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L. Lorenzi

Publications and source records attributed to L. Lorenzi.

13 recordsLinked to original sources
arXiv2025

L^p-quasicontractiveness and Kernel estimates for semigroups generated by systems of elliptic operators

This paper focuses on systems of strongly coupled elliptic operators whose coefficients may be unbounded and are defined on a domain $Ω\subseteq \mathbb{R}^d$. It is shown that a quasi-contractive semigroup in $L^p$-spaces can be associated with such operators for values of $p$ belonging to an interval that contains $2$ as an interior point. Then, under refined assumptions and considering systems of elliptic operators coupled up to first order, new kernel estimates are established with respect to a distance function that accounts for the growth of the diffusion coefficients and the potential term at infinity.

math.AP↗
arXiv2022

Young equations with singularities

In this paper we prove existence and uniqueness of a mild solution to the Young equation $dy(t)=Ay(t)dt+σ(y(t))dx(t)$, $t\in[0,T]$, $y(0)=ψ$. Here, $A$ is an unbounded operator which generates a semigroup of bounded linear operators $(S(t))_{t\geq 0}$ on a Banach space $X$, $x$ is a real-valued $η$-Hölder continuous. Our aim is to reduce, in comparison to [4] and [1] (see also [2,5]) in the bibliography, the regularity requirement on the initial datum $ψ$ eventually dropping it. The main tool is the definition of a sewing map for a new class of increments which allows the construction of a Young convolution integral in a general interval $[a,b]\subset \mathbb R$ when the $X_α$-norm of the function under the integral sign blows up approaching $a$ and $X_α$ is an intermediate space between $X$ and $D(A)$.

math.FA↗
arXiv2021

Generation results for vector-valued elliptic operators with unbounded coefficients in L^p spaces

We consider a class of vector-valued elliptic operators with unbounded coefficients, coupled up to the first-order, in the Lebesgue space L^p(R^d;R^m) with p in (1,\infty). Sufficient conditions to prove generation results of an analytic C_0-semigroup T(t), together with a characterization of the domain of its generator, are given. Some results related to the hypercontractivity and the ultraboundedness of the semigroup are also established.

math.AP↗
arXiv2015

On coupled systems of Kolmogorov equations with applications to stochastic differential games

We prove that a family of linear bounded evolution operators $({\bf G}(t,s))_{t\ge s\in I}$ can be associated, in the space of vector-valued bounded and continuous functions, to a class of systems of elliptic operators $\bm{\mathcal A}$ with unbounded coefficients defined in $I\times \Rd$ (where $I$ is a right-halfline or $I=\R$) all having the same principal part. We establish some continuity and representation properties of $({\bf G}(t,s))_{t \ge s\in I}$ and a sufficient condition for the evolution operator to be compact in $C_b(\Rd;\R^m)$. We prove also a uniform weighted gradient estimate and some of its more relevant consequence.

math.AP↗
arXiv2012

Hypercontractivity and asymptotic behaviour in nonautonomous Kolmogorov equations

We consider a class of nonautonomous second order parabolic equations with unbounded coefficients defined in $I\times\R^d$, where $I$ is a right-halfline. We prove logarithmic Sobolev and Poincaré inequalities with respect to an associated evolution system of measures $\{μ_t: t \in I\}$, and we deduce hypercontractivity and asymptotic behaviour results for the evolution operator $G(t,s)$.

math.AP↗
arXiv2009

Mean Ergodic Theorems for Bi-continuous Semigroups

In this paper we study the main properties of the Cesàro means of bi-continuous semigroups, introduced and studied by Kühnemund in [24]. We also give some applications to Feller semigroups generated by second-order elliptic differential operators with unbounded coefficients in $C_b(\R^N)$ and to evolution operators associated with nonautonomous second-order differential operators in $C_b(\R^N)$ with time-periodic coefficients.

math.FA↗
arXiv2009

Optimal Holder regularity for nonautonomous Kolmogorov equations

We consider a class of nonautonomous elliptic operators ${\mathscr A}$ with unbounded coefficients defined in $[0,T]\times\R^N$ and we prove optimal Schauder estimates for the solution to the parabolic Cauchy problem $D_tu={\mathscr A}u+f$, $u(0,\cdot)=g$.

math.AP↗
arXiv2009

A fully nonlinear equation for the flame front in a quasi-steady combustion model

We revisit the Near Equidiffusional Flames (NEF) model introduced by Matkowsky and Sivashinsky in 1979 and consider a simplified, quasi-steady version of it. This simplification allows, near the planar front, an explicit derivation of the front equation. The latter is a pseudodifferential fully nonlinear parabolic equation of the fourth-order. First, we study the (orbital) stability of the null solution. Second, introducing a parameter $ε$, we rescale both the dependent and independent variables and prove rigourously the convergence to the solution of the Kuramoto-Sivashinsky equation as $ε\to 0$.

math.AP↗
arXiv2009

Asymptotic behavior in time periodic parabolic problems with unbounded coefficients

We study asymptotic behavior in a class of non-autonomous second order parabolic equations with time periodic unbounded coefficients in $\mathbb R\times \mathbb R^d$. Our results generalize and improve asymptotic behavior results for Markov semigroups having an invariant measure. We also study spectral properties of the realization of the parabolic operator $u\mapsto {\cal A}(t) u - u_t$ in suitable $L^p$ spaces.

math.AP↗
arXiv2008

Nonautonomous Kolmogorov parabolic equations with unbounded coefficients

We study a class of elliptic operators $A$ with unbounded coefficients defined in $I\times\CR^d$ for some unbounded interval $I\subset\CR$. We prove that, for any $s\in I$, the Cauchy problem $u(s,\cdot)=f\in C_b(\CR^d)$ for the parabolic equation $D_tu=Au$ admits a unique bounded classical solution $u$. This allows to associate an evolution family $\{G(t,s)\}$ with $A$, in a natural way. We study the main properties of this evolution family and prove gradient estimates for the function $G(t,s)f$. Under suitable assumptions, we show that there exists an evolution system of measures for $\{G(t,s)\}$ and we study the first properties of the extension of $G(t,s)$ to the $L^p$-spaces with respect to such measures.

math.AP↗
arXiv2008

On a class of hypoelliptic operators with unbounded coefficients in ${\matbb R}^N$

We consider a class of non-trivial perturbations ${\mathscr A}$ of the degenerate Ornstein-Uhlenbeck operator in ${\mathbb R}^N$. In fact we perturb both the diffusion and the drift part of the operator (say $Q$ and $B$) allowing the diffusion part to be unbounded in ${\mathbb R}^N$. Assuming that the kernel of the matrix $Q(x)$ is invariant with respect to $x\in {\mathbb R}^N$ and the Kalman rank condition is satisfied at any $x\in{\mathbb R}^N$ by the same $m<N$, and developing a revised version of Bernstein's method we prove that we can associate a semigroup $\{T(t)\}$ of bounded operators (in the space of bounded and continuous functions) with the operator ${\mathscr A}$. Moreover, we provide several uniform estimates for the spatial derivatives of the semigroup $\{T(t)\}$ both in isotropic and anisotropic spaces of (Hölder-) continuous functions. Finally, we prove Schauder estimates for some elliptic and parabolic problems associated with the operator ${\mathscr A}$.

math.AP↗