arXiv subjects

Martin Křepela

Publications and source records attributed to Martin Křepela.

11 recordsLinked to original sources

arXiv2026

Rearrangement-invariant norms commuting with dilations

We study rearrangement-invariant spaces $X$ over $[0,\infty)$ for which there exists a function $h:(0,\infty)\to (0,\infty)$ such that \[ \|D_rf\|_X = h(r)\|f\|_X \] for all $f\in X$ and all $r>0$, where $D_r$ is the dilation operator. It is shown that this may hold only if $h(r)=r^{-\frac1p}$ for all $r>0$, in which case the norm $\|\cdot\|_X$ is called $p$-homogeneous. We investigate which types of r.i. spaces satisfy this condition and show some important embedding properties.

math.FA↗

arXiv2023

Rearrangement-invariant hulls of weighted Lebesgue spaces

We characterize the rearrangement-invariant hull, with respect to a given measure $μ$, of weighted Lebesgue spaces. The solution leads us to first consider when this space is contained in the sum of $(L^1 + L^\infty)(R, μ)$ and the final condition is given in terms of embeddings for weighted Lorentz spaces.

math.FA↗

arXiv2022

A weak-type expression of the Orlicz modular

An equivalent expression of Orlicz modulars in terms of measure of level sets of difference quotients is established. The result in a sense complements the famous Maz'ya-Shaposhnikova formula for the fractional Gagliardo-Slobodeckij seminorm and its recent extension to the setting of Orlicz functions.

math.FA↗

arXiv2021

Lorentz and Gale-Ryser theorems on general measure spaces

Based on the Gale-Ryser theorem for the existence of suitable $(0,1)$-matrices for different partitions of a natural number, we revisit the classical result of G. G. Lorentz regarding the characterization of a plane measurable set, in terms of its cross sections, and extend it to general measure spaces.

math.FA↗

arXiv2021

Discretization and antidiscretization of Lorentz norms with no restrictions on weights

We improve the discretization technique for weighted Lorentz norms by eliminating all "non-degeneracy" restrictions on the involved weights. We use the new method to provide equivalent estimates on the optimal constant $C$ such that the inequality $$\left( \int_0^L (f^*(t))^{p_2} w(t)\,\mathrm{d}t \right)^\frac 1{p_2} \le C \left( \int_0^L \left( \int_0^t u(s)\,\mathrm{d}s \right)^{-\frac {p_1}α} \left( \int_0^t (f^*(s))^αu(s) \,\mathrm{d}s \right)^\frac {p_1}αv(t) \,\mathrm{d}t \right)^\frac 1{p_1}$$ holds for all relevant measurable functions, where $L\in(0,\infty]$, $α, p_1, p_2 \in (0,\infty)$ and $u$, $v$, $w$ are locally integrable weights, $u$ being strictly positive. It the case of weights that would be otherwise excluded by the restrictions, it is shown that additional limit terms naturally appear in the characterizations of the optimal $C$. A weak analogue for $p_1=\infty$ is also presented.

math.FA↗

arXiv2019

Solenoidal difference quotients and their application to the regularity theory of the $p$-Stokes system

We prove existence of a solution to the divergence equation satisfying a new Bogovski-type estimate for the difference quotients. This enables us to give an alternative proof of the interior regularity of the solution to the $p$-Stokes problem, completely avoiding the pressure. Moreover, as a key preliminary result we prove boundedness of Calderón-Zygmnud operators with standard kernels in weighted Lebesgue and Orlicz spaces over a general domain.

math.AP↗

arXiv2019

Weighted inequalities for iterated Copson integral operators

We solve a long-standing open problem in theory of weighted inequalities concerning iterated Copson operators. We use a constructive approximation method based on a new discretization principle that is developed here. In result, we characterize all weight functions $w,v,u$ on $(0,\infty)$ for which there exists a constant $C$ such that the inequality $$ \left(\int_0^{\infty}\left(\int_t^\infty \left(\int_s^{\infty}h(y)\,\text{d}y\right)^mu(s) \,\text{d}s\right)^{\frac{q}{m}}w(t)\,\text{d}t\right)^{\frac{1}{q}} \le C \left(\int_0^{\infty}h(t)^pv(t)\,\text{d}t\right)^{\frac{1}{p}} $$ holds for every non-negative measurable function $h$ on $(0,\infty)$, where $p,q$ and $m$ are positive parameters. We assume that $p\geq 1$ but otherwise $p,q$ and $m$ are unrestricted.

math.FA↗

arXiv2019

Weighted inequalities for discrete iterated Hardy operators

We characterize a three-weight inequality for an iterated discrete Hardy-type operator. In the case when the domain space is a weighted space $\ell^p$ with $p\in(0,1]$, we develop characterizations which enable us to reduce the problem to another one with $p=1$. This, in turn, makes it possible to establish an equivalence of the weighted discrete inequality to an appropriate inequality for iterated Hardy-type operators acting on measurable functions defined on $\mathbb{R}$, for all cases of involved positive exponents.

math.FA↗

arXiv2018

Embeddings and associated spaces of Copson-Lorentz spaces

Let $m,p,q\in(0,\infty)$ and let $u,v,w$ be nonnegative weights. We characterize validity of the inequality \[ \left(\int_0^\infty w(t) (f^*(t))^q \, dt \right)^\frac 1q \le C \left(\int_0^\infty v(t) \left(\int_t^\infty u(s) (f^*(s))^m \,ds \right)^\frac pm \! dt \right)^\frac 1p \] for all measurable functions $f$ defined on $\mathbb{R}^n$ and provide equivalent estimates of the optimal constant $C>0$ in terms of the weights and exponents. The obtained conditions characterize the embedding of the Copson-Lorentz space $CL^{m,p}(u,v)$, generated by the functional \[ \|f\|_{{CL^{m,p}(u,v)}} := \left(\int_0^\infty v(t) \left(\int_t^\infty u(s) (f^*(s))^m \,ds \right)^\frac pm \! dt \right)^\frac 1p, \] into the Lorentz space $Λ^q(w)$. Moreover, the results are applied to describe the associated space of the Copson-Lorentz space ${CL^{m,p}(u,v)}$ for the full range of exponents $m,p\in(0,\infty)$.

math.FA↗

arXiv2017

A counterexample related to the regularity of the $p$-Stokes problem

In this paper we construct a solenoidal vector field $\mathbf{u}$ belonging to $W^{2,q}(Ω)\cap W^{1,s}_0(Ω)$, $s \in (1,\infty)$, $q \in (1,n)$, such that $(1+|\mathbf{Du}|)^{p-2}$, $p\in (1,2)\cup(2,\infty)$, does not belong to the Muckenhoupt class $A_\infty(Ω)$. Thus, one cannot use the Korn inequality in weighted Lebesgue spaces to prove the natural regularity of the $p$-Stokes problem.

math.AP↗

arXiv2016

Boundedness of Hardy-type operators with a kernel: integral weighted conditions for the case $0<q<1\le p<\infty$

Let $U:[0,\infty)^2 \to [0,\infty)$ be a~measurable kernel satisfying: (i) $U(x,y)$ is nonincreasing in $x$ and nondecreasing in $y$; (ii) there exists a~constant $θ>0$ such that $U(x,z) \le θ\left( U(x,y)+U(y,z) \right)$ for all $0\le x 0$ for all $y>0$. Let $0<q<1< p <\infty$. We prove that the weighted inequality \[ \left( \int_0^\infty \left( \int_0^t f(x)U(x,t) dx \right)^q w(t) dt \right)^\frac 1q \le C \left( \int_0^\infty f^p(t)v(t)dt \right)^\frac 1p \] holds for all nonnegative measurable functions $f$ on $(0,\infty)$ if and only if \[ \left( \int_0^\infty \left( \int_t^\infty w(x)dx \right)^\frac{r}{p} w(t) \left( \int_0^t U^{p'}(z,t)v^{1-p'}(z) dy \right)^\frac{r}{p'} dt \right)^\frac 1r <\infty \] and \[ \left( \int_0^\infty \left( \int_t^\infty w(x) U^q(t,x) dx \right)^\frac{r}{p} w(t) \sup_{z\in(0,t)} U^q(z,t)\left( \int_0^z v^{1-p'}(s) ds \right)^\frac{r}{p'} dt \right)^\frac 1r <\infty, \] where $p':=\frac{p}{p-1}$ and $r:=\frac{pq}{p-q}$. Analogous conditions for the case $p=1$ and for the dual version of the inequality are also presented.

math.FA↗