.p. Need Not have the f.p.p" who gave an example of two ...
Ali Taghavi's user avatar
4 votes
0 answers
201 views

Conjecture on convergence of iterated near-matrix square root

Here is a simple problem that has stumped me for some time; sharing with the community, as I suspect it has been solved somewhere, or is immediately implied by the correct theorem. Let $\textbf{diag}: ...
Keith Rush's user avatar
71 votes
3 answers
5k views

Does iterating the derivative infinitely many times give a smooth function whenever it converges?

I am a graduate student and I've been thinking about this fun but frustrating problem for some time. Let $d = \frac{d}{dx}$, and let $f \in C^{\infty}(\mathbb{R})$ be such that for every real $x$, $$g(...
Paul Cusson's user avatar
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Fixed point of a contraction map

This question is a continuation of Is this a contraction mapping for small $T$? Set, for $T, m>0$, $H^m_T:=\{h:[0,T]\to [0,m]:~ h,~h' \mbox{ are both continuous on } [0,T]\}$ endowed with the norm $...
GJC20's user avatar
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2 votes
2 answers
239 views

Measure of non-commutativity of two invertible functions

I need to estimate $|x - f^{-1}(g^{-1}(f(g(x))))|$ for various values of $x$ for two smooth invertible functions $f$ and $g$ on $\mathbb{R}$ (actually some other spaces, but $\mathbb{R}$ will do.) Are ...
Jps's user avatar
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7 votes
1 answer
323 views

The Tarski-Lindenbaum theorem of the middle value

In Ettore Casari, La Matematica della Verita' (2006), the following theorem (attributed to Tarski and Lindenbaum 1926), is stated, which Casari calls the theorem of the middle value of Tarski-...
user65526's user avatar
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1 answer
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Can this fixed point theorem generalize to infinite structures?

Suppose that $n$ is a natural number, $X$ is a set, and $S\subseteq X^{2}$ is a subset such that if $x,y\in X$, then there is a unique tuple $(x_{0},\dots,x_{n})$ where $x_{0}=x,x_{n}=y$ and $(x_{i},...
Joseph Van Name's user avatar
12 votes
2 answers
1k views

A variation of the Ryll-Nardzewski fixed point theorem

Is there a fixed-point theorem that implies the following result? Let $F$ be a nonempty convex set of functions on a discrete group with values in $[0,1]$. Suppose $F$ is invariant with respect to ...
Anton Petrunin's user avatar
0 votes
1 answer
224 views

Continuity of Kakutani fixed points

Let $X$ be a compact and convex space and let $T=[0,1]$ be some parameter space. Let $F:X\times T\rightrightarrows X$ be a correspondence that is compact-valued, convex, and upper-hemicontinous. By ...
tsm's user avatar
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1 vote
1 answer
164 views

Invariant distributions for iterated random variables (stochastic dynamical systems)

This is related to discrete dynamical systems, with the initial condition $X_1$ being a random variable with a non singular distribution. The system is driven by the iteration $X_{n+1} = g(X_n)$ for ...
Vincent Granville's user avatar
14 votes
2 answers
957 views

Is there a square with all corner points on the spiral $r=k\theta$, $0 \leq \theta \leq \infty$?

I've posted this question on Math Stack Exchange, but I want to bring it here too, because 1) the proof seems missing in the literature, although they are some sporadic mentions and 2) maybe it ...
rgvalenciaalbornoz's user avatar
1 vote
0 answers
67 views

Fixed-point theorem for the space of probability flux

Let $\mathcal P_T:=\{\mu=(\mu_t)_{0\le t\le t}: \mu_t\in\mathcal P,~ \forall 0\le t\le 1\}$, where $\mathcal P$ is the space of probability measures on $\mathbb R$. Denote by $\rho$ the metric that is ...
GJC20's user avatar
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2 votes
1 answer
109 views

Smooth dependence in the fixed point theorem between complete Fréchet manifolds

Let $X,Y$ be complete metric spaces, and let $\Sigma:X\times Y\rightarrow Y$ be a continous mapping which satisfies the following property: there exists a $C<1$, such that for all $x\in X$ and $y_{...
MySheperd's user avatar
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1 vote
1 answer
148 views

Classical fixed-point argument and invertible function

Let $n\in\mathbb{N}$ and $W^{1,\infty}(\mathbb{R}^n)=\lbrace f:\mathbb{R}^n\rightarrow \mathbb{R}^n : \text{ f is bounded and Lipschitz continuous } \rbrace$. Suppose $f\in W^{1,\infty}(\mathbb{R}^n)$ ...
Oliver Watt's user avatar
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0 answers
95 views

Banach fixed point theorem / convergence squeeze

I am trying to prove a convergence result on an iterative scheme which has the initial point defined as $$x_1 = \frac{1 - s(x_0)}{s(x_0)}$$ where s(x) is some unknown function. Here is my theorem and ...
Doc Stories's user avatar
2 votes
0 answers
137 views

'Contraction-like' inequality: how to deal with the boundary term?

I am interested in the following problem. Let $D = \mathrm{diag}(d_1, d_2, \ldots, d_n) \in \mathbb{R}^n$ be positive definite, let $B, K \in \mathbb{R}^n$, and let $G\in L^\infty((0, T)\times (0, L);...
user344045's user avatar
3 votes
0 answers
69 views

Non-closed trajectories in convex billiards

This is a weak version of this problem, written down in Lviv Scottish Book. I start with necessary definitions. Let $K=-K$ be a centrally symmetric compact convex body in the Euclidean space $\mathbb ...
Taras Banakh's user avatar
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2 votes
3 answers
549 views

How do I apply Brouwer fixed-point theorem in this claim?

Let $\zeta:\mathbb{R}\to [0,+\infty)$ be a continuous non-negative function such that $\zeta(0)=0$ and $\tau\mapsto \zeta(\tau)\tau$ is a non-decreasing differentiable function whose derivative is ...
Guy Fsone's user avatar
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2 votes
0 answers
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Existence of infinite horizon values in dynamic programming

I am working through the book "Foundations of Stochastic Inventory Theory". One of the results in the book is Theorem 11.2. The background to this theorem is as follows. Given finite state ...
Tryer's user avatar
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2 votes
1 answer
345 views

What is the fixed-point origin?

There is a fixed-point construction used in Anil Gupta & Nuel Belnap, The Revision Theory of Truth, MIT-Press 1993, p. 194: Use only $\wedge, \lnot$ and $\forall$ as primitive connectives and ...
Frode Alfson Bjørdal's user avatar
5 votes
1 answer
856 views

Real eigenvectors of complex matrices

Let $A$ be a nonsingular complex $(3 \times 3)$-matrix (that is, an element of $\mathrm{GL}_3(\mathbb{C})$). Then what are some of the best-known criteria which guarantee $A$ to have real eigenvectors ...
THC's user avatar
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7 votes
2 answers
229 views

Fixed point for a map from $\{0,1\}^N$ to itself

Let $N\geq2.$ Let $F$ be a function from $\left\{ 0,1\right\} ^{N}$ to itself dreceasing for the product order defined by $$ (x_1,x_2,\ldots,x_N)\leq (y_1,\ldots,y_N)\ \text{ if and only if for all }...
Yoyo's user avatar
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