arXiv subjects

Filip Tomić

Publications and source records attributed to Filip Tomić.

9 recordsLinked to original sources

arXiv2026

Extending wavelet regularity beyond Gevrey classes

We construct a smooth orthonormal wavelet $ψ$ such that both $ψ$ and its Fourier transform $\widehatψ$ belong to the extended Gevrey class $\mathcal{E}_σ(\mathbb{R})$ for $σ> 1$, providing an example that lies beyond all classical Gevrey classes. Our approach uses the idea of invariant cycles to extend the initial Lemarié-Meyer support of the low-pass filter $m_0$ from $ [-\frac{2π}{3}, \frac{2π}{3}]$ to $ [-\frac{4π}{5}, \frac{4π}{5}]$. This extension allows us to control the decay rate of $m_0$ near $\frac{2π}{3}$, which yields global decay estimates for $ψ$ and $\hatψ$. In addition, the decay rates are described using special functions involving the Lambert W function, which plays an important role in our construction.

math.FA↗

arXiv2024

An introduction to extended Gevrey regularity

Gevrey classes are the most common choice when considering the regularities of smooth functions that are not analytic. However, in various situations, it is important to consider smoothness properties that go beyond Gevrey regularity, for example when initial value problems are ill-posed in Gevrey settings. Extended Gevrey classes provide a convenient framework for studying smooth functions that possess weaker regularity than any Gevrey function. Since the available literature on this topic is scattered, our aim is to provide an overview to extended Gevrey regularity, highlighting its most important features. Additionally, we consider related dual spaces of ultradistributions and review some results on micro-local analysis in the context of extended Gevrey regularity. We conclude the paper with a few selected applications that may motivate further study of the topic.

math.AP↗

arXiv2024

Almost diagonalization of $Ψ$DO's over various generalized function spaces

Inductive and projective type sequence spaces of sub- and super-exponential growth, and the corresponding inductive and projective limits of modulation spaces are considered as a framework for almost diagonalization of pseudo-differential operators. Moreover, recent results of the first author and B. Prangoski related to the almost diagonalization of pseudo-differential operators in the context of Hörmander metrics are reviewed.

math.FA↗

arXiv2024

Band-limited wavelets beyond Gevrey regularity

It is known that a smooth function of exponential decay at infinity can not be an orthonormal wavelet. Dziubański and Hernández constructed smooth orthonormal wavelets of Gevrey type subexponential decay. We weaken the Gevrey type decay and construct orthonormal wavelets of subexponential decay related to the so-called extended Gevrey classes. The virtue of our construction is that prescise asymptotics of functions from such classes can be given in terms of the Lambert $W$ function.

math.FA↗

arXiv2022

Extended Gevrey regularity via weighted matrices

The main aim of this paper is to compare two recent approaches for investigating the interspace between the union of Gevrey spaces $\mathcal G_t (U)$ and the space of smooth functions $C^{\infty}(U)$. The first approach in the style of Komatsu is based on the properties of two parameter sequences $M_p=p^{τp^σ}$, $τ>0$, $σ>1$. The other one uses weight matrices defined by certain weight functions. We prove the equivalence of the corresponding spaces in the Beurling case by taking projective limits with respect to matrix parameters, while in the Roumieu case we need to consider a larger space then the one obtained as the inductive limit of extended Gevrey classes.

math.FA↗

arXiv2020

Boundary values in ultradistribution spaces related to extended Gevrey regularity

Following the well-known theory of Beurling and Roumieu ultradistributions, we investigate new spaces of ultradistributions as dual spaces of test functions which correspond to associated functions of logarithmic-type growth at infinity. In the given framework we prove that boundary values of analytic functions with the corresponding logarithmic growth rate towards the real domain are ultradistributions. The essential condition for that purpose, condition $(M.2)$ in the classical ultradistribution theory, is replaced by the new one, $\widetilde{(M.2)}$. For that reason, new techniques were performed in the proofs. As an application, we discuss the corresponding wave front sets.

math.FA↗

arXiv2019

A Paley-Wiener theorem in extended Gevrey regularity

In this paper we introduce appropriate associated function to the sequence $M_p=p^{\t p^{\s}}$, $p\in \N$, $\t>0$, $\s>1$, and derive its sharp asymptotic estimates in terms of the Lambert $W$ function. These estimates are used to prove a Paley-Wiener type theorem for compactly supported functions from extended Gevrey classes. As an application, we discuss properties of the corresponding wave front sets.

math.FA↗

arXiv2017

Superposition and propagation of singularities for extended Gevrey regularity

We use sequences which depend on two parameters to define families of ultradifferentiable functions which contain Gevrey classes. It is shown that such families are closed under superposition, and therefore inverse closed as well. Furthermore, we study partial differential operators whose coefficients satisfy the extended Gevrey regularity. To that aim we introduce appropriate wave front sets and derive a theorem on propagation of singularities. This extends related known results in the sense that weaker assumptions on the regularity of the coefficients are imposed.

math.FA↗

arXiv2016

Beyond Gevrey regularity

We define and study classes of smooth functions which are less regular than Gevrey functions. To that end we introduce two-parameter dependent sequences which do not satisfy Komatsu's condition (M.2)', which implies stability under differential operators within the spaces of ultradifferentiable functions. Our classes therefore have particular behavior under the action of differentiable operators. On a more advanced level, we study microlocal properties and prove that $${\rm WF}_{0,\infty}(P(D)u)\subseteq {\rm WF}_{0,\infty}(u)\subseteq {\rm WF}_{0,\infty}(P(D)u) \cup {\rm Char}(P),$$ where $u$ is a Schwartz distribution, $P(D)$ is a partial differential operator with constant coefficients and ${\rm WF}_{0,\infty}$ is the wave front set described in terms of new regularity conditions. For the analysis we introduce particular admissibility condition for sequences of cut-off functions, and a new technical tool called enumeration.

math.AP↗