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Luca Lorenzi

Publications and source records attributed to Luca Lorenzi.

At least 19 recordsLinked to original sources
arXiv2026

Maximal inequalities and Riesz transforms for vector-valued magnetic Schr\"odinger operators

We consider vector-valued magnetic Schr\"odinger operators $-\bm \Delta_{\bm a}+V$ with magnetic potential $\bm a \in L^2_{\mathrm{loc}}(\mathbb{R}^d;\mathbb{R}^d)$ and electric potential $V$ given by a matrix-valued function whose entries belong to $L^1_{\mathrm{loc}}(\mathbb{R}^d)$. We prove maximal inequalities in $L^p(\mathbb{R}^d;\mathbb{C}^m)$, $p\in[1,\infty)$ and the boundedness of the Riesz transforms $(\nabla - i\bm a)(-\bm \Delta_{\bm a}+V)^{-\frac{1}{2}}$ and $V^{\alpha}(-\bm \Delta_{\bm a}+V)^{-\alpha}$ on $L^p(\mathbb{R}^d;\mathbb{C}^m)$ for every $p \in (1,2]$ and every $\alpha\in[0,1/p]$.

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arXiv2025

Bounded $H^\infty$-calculus for vectorial-valued operators with Gaussian kernel estimates

We prove that the vector-valued generator of a bounded holomorphic semigroup represented by a kernel satisfying Gaussian estimates with bounded $H^\infty$-calculus in $L^2(\mathbb R^d;\mathbb C^m)$ admits bounded $H^\infty$-calculus for every $p\in (1,\infty)$. We apply this result to the elliptic operator $-{\rm div}(Q\nabla)+V$, where the potential term V is a matrix-valued function whose entries belong to $L^1_{\rm loc}(\mathbb R^d)$ and, for almost every $x\in \mathbb R^d$, $V(x)$ is a symmetric and nonnegative definite matrix.

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arXiv2024

Regularity results for elliptic and parabolic systems of partial differential equations

We study Cauchy problems associated to elliptic operators acting on vector-valued functions and coupled up to the first-order. We prove pointwise estimates for the spatial derivatives of the semigroup associated to these problems in the space of bounded and continuous functions over $\R^d$. Consequently, we deduce relevant regularity results both in H\"older and Zygmund spaces and in Sobolev and Besov spaces.

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arXiv2024

Space Regularity of Evolution Equations Driven by Rough Paths

In this paper, we consider the linear evolution equation $dy(t)=Ay(t)dt+Gy(t)dx(t)$, where $A$ is a closed operator, associated to a semigroup, with good smoothing effects in a Banach space $E$, $x$ is a nonsmooth path, which is $η$-Hölder continuous for some $η\in (1/3,1/2]$, and $G$ is a non-smoothing linear operator on $E$. We prove that the Cauchy problem associated with the previous equation admits a unique mild solution and we also show that the solution increases the regularity of the initial datum as soon as time evolves. Then, we show that the mild solution is also an integral solution and this allows us to prove a Itô formula.

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arXiv2023

$L^p$ Maximal regularity for vector-valued Schrödinger operators

In this paper we consider the vector-valued Schrödinger operator $-Δ+ V$, where the potential term $V$ is a matrix-valued function whose entries belong to $L^1_{\rm loc}(\mathbb{R}^d)$ and, for every $x\in\mathbb{R}^d$, $V(x)$ is a symmetric and nonnegative definite matrix, with non positive off-diagonal terms and with eigenvalues comparable each other. For this class of potential terms we obtain maximal inequality in $L^1(\mathbb{R}^d,\mathbb{R}^m).$ Assuming further that the minimal eigenvalue of $V$ belongs to some reverse Hölder class of order $q\in(1,\infty)\cup\{\infty\}$, we obtain maximal inequality in $L^p(\mathbb{R}^d,\mathbb{R}^m)$, for $p$ in between $1$ and some $q$.

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arXiv2023

Stability of traveling wave solutions in a credit rating migration Free Boundary Problem

In this paper, we study the stability of traveling wave solutions arising from a credit rating migration problem with a free boundary, After some transformations, we turn the Free Boundary Problem into a fully nonlinear parabolic problem on a fixed domain and establish a rigorous stability analysis of the equilibrium in an exponentially weighted function space. It implies the convergence of the discounted value of bonds that stands as an attenuated traveling wave solution.

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arXiv2023

Strongly coupled Schroedinger operators in L^p(R^d;C^m)

We consider systems of elliptic equations, possibly coupled up to the second-order, on the L^p(R^d;C^m)-scale. Under suitable assumptions we prove that the minimal realization in L^p(R^d;C^m)$ generates a strongly continuous analytic semigroup. We also prove the consistency of the semigroup on the L^p-scale and some spectral results.

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arXiv2022

Generation of semigroups associated to strongly coupled elliptic operator in $L^p(\mathbb R^d;\mathbb R^m)$

A class of vector-valued elliptic operators with unbounded coefficients, coupled up to the second-order is investigated in the Lebesgue space $L^p(\mathbb R^d;\mathbb R^m)$ with $p \in (1,\infty)$, providing sufficient conditions for the generation of an analytic $C_0$-semigroup $T(t)$. Under further assumptions, a characterization of the domain of the infinitesimal generator is given.

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arXiv2021

On weakly coupled systems of partial differential equations with different diffusion terms

We prove maximal Schauder regularity for solutions to elliptic systems and Cauchy problems, in the space $C_b(\mathbb{R}^d;\mathbb{R}^m)$ of bounded and continuous functions, associated to a class of nonautonomous weakly coupled second-order elliptic operators $\bf{\mathcal A}$, with possibly unbounded coefficients and diffusion and drift terms which vary from equation to equation. We also provide estimates of the spatial derivatives up to the third-order and continuity properties both of the evolution operator ${\bf G}(t,s)$ associated to the Cauchy problem $D_t{\bf u}=\bf{\mathcal A}(t){\bf u}$ in $C_b(\mathbb{R}^d;\mathbb{R}^m)$, and, for fixed $\overline t$, of the semigroup ${\bf T}_{\overline t}(τ)$ associated to the autonomous Cauchy problem $D_τ{\bf u}={\bf{\mathcal A}}(\overline t){\bf u}$ in $C_b(\mathbb{R}^d;\mathbb{R}^m)$. These results allow us to deal with elliptic problems whose coefficients also depend on time.

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arXiv2021

Regularity results for non-linear Young equations and applications

In this paper we provide sufficient conditions which ensure that the non-linear equation $dy(t)=Ay(t)dt+σ(y(t))dx(t)$, $t\in(0,T]$, with $y(0)=ψ$ and $A$ being an unbounded operator, admits a unique mild solution which is classical, i.e., $y(t)\in D(A)$ for any $t\in (0,T]$, and we compute the blow-up rate of the norm of $y(t)$ as $t\rightarrow 0^+$. We stress that the regularity of $y$ is independent on the smoothness of the initial datum $ψ$, which in general does not belong to $D(A)$. As a consequence we get an integral representation of the mild solution $y$ which allows us to prove a chain rule formula for smooth functions of $y$ and necessary conditions for the invariance of hyperplanes with respect to the non-linear evolution equation.

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arXiv2021

Carleman estimate for the Navier-Stokes equations and applications

For linearized Navier-Stokes equations, we first derive a Carleman estimate with a regular weight function. Then we apply it to establish conditional stability for the lateral Cauchy problem and finally we prove conditional stability estimates for inverse source problem of determining a spatially varying divergence-free factor of a source term.

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arXiv2019

Stability analysis and Hopf bifurcation at high Lewis number in a combustion model with free interface

In this paper we analyze the stability of the traveling wave solution for an ignition-temperature, first-order reaction model of thermo-diffusive combustion, in the case of high Lewis numbers (${\rm Le} >1$). The system of two parabolic PDEs is characterized by a free interface at which ignition temperature $Θ_i$ is reached. We turn the model to a fully nonlinear problem in a fixed domain. When the Lewis number is large, we define a bifurcation parameter $m=Θ_i/(1-Θ_i)$ and a perturbation parameter $\varepsilon= 1/{\rm Le}$. The main result is the existence of a critical value $m^c(\varepsilon)$ close to $m^c=6$ at which Hopf bifurcation holds for $\varepsilon$ small enough. Proofs combine spectral analysis and non-standard application of Hurwitz Theorem with asymptotics as $\varepsilon\to 0$.

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arXiv2018

On coupled systems of PDEs with unbounded coefficients

We study the Cauchy problem associated to parabolic systems of the form $D_t\boldsymbol{u}=\boldsymbol{\mathcal A}(t)\boldsymbol u$ in $C_b(\mathbb{R}^d;\mathbb{R}^m)$, the space of continuous and bounded functions $\boldsymbol{f}:\mathbb{R}^d\to\mathbb{R}^m$. Here $\boldsymbol{\mathcal A}(t)$ is a weakly coupled elliptic operator acting on vector-valued functions, having diffusion and drift coefficients which change from equation to equation. We prove existence and uniqueness of the evolution operator $\boldsymbol{G}(t,s)$ which governs the problem in $C_b(\mathbb{R}^d;\mathbb{R}^m)$ proving its positivity. The compactness of $\boldsymbol{G}(t,s)$ in $C_b(\mathbb{R}^d;\mathbb{R}^m)$ and some of its consequences are also studied. Finally, we extend the evolution operator $\boldsymbol{G}(t,s)$ to the $L^p$- spaces related to the so called "evolution system of measures" and we provide conditions for the compactness of $\boldsymbol{G}(t,s)$ in this setting.

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arXiv2018

Instabilities in a combustion model with two free interfaces

We study in a strip of $\mathbb R^2$ a combustion model of flame propagation with stepwise temperature kinetics and zero-order reaction, characterized by two free interfaces, respectively the ignition and the trailing fronts. The latter interface presents an additional difficulty because the non-degeneracy condition is not met. We turn the system to a fully nonlinear problem which is thoroughly investigated. When the width $\ell$ of the strip is sufficiently large, we prove the existence of a critical value $Le_c$ of the Lewis number $Le$, such that the one-dimensional, planar, solution is unstable for $0<Le<Le_c$. Some numerical simulations confirm the analysis.

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arXiv2017

Invariant measures for systems of Kolmogorov equations

In this paper we provide sufficient conditions which guarantee the existence of a system of invariant measures for semigroups associated to systems of parabolic differential equations with unbounded coefficients. We prove that these measures are absolutely continuous with respect to the Lebesgue measure and study some of their main properties. Finally, we show that they characterize the asymptotic behaviour of the semigroup at infinity.

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arXiv2017

On invariant measures associated to weakly coupled systems of Kolmogorov equations

In this paper, we deal with weakly coupled elliptic systems $\boldsymbol{\mathcal A}$ with unbounded coefficients. We prove the existence and characterize all the systems of invariant measures for the semigroup $({\bf T}(t))_{t\ge 0}$ associated to $\boldsymbol{\mathcal A}$ in $C_b(\mathbb R^d;\mathbb R^m)$. We also show some relevant properties of the extension of $({\bf T}(t))_{t\ge 0}$ to the $L^p$-spaces related to systems of invariant measures. Finally, we study the asymptotic behaviour of $({\bf T}(t))_{t\ge 0}$ as $t$ tends to $+\infty$.

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arXiv2017

${L^p}$-theory for Schrödinger systems

In this article we study for $p\in (1,\infty)$ the $L^p$-realization of the vector-valued Schrödinger operator $\mathcal{L}u := \mathrm{div} (Q\nabla u) + V u$. Using a noncommutative version of the Dore-Venni theorem due to Monniaux and Prüss, we prove that the $L^p$-realization of $\mathcal{L}$, defined on the intersection of the natural domains of the differential and multiplication operators which form $\mathcal{L}$, generates a strongly continuous contraction semigroup on $L^p(\mathbb{R}^d; \mathbb{R}^m)$. We also study additional properties of the semigroup such as extension to $L^1$, positivity, ultracontractivity and prove that the generator has compact resolvent.

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